• The diagnosis of Toxoplasma gondii infection relies on serologic detection of specific IgM and IgG antibodies.
• Specific IgM antibodies with low IgG (IgG-avidity testing) are consistent with recent infection in immunocompetent persons. Positive IgG antibodies in the absence of IgM in healthy persons indicate past infection and resistance to reinfection.
• Amniocentesis and polymerase chain reaction (PCR)-based analysis (past 18 weeks of gestation) are useful to establish a certain or presumed seroconversion in pregnancy and to determine whether the infection was transmitted to the offspring.
• IgM and/or IgA at birth indicate probable connatal infection.
• Serologic testing is not useful for the diagnosis of toxoplasmic encephalitis in AIDS patients, who should instead have CT or MRI imaging, cerebrospinal fluid (CSF)-PCR testing, or demonstration of tachyzoites by histology.
• Serological screening is helpful in identifying transplant patients at risk, especially seronegative recipients with seropositive donors, and can help in establishing the diagnosis by showing seroconversion. Serological follow-up combined with PCR after allogeneic hematopoietic stem cell transplant (HSCT) is recommended in all patients at risk for toxoplasmosis.
• Universal prophylaxis with TMP/SMX in allo pre-HCTSP patients should be implemented by all transplant programmes.
• All immunocompetent patients as well as non pregnant women with acute toxoplasmosis presenting with lymphadenitis and fever, generally do not require specific antimicrobial treatment because the infection is self-limited and usually subclinical.
• Pyrimethamine (Daraprim) plus sulfadiazine1 plus folinic acid (Leucovorin)1 is the standard and preferred regimen against toxoplasmosis. It is recommended for acute T. gondii infection in immunocompetent adults with acute illness; pregnant women who acquire the infection after 18 weeks of gestation or in whom fetal infection is documented (positive PCR-AF); and immunosuppressed patients, including AIDS patients. Patients who do not tolerate the standard regimen should be given alternative drugs such as clarithromycin (Biaxin),1 azithromycin (Zithromax),1 atovaquone (Mepron),1 and dapsone.1
• Oral spiramycin (Rovamycine)2 is the drug most often used in the prenatal therapy of congenital toxoplasmosis (before 18 weeks of gestation) because of high concentration in placenta without crossing it and of its relative lack of toxicity compared to the teratogenic effects of pyrimethamine. Spiramycin is more efficacious when administered early after maternal seroconversion.
• Trimethoprim-sulfamethoxazole (TMP-SMX; Bactrim)1 should be given as a prophylaxis to HIV-positive patients who have CD4+ cell counts that are less than 100/mm3 and IgG T. gondii antibodies and who are not already receiving a PCP prevention regimen as well as to patients with active toxoplasmosis, transplant patients, and workers following accidental laboratory exposure.
• In ocular toxoplasmosis, corticosteroid therapy without the concomitant use of antimicrobial agents, even in immunocompetent patients, can lead to severe tissue destruction.
• The search for new and efficient pharmacological treatments against toxoplasmosis is under intense and frequent evaluation, in part due to the lack of successful and specific parasitic therapies directed against proliferating tachyzoites or the tissue cyst of Toxoplasma.
• Ethnobotanical and experimental evidence (in vitro/in vivo) was found supporting the use of natural products as a source for the discovery of new therapies against T. gondii.
1 Not FDA approved for this indication.
2 Not available in the United States.
Toxoplasma gondii is a ubiquitous protozoan parasite that is extremely widespread and of great medical importance, infecting all mammalian cells and responsible for human and veterinary diseases. It was initially described in Tunis by Nicolle and Manceaux (1908) in the tissues of the gundi (Ctenodoactylus gundi) and later in Brazil by the microbiologist Alfonso Splendore (1908) in the rabbit. Its identification was rapidly followed by the recognition that it was a human pathogen. In this regard, the Italian bacteriologist Castellani (1914) was probably the first to describe a T. gondii-like parasite in smears of blood and spleen from a 14-year-old Singhalese boy who died from a disease characterized by severe anemia, fever, and splenomegaly.
However, it was not until the 1960s and 1970s that the parasite was identified as a coccidian and the cat recognized as the definite host.
Toxoplasma belongs to the phylum Apicomplexa, which contains many other protozoan pathogens of human and veterinary importance, such as Plasmodium spp. (malaria), Cryptosporidium spp. (cryptosporidiosis), and Eimeria spp. (poultry coccidiosis).
Disease can occur through acute infection after recent contact with T. gondii cysts or oocysts or through endogenous reactivation. Primary infection is usually subclinical, but in some patients cervical or occipital lymphadenopathy or ocular disease is present. Infection acquired during pregnancy can cause severe damage to the fetus if Toxoplasma crosses the placental barrier, and it causes abortion or congenital birth defects if the mother becomes infected for the first time shortly before or during pregnancy.
In AIDS patients and others who are immunocompromised persons, reactivation of latent disease can cause life-threatening encephalitis. Ocular infection by Toxoplasma is a major cause of retinochoroiditis in several geographic areas in both immunocompetent and immunocompromised persons.
Epidemiology and Risk Factors
Toxoplasmosis is a cosmopolitan zoonotic disease that has important implications for public health because it affects one-third of the world’s population. It is also a significant veterinary pathogen that can infect many species of warm-blooded animals.
The incidence of positive serology for Toxoplasma varies greatly around the world and is influenced by different cultures. In Colombia, approximately half of the women of child bearing age have T. gondii antibodies, and the clinical disease in congenitally infected children is more severe than in Europe. In humans, seroprevalence of T. gondii infection rises with age; does not vary greatly between sexes; and is lower in cold regions, hot and arid areas, and at high elevations.
Prevalence rates are thought to depend on food production and harvesting practices, water treatment, environment, climate, and exposure to soil or sand. In the United States, the seroprevalence of T. gondii appears to be declining. Seroprevalence in Europe is high, up to 54% in southern European countries; it decreases with increasing latitude to 5% and 10% in northern Sweden and Norway, respectively.
In Brazil, there is a higher prevalence of T. gondii infection among male patients (79.0%) than among female patients (63.4%), according to data obtained from a blood bank. In general, Toxoplasma infections are especially prevalent in Europe, South America, and Africa.
The sexual cycle of T. gondii occurs in felines. The estimated seroprevalence for T. gondii in domestic cats (Felis catus), worldwide, is 30% to 40%. Most feline infections occur postnatally through ingestion of infected tissue cysts or rarely oocysts, although congenital infections occur. Tens of millions of unsporulated oocysts may be released in the feces of a single cat in a day, depending on the stage of T. gondii ingested. These sporulate in 1 to 21 days and are highly infectious to the parasite’s intermediate hosts (asexual cycle), which include almost any warm-blooded animal, such as birds, humans, and sea otters.
Sporulated oocysts are very resistant to environmental conditions and to disinfectants; however, they are killed within 1 to 2 minutes by heating to 55ºC to 60ºC, and the risk of infection is reduced by deep- freezing meat (− 12°C or lower) before cooking.
Feline infections are typically subclinical. Common symptoms of T. gondii infection in cats can include fever, ocular inflammation, anorexia, lethargy, abdominal discomfort, and neurologic abnormalities. Occasionally, pneumonia, liver damage, and loss of vision develop. Why only some cats show symptoms is not known.
Identification of locally prevalent risk factors is critical for health education, and it is generally important for policy. The most important recognized factors influencing the risk of T. gondii infection are having a cat or a dog, doing household work, having a lower education level, having poor hygiene habits, eating raw vegetables, and working in contact with soil.
Most people are infected inadvertently, and thus the specific route of transmission usually cannot be established. The contact with this obligate intracellular protozoan can occur through the ingestion of oocysts containing sporozoites or cysts containing bradyzoites in contaminated food or water. The major risk factors for T. gondii exposure are directly related to exposure to cats and more specifically to cat feces, which represent the source of ingestion of sporozoites from the environment. Because cats are the primary host for T. gondii, cats in the house or stray cats in and around the house or property are considered a primary risk factor for acquiring this parasite during pregnancy. Moreover, any job or activity that puts a pregnant woman in direct contact with soil, sand, or materials such as fruit, vegetables, or drinking water that could have been contaminated by cat feces puts her at risk for being infected. In this setting, rain and surface water can transport infectious oocysts into drinking water supplies and irrigation waters. Coprophagous insects that can contaminate food and fertilizer also contribute to the spread of oocysts. Climate plays an indirect role in allowing better (in the case of moist and hot climate) or worse (in the case of dry and cold climate) survival of oocysts in the environment.
