• The diagnosis of brucellosis relies on the serologic reaction of antibodies to Brucella’s lipopolysaccharide (LPS) located on the cell surface of smooth Brucella melitensis, Brucella abortus, and Brucella suis.
• Although the rough species, Brucella canis, does not have an intact LPS layer, its detection requires the antigens specifically prepared from B. canis.
• Rose Bengal test is regularly used to diagnose infection with smooth
• When patients with brucellosis symptoms test negative by Rose Bengal test, the microagglutination test and 2-mercaptoethanol rapid slide agglutination test are used to determine whether the patient is infected with rough Brucella spp.
• Three aspects of therapy are critical when using antibiotics to treat brucellosis:
• The antibiotics must be able to penetrate macrophages to take effect because Brucella spp. are intracellular pathogens and normally reside within professional and nonprofessional phagocytes.
• Multiple antibiotics are typically used to minimize relapses commonly seen when only a single antibiotic is used.
• In complicated cases, in addition to antibiotic treatment, surgical intervention may be required to remove infected tissues.
Brucellosis is an ancient disease recorded by the Romans more than 2000 years ago. Since Brucella was identified as the cause of brucellosis in sick soldiers in 1887 by Sir David Bruce, brucellosis has been defined as an emerging disease. Brucella is a zoonotic bacterial pathogen, causing approximately 500,000 new cases annually worldwide. Although rarely fatal (< 5% mortality rate), it causes an undulating fever leading to a dramatic decrease in the quality of life.
Its incubation time can vary between 1 and 8 weeks, and often infection is asymptomatic; however, serious complications can lead to disability or death. Because no human vaccine is available, antibiotic therapy is the only option for treatment.
A total of ten Brucella species have been identified to date (Table 1).
Four of these—Brucella melitensis, Brucella abortus, Brucella suis, and Brucella canis—commonly cause human brucellosis, and their genomes have been sequenced.
Brucella Species, Preferred Animal Hosts, and Human Infection Potential
|Species Host Animals or Reservoir Human Infection?*|
|B. melitensis||Goats, sheep, camels||Yes|
|B. abortus||Cows, bison, camels, yaks||Yes|
|B. suis||Pigs, wild hares, caribou, reindeer, wild rodents||Yes|
|B. cetaceae||Seals, harbor porpoises||Yes|
|B. microti||Common voles, red foxes, soil||Unknown|
|B. pinnipedialis||Minke whales, dolphins||No|
* “Yes” and “no” mean Brucella spp. are able or unable to infect humans, respectively; “unknown” indicates that it is not confirmed whether Brucella spp. are able to infect humans.
Wildlife and domesticated animals can serve as natural reservoirs for Brucella spp. Humans acquire brucellae by exposure to contaminated foods or aerosols or by direct contact. Often contaminated or unpasteurized animal products, such as milk, soft cheese, and butter are consumed, causing oral exposure. Brucella spp. can spread via contaminated aerosols or by aerosolization of contaminated animal by-products, resulting in lung infection. Brucella can also infect by direct contact of skin cuts or abrasions with sick animals or their products and enter into the blood or lymphatic system and, regardless of route, produce infection, causing a bacteremia. Among the three routes, food ingestion and aerosol inhalation can result in brucellosis. Aerosolized brucellae can be used as a bioweapon because the dosage required for infecting 50% of the population is as little as 1885 colony forming units (CFUs).
Currently, brucellosis is endemic to Latin America, Africa, the Mediterranean rim, the Middle East, and Central Asia. To prevent human infection, animal immunization campaigns were adopted by many countries, including the United States and the former Soviet Union. The campaigns successfully curbed animal infection and consequently reduced human infection in these areas. When the programs were stopped, the incidence of brucellosis rapidly increased. Thus, maintenance of animal immunization programs is essential for protecting humans from infection.
Although transmission of brucellae among humans occurs rarely, several cases have been reported. It may be disseminated via sexual contact, milking, and organ transplantation. B. melitensis is the most commonly isolated species from brucellosis patients, followed in descending order by B. abortus, B. suis, and B. canis. B. melitensis and B. abortus can cause acute brucellosis and display the most severe symptoms, which can lead to complications. B. suis can cause extended duration of illness and results in suppurating lesions in infected tissues. Patients with B. canis infections display mild to moderate illness and rarely develop complications.