Consumption of undercooked meat of secondary hosts such as pig and sheep is also a major route of transmission of the disease to humans. The ingestion of undercooked or raw meat during pregnancy is also an important risk factor because the tissue can contain T. gondii cysts that, unless destroyed by cooking or by food-preparation practices, could infect a pregnant woman. Heating at 60°C to 100°C for 10 minutes, freezing at either − 10°C for 3 days or − 20°C for 2 days, or irradiation at doses of 75 to 100 krad is sufficient to kill tissue cysts. Tissue cysts are also killed by gamma irradiation at a dose of 1.0 kGy, but irradiation of meat has not been approved in the European Union (EU). Neither cooking in a microwave oven nor chilling at 5°C for 5 days is sufficient to kill tissue cysts.
Pork was previously identified as a main risk factor in some EU countries such as Norway and Italy, but its importance as a route for infection is now reduced, possibly because pregnant women are most aware of this specific risk. In this regard, it was found that the risk of infection rises in women who taste meat when preparing meals or who eat raw or undercooked beef, lamb, or other meats, but not pork. Eating raw horsemeat imported from non-EU countries can expose consumers to high inocula of highly virulent atypical T. gondii strains, which can cause a life-threatening primary infection or severe congenital toxoplasmosis with atypical outcome. Transmission during breast-feeding or direct human-to-human transmission other than from mother to fetus (discussed later in the chapter) has not been recorded. Drinking unpasteurized milk and consuming milk products also correlates with increased risk of infections. Although oral transmission is considered the main route of infection, it does not explain the common occurrence of toxoplasmosis in a variety of hosts, such as herbivorous animals, birds, and wild rodents. In this regard, T. gondii infection has been discovered in particular ticks (ie. Haemaphysalis longicornis) that could serve as a reservoir for secondary toxoplasmosis transmission to other common hosts, if ingested. Other routes are transplacental infection of the fetus, transfusion of white blood cells, and organ transplantation from a seropositive donor to a seronegative recipient.
It is not uncommon for health care professionals, laboratory workers, pet lovers (especially cat owners), butchers, cooks (those handling raw meat), veterinarians, and farmers to acquire acute toxoplasmosis.
Finally, an interesting hypothesis suggested that toxoplasmosis may be transmitted from infected men to noninfected women during unprotected sexual intercourse resulting in the most dangerous form of disease, the congenital toxoplasmosis. T. gondii tachyzoites in fact, are present in the seminal fluid and tissue of the testes of various animals including humans.
Dogs can also have a role in the transmission of toxoplasmosis as a mechanical vector by rolling in foul-smelling substances and by ingesting fecal material. Unlike in cats, T. gondii does not replicate in the dog’s gut and no cysts are shed. In areas where dogs and cats are plentiful, immunocompromised persons and pregnant women should be warned of the possibility of acquiring T. gondii from dogs as well as from soil contaminated by cats. People should be encouraged to wash their hands after contact with soil, dogs, or cats as well as before eating.
T. gondii infection has important veterinary implications because it causes disease, miscarriage, and congenital malformations in the definitive and intermediate host. Cats, sheep, pigs, and goats are the domestic animal species most seriously affected by the protozoan. In sheep, T. gondii is an important agent of abortion and neonatal mortality in lambs.
Disease can occur through acute infection after recent contact with T. gondii cysts or oocysts or through endogenous reactivation. Following ingestion, the sporozoites or bradyzoites invade the intestinal epithelium and differentiate to tachyzoites, which disseminate and replicate within the new host. Transport of the parasite via the bloodstream can occur intracellularly within dendritic cells or monocytes or as a free tachyzoite. Certain Toxoplasma surface antigens aid in the interaction between the tachyzoite and the host cell. One of this is the Perforin-like Protein 1 (PLP-1) forming pores in the host-cell membrane after binding as well as playing a role in egress. The success of Toxoplasma as a widespread pathogen is due to the effortlessness with which it can be transmitted among the intermediate hosts. Once inside a host, the parasite develops powerful tools to modulate its host cell and to develop into a chronic infection, undergoing bradyzoite development that can evade the host’s immune system as well as, in contrast with acute toxoplasmosis, all known antitoxoplasmic drugs.
Host protection from T. gondii results from a complex cell-mediated immune response involving inflammatory cells, lymphocytes and macrophages, and cytokines. Inoculum size, parasite virulence (strain), genetic background, time of infection (congenital versus postnatal contamination), and sex also contribute to affect the course of infection in human beings and animal models of toxoplasmosis. In particular, the T. gondii genotype affects its replication rate, migration, and tendency to differentiate to bradyzoites, virulence, and epidemiologic pattern of occurrence. Type II genotypes (most strains isolated from AIDS patients and newborns with congenital disease) and type III genotypes (mostly isolated from animals) are generally less virulent and more cystogenic compared to type I genotypes (also found in congenital disease). Ocular toxoplasmosis in humans is associated with type I but not type II or III genotypes.
Other than the three major Toxoplasma clonal lineages designated as types I, II and III, which are only predominant in Europe and North America, there are highly polymorphic strains referred to as atypical in South America and Africa, termed Africa 1, 2 and 3.
Genetic background also plays a significant role in increased susceptibility to T. gondii in humans; HLA-DQ3 appears to be a genetic marker associated with susceptibility to developing toxoplasmic encephalitis in AIDS patients. The mechanisms by which T.gondii invades host cells and forms an intracellular niche have been extensively reviewed, and several aspects of this process are directly relevant to immunity and pathogenesis. During invasion, three successive waves of proteins are secreted from parasite organelles, (the micronemes, dense granules, and rhoptries) into the host cell. Rhoptry proteins are the major virulence factors of Toxoplasma gondii and are located in different parts of the host cells. Blocking the cell intrinsic defense mechanisms of the host, let T. gondii invade, parasitize and proliferate in the host successfully. These proteins can alter host cell function and inhibit the immune response directed toward the parasite.
During infection in the intermediate host, T. gondii undergoes stage conversion between the rapidly dividing tachyzoite that is responsible for acute toxoplasmosis and the slowly replicating, encysted bradyzoite stage. This process of tachyzoite-bradyzoite interconversion is central to the pathogenesis and longevity of infection. In normal conditions, the tachyzoite stage is thwarted by the prompt and efficient interferon (IFN)-γ-dependent cell-mediated immune response, which eventually kills off the majority of the disseminating tachyzoites before eventually entering the persistent form, the bradyzoite. Bradyzoites are encysted within various tissues, most notably the brain but also muscle, eye, and lung, and are infectious to another intermediate host or the cat if eaten. Cell- mediated immune mechanisms thus play a major role in the control of T.gondii infection because the parasite is exclusively localized intracellularly.
Following infection, T. gondii evokes a powerful and persistent T- helper-1 (Th1) response (dendritic cell activated) together with neutrophils, inflammatory monocytes, and macrophages. The response is characterized by production of proinflammatory cytokines, including interleukin (IL)-12, INF-α and tumor necrosis factor (TNF)-α, which, together with other immunologic mechanisms, protect the host against rapid replication of tachyzoites and subsequent pathologic changes. The outcome of toxoplasmic infection depends on a balance between proinflammatory (IL-12, TNF-α) and downregulatory (IL-10, IL-27) cytokines that suppress parasite proliferation and control the inflammatory response, respectively.
INF-γ, produced by both CD4 and CD8 cells in response to Toxoplasma is the major cytokine involved with acute and chronic resistance to T. gondii infection in immunocompetent hosts because it controls tachyzoite growth and subsequent pathologic changes.
Macrophages activated by INF-γ inhibit parasite replication through a number of potent microbicidal systems, including oxidative and nonoxidative mechanisms as well as the induction of indoleamine 2-3- dioxygenase that degrades tryptophan, which is required for T. gondii replication.
The toll-like receptor (TLR) is another critical pathway in initiating defense against this opportunistic protozoan, which can also be a mediator of pathology during immune dysfunction. In fact, the innate production of IL-12 requires that the parasite first be sensed by the host; innate TLRs, particularly 2, 4, 9, and 11, have an important role in this process. TLR4 plays important roles in the recognition and stimulation of immune responses against T. gondii. To combat the host immune system, the parasite targets TLR4 and its intracellular signaling. TLR4 also participates in the pathogenesis of Toxoplasmosis. A therapeutic strategy using the TLR4 pathway to treat T. gondii would be possible, but treatment would need to be followed carefully to avoid the detrimental consequences associated with this pathway.
Clinical Presentation in the Immunocompetent
Acute acquired toxoplasmosis has traditionally been considered an oligosymptomatic and self-limited infection in previously healthy patients. The typical clinical presentation of acute T. gondii infection is a short flu-like or mononucleosis-like illness that includes prolonged fever, headache, persistent enlarged but firm lymph nodes (rarely painful, but initially tender) unattached to the overlying skin, and, occasionally, myalgia and gastrointestinal symptoms that rarely need treatment. Special consideration must be given to other infectious or noninfectious diseases that cause similar symptoms and should always be included in the differential diagnosis: mononucleosis, cat scratch disease, tuberculosis, primary HIV infection, sarcoidosis, metastatic cancer, and lymphoma. In these cases, serologic testing is the initial and primary method of diagnosis.
Lymph node biopsy should be reserved for when Toxoplasma serology is uncertain and all other infectious serologic investigations are negative. A form of the disease characterized by chronic lymphadenopathy has been described, and lymph node enlargement can fluctuate for months. Hepatomegaly and hepatitis with a moderate increase in liver enzyme levels (a 5- to 10-fold elevation) are common. The incidence of hepatitis in persons with Toxoplasma infection varies between 11% and 89%. Lymphocytosis and atypical lymphocytes are other laboratory hallmarks of toxoplasmosis. The spleen appears to be involved early in toxoplasmosis, but palpable splenomegaly is uncommon.
Clinical pulmonary involvement is rarely described in immunocompetent hosts, in whom it usually manifests as atypical pneumonia. Myocarditis, polymyositis, or encephalitis can be observed very infrequently in otherwise healthy persons. Acute Toxoplasma infection during pregnancy is asymptomatic in most women.
Although malaria and dengue are the most common etiologies of fever in travelers, toxoplasmosis should be considered in the differential diagnosis of fever, particularly in travelers with non- specific symptoms such as fatigue, fever, headache and lymphadenopathy. In 72 prospectively studied travelers with the mononucleosis-like syndrome, toxoplasmosis was the cause in 22% and it was clinically indistinguishable of other causes. Atypical presentations (such as pneumonitis or ocular toxoplasmosis) should be kept in mind in persons who return from a travel.
Latent toxoplasmosis is characterized by the lifelong presence of cysts of the parasite in different host tissues, including the nervous system, and by the presence of anamnestic Toxoplasma immunoglobulin (Ig)G antibodies in the serum. Long considered asymptomatic, latent toxoplasmosis might increase the risk of schizophrenia and Parkinson’s disease; influence human personality and behavior; impair psychomotor performance; and increase the risk of suicide, traffic accidents, and the probability of the birth of male offspring. Toxoplasmosis should also be regarded as an epilepsy risk factor. Epileptogenic mechanisms are probably multifactorial (direct lodgment of parasite, direct modulation of neuronal functions, abnormalities in GABA, role of calcium, etc). etc. Recently, it was suggested that chronic latent neuroinflammation caused by the parasite may be responsible for the development of several neurodegenerative diseases manifesting with the loss of smell.
Olfactory dysfunction reported in Alzheimer’s disease, multiple sclerosis, and schizophrenia was frequently associated with the significantly increased serum anti-T gondii immunoglobulin G antibody levels. Higher rates of T. gondii seropositivity have been reported for several psychiatric conditions in multiple parts of the world. A series of studies comparing the prevalence rates of the parasite with the national suicide rates in 20 European countries found that suicide rates were higher in countries with greater T. gondii prevalence, regardless of national wealth or gross domestic product. Many of the observed behavioral effects of toxoplasmosis might be a result of the impairment of the immune system and of increased level of dopamine in the brain tissue in response to IL-2 produced by immune cells in the sites of local inflammation in the infected brain. However, RhD phenotype also plays an important role in the strength and direction of association between latent toxoplasmosis and personality and intelligence. Recently, Toxoplasma has been implicated as possible cause of some cases of psychosis in patients with Bipolar Disorder type I with a latent T. gondii infection. Another recent finding is that people seropositive for both Helicobacter pylori and latent toxoplasmosis – both of which appear to be common in the general population – appear to be more susceptible to cognitive deficits than are people seropositive for either Helicobacter pylori and or latent toxoplasmosis alone, suggesting a synergistic effect between these two infectious diseases on cognition in young to middle-aged adults.
Finally, a very important aspect that has emerged in recent years concerns the possible occurrence of toxoplasmosis in patients receiving biotherapies, particularly anti-TNF-α agents for the treatment of rheumatologic diseases and other conditions. In this setting, rituximab (Rituxan) induces B-cell depletion and influences T- cell immunity, which could consequently predispose patients to serious infectious complications, including cerebral toxoplasmosis.
Toxoplasma serology should be performed in patients before treatment with TNF-α antagonists is initiated, and patients should be advised to avoid situations that increase risk of exposure to Toxoplasma.
Clinical Presentation in Immunocompromised Patients
The presence of latent bradyzoite cysts, along with the ability of bradyzoites to reconvert into the active and rapidly growing tachyzoites that can result in often-fatal injury, explains the high incidence of acute toxoplasmosis often observed in immunocompromised persons. In immunocompromised hosts and in AIDS patients, the central nervous system (CNS) is the site most typically affected by infection, and toxoplasmic encephalitis is the most common manifestation. This is also the third most common condition associated with AIDS in Brazil, still accounting for high mortality and morbidity despite free access to Antiretroviral therapy (ART).
However, as in transplant patients or those with malignant hemolymphopathies or solid tumors, other organs such as the lungs or eyes may be involved. Most of these cases result from reactivation of latent infection, although reinfection with a different T. gondii strain in the transplanted organs can also occur.
Although the mortality of pulmonary toxoplasmosis was high before the advent of ART, AIDS patients and recipients of bone marrow transplants are also at risk for developing pulmonary toxoplasmosis, especially patients with very low CD4 cell counts. The clinical manifestations are nonspecific and are similar to those of Pneumocystis jirovecii.
In immunocompromised patients with suspected pulmonary toxoplasmosis, conventional staining is the most appropriate method to diagnose for experienced microbiology laboratories, whereas T. gondii-specific PCR may be useful for laboratories with less experience in parasitology.
The predictive value of elevated blood levels of lactate dehydrogenase (LDH) is of uncertain value. Patients with toxoplasmic encephalitis typically present with headache, confusion, altered mental status, and motor weakness. Focal neurologic deficits or seizures, weakness, sensory abnormalities, cerebellar signs, and neuropsychiatric manifestations are also common. Fever is usually, but not reliably, present. Accompanying nausea or vomiting usually indicates elevated intracranial pressure. Computed tomography (CT) scan or magnetic resonance imaging (MRI) of the brain typically shows multiple contrast-enhancing lesions, often with associated edema. These should be distinguished from other infectious or noninfectious CNS diseases in the course of AIDS.
In immunocompromised patients, the infection can disseminate rapidly and manifests with nonspecific symptoms such as fever and malaise. It can affect a number of organs, including the brain, cerebellum (an uncommon presentation) eye, liver, and lungs as well as skeletal muscle, bone marrow, bladder, and spinal cord. Cardiac toxoplasmosis can occur during the course of multivisceral dissemination. However, in patients with AIDS, it is usually asymptomatic and found only at autopsy, typically in the setting of widely disseminated infection. Acquired toxoplasmosis with cutaneous involvement can also occur in the pediatric population, particularly in immunocompromised patients after stem cell transplantation. Early diagnosis and treatment of this life-threatening opportunistic infection may improve patient outcomes.
Toxoplasmosis of the Eye
Toxoplasmosis can affect the retina and the underlying choroid, causing retinochoroiditis, the most common manifestation of ocular toxoplasmosis. Ocular toxoplasmosis is the major cause of visual impairment in high T. gondii endemic regions of the United States and Europe, where it accounts for 30% to 50% of the posterior uveitis.
Ocular involvement can be a result of acquired infection or, more commonly, a recurrence of the congenital form of the disease.