The susceptible population includes those who live or travel in the endemic areas, particularly veterinarians, microbiologists, and those who work with or process livestock. The current animal vaccines are all infectious to human, and consequently veterinarians who administer these vaccines are at high risk for being infected.
Microbiologists or laboratory workers also can acquire brucellosis through close contact, and indeed laboratory outbreaks are frequently reported. Thus, Brucella is also considered an occupational disease or a laboratory-acquired disease.
Brucella can be phagocytized by nonprofessional and professional phagocytic cells to establish an intracellular replication niche. Once inside host cells, brucellae stay in early-formed phagosomes, where they prevent phagosomes from fusing with late-formed endosomes and lysosomes and survive intracellularly. Antibiotics must be able to penetrate the host cell membrane to kill brucellae or inhibit their growth.
Brucella employs numerous mechanisms to evade detection by the immune system. Unlike LPS from other gram-negative bacteria, Brucella LPS does not trigger an inflammatory response or activate the alternative complement system, so it helps brucellae avoid recognition. Brucella‘s LPS endows brucellae with a phenotype highly resistant to cationic bactericidal peptides. The intracellular brucellae do not cause cell death in macrophages or apoptosis in respiratory epithelial cells. Thus, the LPS reduces the chances of exposure to the immune system, which aids brucellae survival and meanwhile diminishes the stimulation of the host immune system. Brucella inhibits the macrophage response to interferon-γ (IFN-γ), a critical cytokine for stimulating innate and adaptive immunity. Additionally, to maintain vigorous intracellular survival and replication, brucellae equip their outer membranes with phosphatidylcholine and synthesize cyclic β-1,2-glucans to preclude fusion between phagosome and lysosome.
Accurate diagnosis of brucellosis is always a challenge because brucellosis exhibits symptoms similar to numerous other febrile illnesses (Table 2). In many patients, the symptoms are mild and moderate; thus, in a number of cases the diagnosis might not be even considered.
Symptom Pattern of Human Brucellosis
|Lack of energy||95|
|Joint and back pain||86|
|Loss of appetite||78|
|Testicular pain, epididymoorchitis||21†|
|Central nervous system disorder||4|
Data from Corbel MJ: Brucellosis in humans and animals. Geneva, World Health Organization, 2006.
• Percentage based on 500
† Percentage based on 290 male patients.
Isolation of brucellae from blood, bone marrow, urine, cerebrospinal fluid, or joint aspirate serves as the gold standard for diagnosing brucellosis. However, under many circumstances, brucellae cannot be cultured from patients’ samples. The rate of blood culture positivity ranges from 16% to 90%. Hence, other indices are used to identify brucellosis. Testing for antibodies against Brucella is commonly used. Brucella agglutination titer of 1:160 is considered a clear diagnostic index as long as the patient presents signs and symptoms of the disease. Recommended tests are the Rose Bengal test, the serum agglutination test alone or with 2-mercaptoethanol or dithiothreitol reduction, Coombs’ antiglobulin, the complement- fixation test, and the enzyme-linked immunosorbent assay (ELISA).
The results of a combination of tests such as the serum agglutination test and Coombs’ antiglobulin can be used to assess the stage of evolution of the disease at the time of diagnosis. In the blood sample, infection may also be associated with low levels of red and white blood cells, low platelets, and elevated liver function. A biopsy of body tissues can also assist in making the diagnosis because patients can experience bone marrow hypoplasia and/or liver fibrosis and cirrhosis.
Depending on the patient’s symptoms and severity of the illness, an investigation may be undertaken. Examinations may include computed tomography (CT) scan or magnetic resonance imaging (MRI) to identify signs of inflammation or abscesses in the brain or other tissues. Electrocardiogram (ECG) may be performed to investigate heart infection or damage, and x-rays can show bone and joint deformations.