Ocular toxoplasmosis is a progressive and recurrent disease with vision-threatening complications, including retinal detachment, chorioretinal anastomosis, and choroidal neovascularization, which can occur any time in the clinical course of the disease, even after treatment. Periodic follow-up is therefore necessary to reduce the occurrence of late complications. Many lesions are self-limiting and heal, forming characteristic unilateral or bilateral pigmented scars in the retina, where Toxoplasma cysts are found. Ocular lesions can recur in adolescence and adulthood, even after treatment in infancy. The severity of the disease is mainly a function of the parasite genotype and the host’s immune status.
In clinical practice, diagnosis in immunocompetent patients is based on the findings of typical ocular manifestations, including eye pain and decreased visual acuity, and may be confirmed by biological tools applied to ocular fluids. The value of serology is limited. However, there are many asymptomatic seropositive persons in areas where the parasite is endemic, and they have atypical lesions that are similar to other necrotizing forms of retinitis, specifically acute retinal necrosis and cytomegalovirus retinitis. Atypical lesions also occur and are seen especially in elderly persons and in those with underlying immunodeficiency. In these cases, the toxoplasmic origin can be demonstrated only by laboratory testing or by a positive response to treatment. Secretory IgA antibodies in tears have been suggested as a reliable marker of acute ocular toxoplasmosis.
Symptoms and signs in immunocompromised patients do not distinguish ocular toxoplasmosis from other ocular infections in HIV, including tuberculosis, P. jirovecii pneumonia, and cytomegalovirus (CMV) retinitis. Toxoplasmic chorioretinitis appears as raised yellow- white, cottony lesions in a nonvascular distribution, unlike the perivascular exudates of CMV retinitis. Vitreal inflammation is usually present, in contrast to ocular toxoplasmosis in immunocompetent patients. Up to 63% of AIDS patients with Toxoplasma chorioretinitis have concurrent CNS lesions.
Maternal Infection and Congenital Toxoplasmosis
Infection with T. gondii is particularly dangerous for pregnant women because it can lead to the transplacental passage of the parasite from the circulation of the primarily infected mother. During pregnancy, the prevalence of toxoplasmosis increases throughout the second and third quarter of gestation, simultaneously progesterone and 17!- estradiol also increase. Thus, it has been suggested that these hormones can aggravate or reduce parasite reproduction. Infection of the placenta is a prerequisite for congenital transmission. More than 60% of infected pregnant women do not experience any symptoms or signs, and the clinical features, when present, are the same as in other immunocompetent persons.
Currently, congenital toxoplasmosis is the second most common intrauterine infection and remains a public health problem throughout the world. The global annual incidence of congenital toxoplasmosis has been estimated to be 190,100 cases (95% credible interval, CI: 179,300–206,300). This is equivalent to a burden of 1.20 million DALYs (95% CI: 0.76–1.90). High burdens are seen in South America and in some Middle Eastern and low-income countries. The risk of transmission of T. gondii to the fetus ranges from 0.6 to 1.7 per 1,000 pregnant women. In France, about 300 cases are reported each year to the National Reference Center. The frequency of transmission and severity of disease are inversely related.
The development of possible consequences depends on many factors, including the degree of parasitemia in the mother, the maturity of the placenta, the age of the fetus, and immunologic maturity. Early maternal infection (first trimester) can cause severe congenital toxoplasmosis and can result in death of the fetus in utero and spontaneous abortion. By contrast, late maternal infection (acquired during the third trimester) usually results in a normal- appearing newborn who may be at high risk for seizures, mental retardation, and chorioretinitis. Infection during the second trimester also can result in symptomatic infection, but the clinical manifestations vary from mild to severe and depend on individual factors. Nevertheless, most neonates (70%–90%) with congenital toxoplasmosis are asymptomatic or have subclinical infection; overall, incidence is as high as 85%. Infection initially goes unnoticed, but if it is not treated, these children can later develop chorioretinitis or experience a delay in growth in the second or third decade of life.
Clinical manifestations of congenital toxoplasmosis observed in infancy or later in life are numerous and include jaundice, rash, hepatosplenomegaly, anemia, thrombocytopenia, hydrocephalus, microcephalus, intracranial calcifications, convulsions, psychomotor and mental retardation, chorioretinitis, microphthalmia, blindness, and strabismus. All these signs and symptoms are included in the general work-up of suspected congenital TORCH infections: toxoplasmosis, other (syphilis, varicella-zoster, parvovirus B19, HIV infection, listeriosis, hepatitis B), rubella, cytomegalovirus, and herpes. The classic triad of bilateral chorioretinitis, hydrocephalus, and cerebral calcifications is exceptional.
A significant correlation has been found between toxoplasmosis cases and the number of pregnancies in a woman. In multiparous women, the risk of infections was twice as high as in nulliparas.
The clinical course of the disease in a child with congenital toxoplasmosis is not influenced by whether the mother showed any clinical symptoms of the disease or was entirely asymptomatic. Infants born to women infected simultaneously with HIV and T. gondii should be evaluated for congenital toxoplasmosis, considering the increased risk of reactivation of parasitemia and disease in these mothers.
Severely immunocompromised mothers chronically infected with T. gondii can transmit the disease to the fetus as a result of reactivation. However, the rate of vertical transmission in this setting seems to be moderately low.
Diagnosis in the Immunocompetent
In immunocompetent persons, confirmation of acute infection can exclude other potentially more serious etiologies. In other clinical settings where Toxoplasma infection can result in severe sequelae, the interpretation of laboratory findings poses significant challenges. The parasite can in fact be present in acute, chronic, latent, or reactivated form. Thus, discrimination of these forms is often crucial in understanding clinical relevance. A schematic diagnostic pathway that can be followed for the diagnosis of toxoplasmosis is shown in Figure 1. Methodical serologic screening for T. gondii IgG and IgM antibodies in adult symptomatic immunocompetent persons, in pregnant women as early in gestation as possible (preferably in first trimester), and in seronegative women each month or trimester thereafter is optimal. Such screening allows detection of seroconversion and early initiation of treatment.
FIGURE 1 A schematic diagnostic pathway for the diagnosis of toxoplasmosis. *No treatment is required. †Repeat the tests weekly to detect the appearance of IgG antibodies to confirm the seroconversion.
‡Given that IgM may persist for several months, try to date the beginning of infection through education and counseling to guide the prenatal diagnosis of the patient. §PCR sensitivity is significantly higher for infections occurred between the 17th and 21st week of gestation. ||If ecographic abnormalities, consider abortion. ¶A negative PCR cannot completely rule out congenital infection; consider follow-up through ultrasounds and continue prophylaxis with spiramycin (Rovamycine)2 until delivery and neonatal testing. **For confirmation of PCR results; however, they are complex, expensive, and relatively insensitive.
††Limitations because IgA antibodies may persist for several months and are not always produced. ‡‡This test is not widely used mainly because of its technical complexity and high price. §§Administer pyrimethamine (Daraprim)-sulfadiazine1 therapy and serologic controls after cessation of therapy and every 6 months for the first 2 years.
The laboratory tests used most commonly for initial investigation are serologic, targeting detection of IgG, IgM, and IgA specific for T. gondii by available commercial kit. In addition to confirming infection, these tests can aid in determining prognosis, influence management, and assist in monitoring response to treatment. Typically, acute-phase IgM appears first about 1 to 2 weeks after infection, closely followed by IgA and IgE. Generally, IgM peaks at about 2 months. The time at which immunoglobulins can no longer be detected is highly variable depending on the test employed, usually about 6 to 9 months. In a small minority of cases IgM can persist at high levels for up to 18 months or for years, leading to an inaccurate assessment of when the exposure occurred. This circumstance can be problematic because congenital toxoplasmosis can occur if the mother was infected during her pregnancy, and it is thus important to ascertain in which trimester of pregnancy the infection occurred. IgA antibodies are considered to be a marker of acute toxoplasmosis as their kinetics are faster than those of IgM antibodies; however, they can also persist for more than a year, and their detection, together with IgM detection, strongly suggests neonatal infection. IgE serology is highly specific in pregnant women but has low sensitivity, remaining detectable for less than 4 months after infection; moreover, it is not useful in samples from newborns.
To date, an IgM test is still used by most laboratories to determine if a patient has been infected recently or in the distant past, but confirmatory testing (double sandwich or capture IgM-ELISA [enzyme-linked immunosorbent assay] kits and the immunosorbent agglutination assay [IgM-ISAGA]) should always be performed owing to the difficulties in interpreting a positive IgM test result for the relatively high incidence of false-positive results (due to the rheumatoid factor and antinuclear antibodies in some IgM-IFA tests).
Thus, a positive IgM test result in a single serum sample can be interpreted as a true-positive result in the setting of a recently acquired infection, a true-positive result in the setting of an infection acquired in the distant past, or a false-positive result. IgM levels decline more rapidly than IgG antibodies, which typically reach maximal levels at about 4 months, then decline to a lower level over the next 12 to 24 months but persist for decades. Elevated IgG levels confirm if a patient has been exposed to the parasite, but they do not differentiate between a recent or past exposure, because IgG persists at a low level throughout the life of the patient. The absence of IgG antibodies in early pregnancy indicates that women are at risk for acquiring toxoplasmosis.
The most commonly used tests for the measurement of IgG antibody are the Sabin-Feldman dye test (still considered the gold standard diagnostic test; it measures primarily IgG antibodies to T. gondii, but it requires viable Toxoplasma organisms), the ELISA, the IFA, and the modified direct agglutination test.
In recent years, significant progress has been made toward improving the ability to diagnose recently acquired T. gondii infection. In this setting, the introduction of IgG avidity testing based on the increase in functional affinity (avidity) between T. gondii-specific IgG and the antigen over time, as the host immune response (and specific B-cell selection) evolves, represents an irreplaceable test. The utility of the avidity test is based on the observation that Toxoplasma IgG antibodies from patients with a recently acquired T. gondii infection bind antigens weakly (low avidity), whereas IgG antibodies from chronically infected patients have stronger binding capacity (high avidity).
Depending on the method used, the avidity tests currently available are helpful primarily to rule out that a patient’s infection occurred within the prior 4 to 5 months and that the fetus is not at risk for congenital toxoplasmosis. The avidity test is most important when only a single serum sample is available at the time when critical decisions must be made. A major problem with this test is that the maturation of the IgG response after a primary Toxoplasma infection varies considerably among patients; in fact, low-avidity results can persist for as long as 1 year, and substantial numbers of patients have borderline or equivocal results. These equivocal cases require that in addition to a more careful interpretation of all laboratory test results in conjunction with other clinical findings, other serologic methods should then be undertaken in serology reference laboratories.
Interesting diagnostic approaches that rely on antigens produced by recombinant DNA technologies and specifically expressed during either the primary phase (granule dense proteins [GRA-7, GRA-4]) or the latent phase (GRA-1) of infection can lead to a more informative serologic diagnosis, even based on a single serum sample. Recently, a recombinant rhoptry protein 2 (rROP2) antigen has shown to be recognized by antibodies produced in both the acute and chronic phases of T. gondii infection acquired during pregnancy. Thus, the combination of rROP2 with other recombinant antigens could be employed to differentiate the phases of toxoplasmosis in pregnant women.
The employment of polymerase chain reaction (PCR) to identify T. gondii DNA in amniotic fluid (AF) obtained by amniocentesis at 18 weeks and later represents a milestone in the early diagnosis of intrauterine T. gondii infection, thereby avoiding the use of more- invasive procedures on the fetus. Sensitivity and specificity of PCR may even increase when performed on AF samples obtained from the sixteenth week of pregnancy onwards. The specificity and positive predictive value of PCR on AF samples is close to 100%, although different protocols influence its sensitivity and specificity. PCR is generally carried out with various gene targets, of which the most widely used is the 35-fold repetitive gene B1. With the recent employment of sensitive and specific real-time PCR techniques, which use as target regions of the gene AF146527 repeated 300 times in the genome of T. gondii, it is possible to perform a quantitative study and follow the parasite load, allowing determination of parasite count and its correlation with clinical symptoms and impact of treatment.
However, a definitive correlation between the number of Toxoplasma organisms in AF and the severity of congenital infection has not yet been demonstrated. PCR should be considered for pregnant women (without a contraindication for the procedure) with positive diagnostic serology or highly suggestive of an infection because of ultrasonographic abnormalities in the fetus acquired during gestation or shortly before conception. If the PCR-AF test result is negative, the fetus should be unaffected, presumably because T. gondii has not yet crossed the placental barrier even if there is the theoretical possibility that fetal infection can occur later in pregnancy from a placenta that was infected earlier in gestation. Samples obtained by cordocentesis, funipuncture, or periumbilical fetal blood sampling should not be used because fetal risk of contamination with maternal blood is higher than with amniocentesis, and cordocentesis is less sensitive.
In the newborn, diagnosis of infection is based on detection of serum anti-Toxoplasma IgM and IgA antibodies that do not cross the placenta, unlike maternal IgG antibodies. Better results are obtained if the ISAGA method is used. Maternal IgG antibodies present in the newborn can reflect either past or recent infection in the mother and require serologic follow-up of the newborn for the 1st year of life until the complete disappearance of these antibodies. A negative T. gondii– specific IgG test result at 1 year of age essentially rules out congenital toxoplasmosis. Detection of IgG in oral fluid appears to be a promising tool for monitoring infants with suspected congenital toxoplasmosis.
Detection of specific IgA antibodies appears to be more sensitive than detection of IgM antibodies for establishing infection in the newborn, because these antibodies may be present when there is no T. gondii-specific IgM, and the converse can also occur. However, transmission of maternal IgM or IgA antibodies can occur during the birth process. Because of the relatively brief half-life of IgM and IgA, positive tests for these antibodies usually must be confirmed by repeat testing at 2 to 4 days of life in the case of IgM antibodies and at 10 days of life for IgA antibodies.
The suspected infection can be verified by a mother and child comparative immunologic profile analysis using the Western blot assay and two-dimensional immunoblot of serum from mother-baby pairs. This technique, although laborious and expensive, permits early postnatal diagnosis irrespective of the time of maternal infection and can identify newborns who have congenital infection with a sensitivity that reaches 85% within the 3 months of life.
Although complex and expensive, other methods that may be successfully employed to diagnose the infection in infants include direct demonstration of the organism by isolation of the parasite (e.g., inoculation in tissue cultures of placental tissue or mouse inoculation) and amplification of T. gondii-specific DNA.
Evaluation of infants with suspected congenital toxoplasmosis should always include ophthalmologic examination, electroencephalogram, hearing test, blood tests, noncontrast CT scanning or ultrasound of the brain, and examination of CSF. Although CT scanning is the first-line diagnostic method used in the United States to detect CNS abnormalities caused by toxoplasmosis, ultrasound can be used as an alternative diagnostic method to avoid the effects of radiation in the neonatal period. Ultrasound is recommended for women with suspected or diagnosed acute infection acquired during or shortly before gestation. This technique can reveal fetal abnormalities, including hydrocephalus, brain or hepatic calcifications, splenomegaly, and ascites. Because most infants do not show signs of toxoplasmosis at birth, a negative ultrasound does not rule out the possibility of infection and might need to be followed by a CT scan for confirmation because ultrasound results can vary depending on the examiner and technology used. Newborns thus need a thorough examination after birth and follow-up blood tests during the first year of life.
Serological screening is also helpful in identifying transplant patients at risk, especially seronegative recipients with seropositive donors, and can help in establishing the diagnosis by showing seroconversion.
In this setting, the incidence of toxoplasmosis in seronegative heart recipients receiving an organ from a seropositive donor (mismatch D+/R−) has been reported as high as 50%–75% in the absence of prophylaxis, and prevention with cotrimoxazole is recommended for patients at a higher risk for toxoplasmosis.
Toxoplasmosis following allogeneic hematopoietic stem-cell transplant (HSCT) remains a cause of severe infection associated with a high mortality rate. The highest risk of transmission resulting from reactivation is observed in seropositive recipients following allogeneic HSCT.
There is no risk of transmission in cases of HSCT in mismatch R−/D+ (chronically infected). Serological follow-up combined with PCR after allogeneic HSCT is recommended in all patients at risk for toxoplasmosis.
Diagnosis in the Immunocompromised
In immunocompromised persons and AIDS patients with toxoplasmic encephalitis, indirect serologic methods widely used in immunocompetent patients are unreliable because they fail to produce significant titers of specific antibodies. Although incidence of toxoplasmic encephalitis among HIV-infected persons directly correlates with the prevalence of anti-T. gondii antibodies, the absence of IgG antibody makes a diagnosis of toxoplasmosis unlikely, but not impossible. Anti-Toxoplasma IgM antibodies are usually absent. CSF findings are normal or show nonspecific alterations such as lymphocytic pleocytosis and discrete CSF hyperproteinorrachia.