Because human brucellosis mimics the symptoms of many other diseases, disease complications vary from patient to patient, and the latency time between exposure and the occurrence of symptoms is irregular and relatively long, making the differential diagnosis tedious and difficult. Therefore, prior to diagnosis, answers to three questions can help narrow the focus to brucellosis:
• Did the patient have direct contact with large or small ruminants, their carcasses, or their products?
• Did the patient consume unpasteurized dairy products?
• Does the patient live in or travel to areas where brucellae are endemic in humans or epidemic in animals?
In spite of the difficulties in diagnosing brucellosis, some clinical features can still be used to distinguish brucellosis from other infectious diseases. If left untreated, fever of brucellosis displays an undulating pattern. In about 50% of patients, the fever of brucellosis is associated with musculoskeletal symptoms; however, these symptoms are rarely observed in typhoid and malaria fevers.
In patients with hepatosplenomegaly or lymphadenopathy, the differential diagnosis includes glandular fever–like illnesses, such as cytomegalovirus (CMV) infection, Epstein-Barr virus (EBV) infection, HIV infection, toxoplasmosis, and tuberculosis (TB). CMV patients can develop antibodies to the virus. Also, in the active infection phase, CMV can be detected from the blood, saliva, urine, or other body tissues. These indices can be used to differentiate CMV infection from the brucellosis. EBV patients have an elevated white blood cell count, an increased total number of lymphocytes, and greater than 10% atypical lymphocytes, but brucellosis patients usually show a decrease in the white blood cell count.
HIV infection can induce antibodies against HIV, and virologic tests can detect HIV antigens or RNA. HIV infection can be further confirmed by a supplemental antibody test, such as Western blot and indirect immunofluorescence assay. Brucellosis patients’ sera do not display any reaction in these tests. For toxoplasmosis patients, serum immunoglobulin (Ig)G and IgM titers can be used to detect whether the patient is infected by Toxoplasma.
For TB patients, a skin test or a special TB blood test can be used for diagnosis, and other tests such as chest x-ray and a sample of sputum coughed up from deep in the lungs may be used for confirmation. In patients with osteomyelitis or septic arthritis, the most important alternative diagnosis is TB.
In patients with acute epididymoorchitis, the differential diagnosis includes mumps and surgical problems, such as torsion. For mumps patients, buccal swab specimens are collected for viral detection via culturing virus in cell lines or using real-time polymerase chain reaction (RT-PCR) to detect mumps viral RNA. For testicular torsion in men, an ultrasound examination of the spermatic cord can provide valuable information regarding whether the patient requires emergency surgery.
To date, the only option for treating brucellosis is by means of antibiotics. However, in cases of complications, such as heart brucellosis and spinal brucellosis, antibiotic treatment in combination with surgical intervention may be needed. Antibiotics commonly used are doxycycline (Vibramycin), rifampin (Rifadin),1 streptomycin, cotrimoxazole (TMP-SMX [Bactrim]),1 and gentamicin (Garamycin)1 (Table 3). Regimens of a combination of 2 or 3 antibiotics are recommended to reduce the unacceptably high relapse rates with monotherapy. Antibiotic regimens vary and depend on the patient’s age, the severity of the disease, pregnancy, cost of the medicine, and availability of the medicine.
Treatment for Brucellosis
Abbreviations: CrCl = creatinine clearance; DS = double strength.
1 Not FDA approved for this indication.
3 Exceeds dosage recommended by the manufacturer.
Doxycycline combined with rifampin1 for a full 6-week course is a commonly used therapy recommended by the World Health Organization. It is considered the most effective regimen, particularly when combined with an aminoglycoside. In patients with spondylitis or sacroiliitis, doxycycline plus streptomycin is an effective combination. For pediatric patients older than 8 years, doxycycline plus gentamicin1 is the recommended therapy. For children younger than 8 years, trimethoprim-sulfamethoxazole (TMP-SMX) therapy for 3 weeks followed by a 5-day course of gentamicin1 is most effective.
Once brucellosis is diagnosed, immediate therapy is critical because it can alleviate symptoms and also prevent the development of complications. Nevertheless, even when treatment is executed according to the doctor’s prescription, rates of relapse can still reach up to 5% to 10%. Depending on the severity or complications of the illness and the treatment time applied, the recovery time can last from several weeks to several months.