In clinical practice, the diagnosis of toxoplasmic encephalitis is presumptive and mainly based on clinical presentation, imaging, and laboratory test results. Imaging includes CT or MRI, preferably with gadolinium contrast, and shows isodense or hypodense single or multiple lesions with a mass effect, taking up the contrast dye in a ringlike or nodular manner in more than 90% of cases. However, as neither CT nor MRI can reliably distinguish CNS infections, such as toxoplasmosis, from lymphoma in HIV-1-positive patients, the use of FDG-PET has been shown to offer better sensitivity to noninvasively differentiate cerebral toxoplasmosis and other infectious diseases from primary CNS lymphoma. Laboratory results include the presence of serum-specific T. gondii IgG antibodies. Diagnosis is also made from an objective response, on the basis of clinical and radiographic improvement, to specific anti-T. gondii therapy in the absence of a likely alternative diagnosis. In general, the occurrence of low CD4+ T- cell count (less than 150–200/mm3) and the presence of T. gondii IgG antibody is habitually accepted as being a good predictor of toxoplasmic encephalitis reactivation, although less than 6% of toxoplasmic encephalitis patients show negative tests. If the suspected diagnosis of toxoplasmosis is correct, clinical or radiographic improvement should become evident by more than 50% within 7 to 14 days. If symptoms persist, a brain biopsy should be performed. This should be considered in immunocompromised patients with presumed CNS toxoplasmosis with a single MRI lesion, absence of IgG antibodies, or an unsatisfactory clinical response to specific anti-T. gondii treatment.
Diagnosis of toxoplasmic encephalitis is crucial because other brain diseases—such as CNS lymphoma, bacterial abscess, progressive multifocal leukoencephalopathy, viral or fungal encephalitis, neurotuberculosis, cytomegalovirus encephalitis, and focal lesions caused by fungi (Cryptococcus neoformans, Aspergillus spp., and Nocardia spp.)—could share similar clinical and CT scan signs. In general, no imaging technique is completely specific. Thallium single- photon emission computed tomography (SPECT) and positron emission tomography (PET) can be useful in distinguishing toxoplasmosis or other infections from CNS lymphoma. CNS lymphoma has greater thallium uptake on SPECT and greater glucose and methionine metabolism on PET than neurotoxoplasmosis or other infections.
Definitive diagnosis of toxoplasmic encephalitis is made by pathologic examination of brain tissue obtained by open or stereotactic CT-guided brain biopsy, although there is morbidity and even mortality associated with the biopsy procedure. Tachyzoites are demonstrated on hematoxylin and eosin stains or immunoperoxidase staining, which can increase diagnostic sensitivity, or by Giemsa on cytocentrifuged CSF samples. Toxoplasma organisms must be distinguished from other intracellular organisms such as Histoplasma, Trypanosoma cruzi, and Leishmania.
Since the turn of the twenty-first century, molecular techniques have allowed significant improvement and have been shown to be an important diagnostic tool in laboratory diagnosis of toxoplasmic encephalitis. Their use is principally appropriate for immunocompromised patients, because these techniques are not affected by the immunologic status of the host. PCR-based assays, in particular, have been shown to be rapid, sensitive, and specific enough to be used as a front-line test for detecting CSF T. gondii DNA in most patients with CNS infection, thus avoiding invasive and expensive brain biopsy procedures. However, results usually are negative once specific anti-Toxoplasma therapy has been started.
Moreover, parasite levels in blood and CSF may be very poor in some patients and cause difficulties in interpreting the PCR product. For this reason, a number of quantitative real-time PCRs with specific T. gondii genome sequence have been developed and employed in patients with AIDS and CNS damage with different rates of sensitivity and specificity. In the recent years, the use of PCRs with primers targeting the genes SAG4 and MAG1 (located in the bradyzoites) and SAG1 (located in tachyzoites), have shown to be useful to identify relapses of T. gondii encephalitis in patients in whom the use of PCR with B1 gene failed to detect DNA in AIDS patients and other immunocompromised patients especially when prophylaxis or treatment has been started.
PCR testing for other pathogens (e.g., Epstein-Barr virus [EBV], JC virus, Mycobacterium tuberculosis, C. neoformans) can be considered in patients who have focal brain lesions and who are already taking prophylactic antibiotics for toxoplasmosis or are seronegative.
In general, PCR enables detection of T. gondii DNA in vitreous and aqueous fluids, bronchoalveolar lavage (BAL) fluid, and blood and in patients with AIDS with other toxoplasmic localizations, and thus it is useful in the diagnosis and assessment of this disease. Currently, the search for parasites in clinical specimens by PCR has overtaken direct inoculation into mice.
Diagnosis of toxoplasmic encephalitis in AIDS patients can also be supported by demonstration of intrathecal antibody production based on detection of oligoclonal bands in CSF with antibody-specific index (ASI) and affinity-mediated immunoblot (AMI) techniques. This approach has also been shown to discriminate toxoplasmic encephalitis from other opportunistic CNS infections in AIDS. In non AIDS immunocompromised patients, regular PCR follow-up of allogeneic hematopoietic stem-cell transplant (allo-HSCT) patients could guide pre-emptive treatment and improve outcome.
The main goal of treatment is to interrupt the replication of the parasite and prevent further damage to the organs involved.
Nonpregnant immunocompetent patients with acute toxoplasmosis plus lymphadenitis generally do not require antimicrobial therapy because the infection is self-limited and usually subclinical. Chronic lymphadenopathy accompanied by fever and marked weakness can be cured with specific therapy. Patients whose T. gondii infection occurred during laboratory accidents or blood transfusions need specific treatment. Pregnant women must be treated to reduce the risk and severity of congenital infection. Immunocompromised patients, including AIDS patients and transplant recipients, should always be treated with antitoxoplasmic agents. For eye disease, treatment usually includes anti-Toxoplasma agents plus systemic corticosteroids. The most important treatment and prophylactic regimens for the therapy of toxoplasmosis are shown in Tables 1 and 2.
Treatment Regimens for Toxoplasmosis
Prophylaxis Regimens for Toxoplasmosis
Abbreviations: ART, highly active retroviral therapy; PCR, polymerase chain reaction; TMP- SMX, trimethoprim-sulfamethoxazole.
1 Not FDA approved for this indication.
* For side effects, see Table 1.
† In patients with AIDS, primary and secondary prophylaxis are generally discontinued when the patient’s CD4+ cell count has returned to more than 200 cells/μL and HIV PCR peripheral blood viral load has been reasonably controlled for at least 6 months following ART.
‡ Preemptive antiparasite treatment should be considered for all symptomatic, seropositive, immunocompromised patients in whom toxoplasmosis is suspected.
§ There are no clear recommendation for anti-T. gondii prophylaxis in allogeneic stem cell transplantation because the optimal regimen has not yet been determined.
” Not specifically codified.
¶ Alternative regimen in sulfonamide-allergic patients.
** Presumptive therapy should be given with the same dosages as for acute infection and monitored serologically for several months after the exposure or until seroconversion is noted; that is, testing immediately after the exposure, weekly for at least 1 month, and at least monthly thereafter. Seroconversion can occur despite presumptive therapy. Although presumptive therapy typically prevents disease or at least substantial morbidity, it does not necessarily prevent infection.
The standard treatment for acquired toxoplasmosis in both immunocompetent and immunodeficient patients is the synergistic combination of pyrimethamine (Daraprim) (the most effective anti- Toxoplasma agent available) and sulfonamides. Pyrimethamine plus sulfadiazine1 plus folinic acid (Leucovorin)1 is the preferred regimen. Azithromycin (Zithromax)1 and atovaquone (Mepron)1 have been shown to be partially effective against tissue cysts in experimental studies. In these last years however, the search for new and efficient pharmacological treatments against toxoplasmosis is under intense and frequent evaluation, in part due to the lack of successful and specific parasitic therapies directed against proliferating tachyzoites or the tissue cyst of Toxoplasma.
Treatment during Pregnancy
Management of maternal and fetal infection of T. gondii varies considerably between different countries and even centers in the same country. According to the European Multicentre Study on Congenital Toxoplasmosis (EMSCOT) data, which suffer from the lack of a large- scale controlled clinical trial, treatment for the fetus should be started immediately (within 3 weeks of infection) after diagnosis of recently acquired maternal infection. This has been shown to significantly reduce sequelae and to have a beneficial effect when therapy is begun soon after birth. Later treatment has been shown to have no effect.
The macrolide antibiotic spiramycin (Rovamycine)2 is prescribed immediately after diagnosis of maternal infection in most centers in Europe and the United States, and there has been no evidence that this drug is teratogenic. Because spiramycin does not cross the placenta but is concentrated in the placenta. It protects the fetus from contact with the parasite. It has been estimated to reduce the incidence of vertical transmission of T. gondii by about 60%. It is used 1 g every 8 hours in many EU countries, especially France, and in Asia and South America and does not seem to have any fetal effects, although a small percentage of women develop gastrointestinal side effects.
Spiramycin is theoretically most beneficial for patients with negative PCR-AF testing; it should be continued as prophylaxis until delivery, along with periodic ultrasound examination, because the placenta could have been infected earlier in gestation.
In the case of positive PCR-AF at, or immediately after, 18 weeks of gestation, or with high probability of fetal infection acquired late in the second trimester (after 18 weeks) or during the third trimester of gestation (because of ultrasound abnormalities), and in women who cannot undergo amniocentesis, the spiramycin regimen should be replaced with the drug combination of sulfadiazine1 and pyrimethamine plus folinic acid (Leucovorin).1 This treatment strategy varies in some U.S. and European centers. Folinic acid prevents the toxicity of pyrimethamine without activity against Toxoplasma. This combination therapy is used only after confirmed or strongly suspected fetal infection and is never administered in the first trimester of pregnancy due to its teratogenic and hematological adverse effects, in addition to symptoms of nausea in the mother.
Throughout the pregnancy, therapy with pyrimethamine in conjunction with leucovorin should be continued to prevent hematologic toxicities, along with monitoring of blood cell counts and rigorous periodic ultrasound examination. Because spiramycin does not readily cross the placenta, it is not reliable for the treatment of infection in the fetus. In the absence of clinical and laboratory signs suggestive of congenital toxoplasmosis, therapy is not indicated in infants, but it is necessary to inform the specialist and to plan, with the specialist, periodic clinical and serologic controls. In symptomatic infants, the combination of pyrimethamine plus sulfadiazine1 plus leucovorin1 is recommended. The duration of this treatment is not specifically defined, but it must be continued for up to a year, possibly alternating cycles of antifolate for 4 weeks with spiramycin cycles of equal duration.
Although no treatment regimen seems to decrease the rate of chorioretinitis, pyrimethamine, sulfadiazine1 plus folinic acid (leucovorin),1 and corticosteroids form the most common drug combination currently used to treat ocular toxoplasmosis. Patients may also be treated with TMP-SMX (Bactrim),1 which appears to be a safe and effective substitute for pyrimethamine, sulfadiazine, and folinic acid, or with azithromycin (Zithromax)1 or intravitreal clindamycin (Cleocin)1 and prednisone for 4 to 6 weeks.
Pyrimethamine plus azithromycin is another drug combination that is similar to the standard treatment and can be considered an acceptable alternative treatment for sight-threatening ocular toxoplasmosis.
Recurrent toxoplasmic retinochoroiditis, probably related to the rupture of the dormant retinal cyst or Toxoplasma circulating in peripheral blood, remains a major health crisis and can be associated with severe morbidity if the disease extends to structures critical for vision, including the macula and optic disk. Severe morbidity can also occur if there is damage to the eye from inflammation or if there are complications such as retinal detachment or neovascularization. In patients with frequent recurrences, the rate of recurrent toxoplasmic retinochoroiditis can be significantly reduced by long-term intermittent treatment with TMP-SMX 160 mg/800 mg (Bactrim DS),1 with one tablet administered three times a week as a prophylactic regimen. Intravitreal clindamycin injection1 and possibly steroids may be an acceptable alternative to the classic treatment in ocular toxoplasmosis. It can offer the patient more convenience, a safer systemic side-effect profile, greater availability, and fewer follow-up visits and hematologic evaluations. Although research has identified wide variation in practices regarding the use of corticosteroids, a recent Cochrane review did not identify evidence from randomized controlled trials for the role of corticosteroids in the management of ocular toxoplasmosis. The question of foremost importance, however, is whether they should be used as adjunct therapy (that is, in addition) to antiparasitic agents. There is no evidence to support that one antibiotic regimen is superior to another so choice needs to be informed by the safety profile. Intravitreous clindamycin with dexamethasone1 seems to be as effective as systemic treatments. There is currently level I evidence that intermittent trimethoprim-sulfamethoxazole prevents recurrence of the disease.
Toxoplasmosis in Immunosuppressed Patients
With the advent of the ART, the natural course of HIV infection has markedly changed, and opportunistic infections including toxoplasmosis have declined and changed in presentation, outcome, and incidence. However, toxoplasmic encephalitis is a major cause of morbidity and mortality, especially in resource-poor settings, and it is a common neurologic complication in some countries despite the availability of ART and effective prophylaxis.
The initial therapy of choice in toxoplasmic encephalitis patients consists of the combination of pyrimethamine, which is able to penetrate the brain parenchyma efficiently even in the absence of inflammation, plus sulfadiazine or clindamycin1 plus leucovorin.1
Adjunctive corticosteroids should be administered when clinically indicated only for treatment of a mass effect associated with focal lesions or associated edema. Because of the potential immunosuppressive effects of corticosteroids, they should be discontinued as soon as clinically feasible. Patients receiving corticosteroids should be closely monitored for the development of other effects, including CMV retinitis and tuberculosis.
Anticonvulsants should be administered to patients with a history of seizures, but should not be administered prophylactically to all patients. Drug interactions between anticonvulsants and antiretroviral agents should be carefully evaluated, and doses should be adjusted according to established guidelines. The preferred alternative regimen for patients unable to tolerate or who fail to respond to first-line therapy is pyrimethamine plus clindamycin plus leucovorin.
Common clindamycin toxicities include fever, rash, nausea, diarrhea (including pseudomembranous colitis or diarrhea related to Clostridium difficile toxin), and hepatotoxicity. Other alternative regimens shown in Table 1 are active in the treatment of toxoplasmic encephalitis and may be used in sulfa-intolerant patients or those who have not responded to treatment, although their relative efficacy compared with the previous regimens is unknown.
The combination of atovaquone (Mepron)1 with either pyrimethamine or sulfadiazine has demonstrated utility for the treatment of acute toxoplasmic encephalitis in patients with a Karnofsky Performance Status Score greater than 30, which combines the ability to work, to undertake normal activities without external assistance, and to take care of personal needs. Acute therapy should be continued for at least 6 weeks, if there is clinical and radiologic improvement. This should be followed by maintenance therapy (lifelong secondary prophylaxis), usually with the same regimen that was used in the acute phase but at half doses (see Table 2).
Currently, maintenance treatment should be continued for the life of the patient or until underlying immunosuppression has ceased (immune reconstitution resulting from ART). Adult and adolescent patients appear to be at low risk for recurrence of toxoplasmic encephalitis when they have successfully completed initial therapy, remain asymptomatic with respect to signs and symptoms, and have a sustained (i.e., more than 6 months) increase in their CD4+ T- lymphocyte counts to greater than 200 cells/µL on ART. AIDS patients with CD4+ T cell counts less than 100/µL and IgG Toxoplasma antibody should receive primary prophylaxis for toxoplasmosis. Fortunately, the same daily regimen of one double-strength tablet of TMP/SMX (Bactrim DS)1 used for Pneumocystis jirovecii prophylaxis provides adequate primary protection against toxoplasmosis. Secondary prophylaxis for toxoplasmosis may be discontinued in the setting of effective ART and when CD4+ T-cell counts increase to more than 200/ µL for 6 months.
Prophylaxis and treatment measures of disease in bone marrow transplant recipients as well as in those receiving organ transplantation are shown in Table 2. Universal prophylaxis with TMP/SMX in allo pre-hematopoietic cell transplant seropositive recipients (HCTSP), should be implemented by all transplant programmes. Preemptive treatment with routine blood PCR monitoring is an option if prophylaxis cannot be used.