After antibiotic therapy is initiated, patients are periodically monitored by doctors to evaluate whether the therapeutic regimen is effective and whether relapse occurs. Because relapse is indicated by the recurrence of a positive blood culture result during the post- therapy period and/or signs and symptoms of brucellosis infection, routine examination of the patient includes serum culture for Brucella organism and assessment for brucellosis symptoms after treatment phase. In addition to monitoring brucellosis symptoms, both doctors and patients should monitor any adverse effects of medication. For instance, adverse reactions to TMP-SMX occur in 50% to 100% of patients with AIDS compared to about 14% of patients without AIDS. Up to 57% of AIDS patients treated with TMP-SMX1 require a change in therapy owing to the adverse effects.
Generally, brucellosis patients should be followed clinically for up to 2 years to detect relapse. Patients should be monitored for regaining of body weight. IgG antibody should be checked by serum agglutination test for levels that remain in the diagnostic range for more than 2 years. Complement fixation titers should fall to normal within 1 year of treatment. Relapse should respond to a prolonged course of the same therapy originally used.
Brucellae are transported into the lymphatic system and can replicate in spleen, liver, kidney, breast tissue, and joints to cause both localized and systemic infections. Infection of the reproductive system can cause fetal abortion. Owing to the low virulence, low toxicity, and multiple mechanisms to protect them from the immune system, brucellae can survive and reproduce in nearly any tissues or organs. At one year following infection, the disease can develop into chronic brucellosis that can further cause one or multiple complications in one organ or the whole body.
There are seven major types of complications from Brucella infection.
• Endocarditis: Brucellae can infect the heart’s inner lining, which can destroy the heart valves and, if left untreated, will lead to death of the patient.
• Meningitis and encephalitis: Brucellae can infect the central nervous system to cause an inflammation of the brain, the membranes surrounding the brain, and the spinal cord. This is fatal if the patient is not treated in time.
• Arthritis: Osteoarthritis caused by Brucella infection is typically associated with pain, stiffness, and swelling of the joints, such as the knees, hips, ankles, wrists, spine, shoulder, elbow, sternoclavicular, and small joints. Spondylitis caused by brucellae is characterized by joint inflammation between the vertebrae bones of the spine or between the spine and pelvis. Spondylitis is difficult to treat and can cause lasting damage. These osteoarticular complications are the clinical forms most commonly observed.
• Epididymitis and epididymoorchitis: Brucellae can infect the epididymis to cause swelling and pain of the testicle.
• Cutaneous complications: Brucellae can infect skin to cause lesions, rashes, nodules, erythema nodosum, papules, petechiae, and purpura.
• Respiratory complications: Inhalation of brucellae can result in lung infection, which can lead to pneumonia, bronchopneumonia, pleural effusion with a predominance of monocytic or lymphocytic infiltrates, and paroxysmal dry cough.
• Hematologic complications: Aspartate aminotransferase (AST) or alanine aminotransferase (ALT) levels can be elevated and are associated with pain in the right upper quadrant or jaundice.
Complications are common in brucellosis patients. Young patients tend to have cutaneous, hematologic, and respiratory complications. Adult patients tend toward osteoarticular and cardiac complications. Middle-aged patients tend to develop genitourinary, neurologic, and gastrointestinal complications.
Maintaining hygienic habits is very important for avoiding Brucella infection. These include consuming only pasteurized milk and cheese. Meat must be cooked thoroughly before consumption. In these regards, education is beneficial for preventing infection by this pathogen. People who handle animals or animal products should wear personal protective equipment, including glasses, rubber gloves, and clothing to protect skin and eyes from exposure or direct contact. Laboratory workers should use a Biosafety Level 3 (BSL-3) facility to handle the Brucella organisms and work according to the laboratory’s standard operating procedures. Animal immunization programs must be maintained all over the world to cut off the transmission chain from livestock to humans. In addition, primary care physicians should be familiar with the clinical and laboratory findings of brucellosis symptoms and complications.
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1 Not FDA approved for this indication.