Toxoplasma gondii Infection during Pregnancy in HIV-Positive Women
Immunocompromised women with chronic infection do rarely transmit the parasite to the fetus, resulting in congenital infection. This includes women with AIDS as well as those on immunosuppressive treatment. Vertical transmission occurs in up to 4% of cases, particularly when the CD4+ count is less than 100/mm3. The risk of transmission is low when the CD4+ count is greater than 200/mm3 (see Table 2).
Consideration should be given to screening all immunosuppressed women for evidence of maternal T. gondii serologic status to establish an early diagnosis of reactivation. Seropositive women with low CD4 counts should receive TMP-SMX (Bactrim)1 to prevent reactivation of Toxoplasma. Because of reports of congenital toxoplasmosis in mildly or moderately immunosuppressed women, it is recommended that immunocompromised women not infected with HIV and women who are infected with HIV and who have CD4 counts less than 200/mm3 be treated with spiramycin (Rovamycine)2 for the duration of the pregnancy. A nonpregnant woman who has been diagnosed with an acute T. gondii infection should be counseled to wait 6 months before attempting to become pregnant. Each case should be considered separately in consultation with an expert.
Detailed ultrasound examination of the fetus specifically evaluating for hydrocephalus, cerebral calcifications, and growth restriction should be done for HIV-1-infected women with suspected primary or symptomatic reactivation of T. gondii during pregnancy.
Strategies for the primary prevention of toxoplasmosis in pregnant women and in immunosuppressed patients with negative serology are shown in Box 1.
|Strategies for the Primary Prevention of Toxoplasmosis in Pregnant Women and in Immunosuppressed Patients with Negative Serology|
|• Check immunity to toxoplasmosis. If the patient has no protection (absence of anti-Toxoplasma IgG), take direct or indirect prophylactic measures against cats, food, and water.
• Universal prophylaxis with TMP/SMX in all pre-HCTSP patients should be implemented by all transplant programs. Preemptive treatment with routine blood PCR monitoring is an option if prophylaxis cannot be used.
• Avoid contact with food (including unwashed fruit or vegetables) or water potentially contaminated with cat feces. Wash before consuming.
• Avoid any job or activity that requires contact with dirt, soil, or other material potentially contaminated with cat feces (e.g., gardening or handling cat litter). Wear gloves when gardening or handling cat litter.
• Keep cats inside. Do not adopt or handle stray cats. Do not feed cats raw or undercooked meat but only canned or dried commercial food or well-cooked food. Do not obtain a new kitten or cat during pregnancy.
• Disinfect the cat litter box with near-boiling water for 5 min before handling.
• Wash hands thoroughly after contact with raw meat.
• Kitchen surfaces and utensils that have come in contact with raw meat should be washed.
• Avoid ingesting raw or dried meat, raw eggs, or unpasteurized milk.
• Avoid eating raw oysters, clams, or mussels.
• Protect foods from flies, cockroaches, and other insects.
• Avoid touching the eyes or face or any mucous membrane during or immediately after preparing food.
• Cook meat thoroughly. Pork, lamb, beef, veal, and poultry should be cooked until the meat reaches 80°C in the center.
• Refrain from skinning animals.
• Avoid children’s sandboxes.
• Avoid contact with stray dogs (xenosmophilia); if this is not possible, wash your hands after contact.
All the primary measures for preventing congenital toxoplasmosis concern mainly agricultural areas, veterinary practices, zoological shops, and gastronomy. Efforts toward health education are focused on avoiding personal exposure to the parasite (hygienic and culinary practices during pregnancy). Although T. gondii can be avoided by implementing relatively simple strategies in daily life, the majority of pregnant women are unaware of how to prevent exposure.
Most women are aware that toxoplasmosis is associated with cat litter. Contact with cat litter should be avoided if possible; if not, gloves should be worn while changing the litter box and hands should be washed thoroughly afterward. Litter should be changed frequently because it takes several days for oocysts to become infectious, and the box should be thoroughly cleaned with disinfecting agents.
Preventing a cat from hunting outdoors or eating raw meat also can prevent the feline from being infected with T. gondii. Practitioners should encourage pregnant women to keep indoor-only cats and to feed them only canned or dry food that has been bought in a store.
Contact with any utensils that might have been contaminated with cat feces and cat litter should be strictly avoided. Hands should be washed after contact with soil, dogs, and cats, and before meals.
Women should also devote particular attention to environmental exposure. Sporulated oocysts can be found in dirt, sand, or soil and on the skins of raw fruits and vegetables grown in these substrates.
Limiting contact with dirt, sand, or soil can help prevent the ingestion of oocysts from the environment, and if contact occurs, an expectant mother should be taught to thoroughly wash her hands to avoid ingesting the parasite. Wearing gloves while gardening, for example, also can limit the contact a pregnant woman might have with these environmental hazards. The skins of all raw fruits and vegetables should be washed and then peeled away because oocysts may be attached to these parts of the food and could be ingested. Hand washing should be strongly emphasized after handling any raw food including fruits, vegetables, and meat products. Other exposure risks include eating raw oysters, clams, or mussels. T. gondii cysts can reside in the meat of many different types of mammals, including imported horse meat containing T. gondii atypical strains, or birds. In the United States, it is estimated that 8% of beef and 20% of lamb and pork contains T. gondii tissue cysts. All pregnant women should be taught to never ingest raw meat and to cook all meat to an internal temperature of at least 80°C (175°F) to destroy the tissue cysts.
Considering the diverse clinical spectrum of toxoplasmosis with atypical presentations (such as pneumonitis or ocular toxoplasmosis), the measures that travelers should abide include avoiding to eat meat still pink in the center or undercooked poultry, smoked or dried meat, or fresh seafood; peeling or thorough washing of fresh fruits and salads before eating; and drinking treated, preferably bottled, water.
Secondary prevention by serologic monitoring of seronegative pregnant women should be closely connected with detailed primary prevention. Routine toxoplasmosis screening programs for pregnant women have been established in France, Austria, and Brazil. In France, guidelines recommend that nonimmune pregnant women be tested every month throughout pregnancy to detect seroconversion to Toxoplasma infection. In the United States, routine serologic screening is not performed. In Italy, serologic screening is recommended to prevent congenital toxoplasmosis as part of the antenatal care protocol. Serologic screening of women should begin before conception, with follow-up monthly tests during pregnancy to detect seroconversion. This is the basis for the French screening program and the Austrian Toxoplasmosis Prevention Program, which recommend routine serologic testing. Treatment is recommended if one of the tests suggests definite or probable primary maternal infection.
Vaccination is one of the most efficient strategies to prevent and control the spread of toxoplasmosis by immunization of humans and animals (the source of infection). To date, there is much debate about whether a human vaccine is feasible. Costs, target population, method for testing the vaccine, and risks associated with the vaccine are important factors affecting development of the vaccine. Much of the work has been focused on SAG1, a surface antigen expressed on tachyzoites, in attempts to induce protective immune response (mainly T-helper response) when introduced to the host with various adjuvants. SAG1 is primarily involved in adhesion, signal transduction, invasion, material transport and host immune responses. Thus, this protein may be crucial for both the diagnosis of T. gondii infection and the ability to immunize against this parasite. This exciting progress on SAG1 will lay a solid foundation to combat toxoplasmosis.
In general, a simpler route to prevent human disease would be to vaccinate animals (cats and intermediate hosts) that are responsible for transmission to humans, as demonstrated using a live attenuated vaccine developed for the prevention of chronic infection in sheep.
However, it cannot be used in humans because of the risk of reversion to a pathogenic form. Vaccine development to prevent feline oocyst shedding is ongoing, mostly involving live vaccines. More recently, T. gondii dense granule proteins including GRA-6, a secretory vesicular organelle that produces proteins that participate in the modification of the parasitophorous vacuole, have been shown to be suitable as possible DNA vaccines for immunity against toxoplasmosis. Aspartic protease 1 (Asp 1) has also shown to play an essential role in the T. gondii lifecycle. This could be a novel vaccine candidate against toxoplasmosis.
In summary, the immune evasive nature and complex life cycle of T. gondii has made the design of immunotherapeutics aimed at providing complete protection, a discouraging challenge. Generally, vaccine studies thus far have shown that multi-antigenic formulations confer better protection than single-subunit vaccines.
The discovery of atypical strains in the South American region which are more virulent than the classic European types I, II and III strains and causes severe illness, would imply that next generation vaccine efforts should also consider designing assays for cross- protection evaluation. A universal or multi-type anti-T. gondii immunotherapeutics is a highly valuable goal for global disease control.
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