The causative organisms of human brucellosis are B. abortus (from cattle), B. melitensis (from sheep and goats), and B. suis (from hogs). B. canis (from dogs) has caused sporadic infections. Generally, B. melitensis and B. suis are more pathogenic than other Brucella species.
The most common sources of infection are farm animals and raw dairy products. Deer, bison, horses, moose, caribou, hares, chickens, and desert rats may also be infected; humans can acquire the infection from these animals as well.
Brucellosis is acquired by
Direct contact with secretions and excretions of infected animals
Ingesting undercooked meat, raw milk, or milk products containing viable organisms
Inhaling aerosolized infectious material
Rarely, person-to-person transmission
Most prevalent in rural areas, brucellosis is an occupational disease of meatpackers, veterinarians, hunters, farmers, livestock producers, and microbiology laboratory technicians. Brucellosis is rare in the US, Europe, and Canada, but cases occur in the Middle East, Mediterranean regions, Mexico, and Central America.
Because very few organisms (perhaps as few as 10 to 100) may cause infection via aerosol exposure, Brucella species are potential agents of biological terrorism Biological Agents as Weapons Biological warfare (BW) is the use of microbiological agents for hostile purposes. Such use is contrary to international law and has rarely taken place during formal warfare in modern history... read more .
Patients with acute, uncomplicated brucellosis usually recover in 2 to 3 weeks, even without treatment. Some go on to subacute, intermittent, or chronic disease.
Complications of brucellosis are rare but include subacute bacterial endocarditis Infective Endocarditis Infective endocarditis is infection of the endocardium, usually with bacteria (commonly, streptococci or staphylococci) or fungi. It may cause fever, heart murmurs, petechiae, anemia, embolic... read more , neurobrucellosis (which includes acute and chronic meningitis Overview of Meningitis Meningitis is inflammation of the meninges and subarachnoid space. It may result from infections, other disorders, or reactions to drugs. Severity and acuity vary. Findings typically include... read more , encephalitis Encephalitis Encephalitis is inflammation of the parenchyma of the brain, resulting from direct viral invasion. Acute disseminated encephalomyelitis is brain and spinal cord inflammation caused by a hypersensitivity... read more , and neuritis), orchitis Orchitis Orchitis is infection of the testes, typically with mumps virus. Symptoms are testicular pain and swelling. Diagnosis is clinical. Treatment is symptomatic. Antibiotics are given only if bacterial... read more , cholecystitis Acute Cholecystitis Acute cholecystitis is inflammation of the gallbladder that develops over hours, usually because a gallstone obstructs the cystic duct. Symptoms include right upper quadrant pain and tenderness... read more , hepatic suppuration, and osteomyelitis Osteomyelitis Osteomyelitis is inflammation and destruction of bone caused by bacteria, mycobacteria, or fungi. Common symptoms are localized bone pain and tenderness with constitutional symptoms (in acute... read more (particularly sacroiliac or vertebral).
Symptoms and Signs of Brucellosis
The incubation period for brucellosis varies from 5 days to several months and averages 2 weeks.
Onset may be sudden, with chills and fever, severe headache, joint and low back pain, malaise, and occasionally diarrhea. Or onset may be insidious, with mild prodromal malaise, muscle pain, headache, and pain in the back of the neck, followed by a rise in evening temperature.
As the disease progresses, temperature increases to 40 to 41° C, then subsides gradually to normal or near-normal with profuse sweating in the morning.
Typically, intermittent fever persists for 1 to 5 weeks, followed by a 2- to 14-day remission when symptoms are greatly diminished or absent. In some patients, fever may be transient. In others, the febrile phase recurs once or repeatedly in waves (undulations) and remissions over months or years and may manifest as fever of unknown origin.
After the initial febrile phase, anorexia, weight loss, abdominal and joint pain, headache, backache, weakness, irritability, insomnia, depression, and emotional instability may occur. Constipation is usually pronounced. Splenomegaly appears, and lymph nodes may be slightly or moderately enlarged. Up to 50% of patients have hepatomegaly.
Brucellosis is fatal in < 5% of patients, usually as a result of endocarditis or severe central nervous system complications.
Diagnosis of Brucellosis
Blood, bone marrow, and cerebrospinal fluid (CSF) cultures
Acute and convalescent serologic testing (not reliable for B. canis) and polymerase chain reaction (PCR) assay
Blood cultures should be obtained; growth may take > 7 days, and subcultures using special media may need to be held for up to 3 to 4 weeks, so the laboratory should be notified of the suspicion of brucellosis.
Samples of bone marrow and CSF may also be cultured.
Acute and convalescent sera should be obtained 3 weeks apart. A 4-fold increase or an acute titer of 1:160 or higher is considered diagnostic, particularly if a history of exposure and characteristic clinical findings are present. The WBC count is normal or reduced with relative or absolute lymphocytosis during the acute phase. Serologic testing is not reliable for B. canis.
PCR assay can be done on blood or any body tissue and can be positive as early as 10 days after inoculation.
Treatment of Brucellosis
Doxycycline plus either rifampin, an aminoglycoside (streptomycin or gentamicin), or ciprofloxacin
Activity should be restricted in acute cases of brucellosis, with bed rest recommended during febrile episodes. Severe musculoskeletal pains, especially over the spine, may require analgesia. Brucella endocarditis often requires surgery in addition to antibiotic therapy.
If antibiotics are given, combination therapy is preferred because relapse rates with monotherapy are high. Oral doxycycline 100 mg 2 times a day for 6 weeks plus streptomycin 1 g IM every 12 to 24 hours (or gentamicin 3 mg/kg IV once a day) for 14 days lowers the rate of relapse. For uncomplicated cases, oral rifampin 600 to 900 mg 2 times a day for 6 weeks can be used instead of an aminoglycoside. Regimens using oral ciprofloxacin 500 mg 2 times a day for 14 to 42 days plus rifampin or doxycycline instead of an aminoglycoside have been shown to be equally effective.
In children < 8 years, trimethoprim/sulfamethoxazole (TMP/SMX) and oral rifampin for 4 to 6 weeks have been used.
Neurobrucellosis and endocarditis require prolonged treatment.
Even with antibiotic treatment, about 5 to 15% of patients relapse, so all should be followed clinically and with repeat serologic titers for 1 year.
Prevention of Brucellosis
Pasteurization of milk helps prevent brucellosis. Cheese that is made from unpasteurized milk and is aged < 3 months may be contaminated.
People handling animals or carcasses likely to be infected should wear goggles and rubber gloves and protect skin breaks from exposure. Programs to detect infection in animals, eliminate infected animals, and vaccinate young seronegative cattle and swine are required in the US and in several other countries.
There is no human vaccine; use of the animal vaccine (a live-attenuated preparation) in humans can cause infection. Immunity after human infection is short-lived, lasting about 2 years.
Postexposure antibiotic prophylaxis is recommended for high-risk patients (eg, those who have unprotected exposure to infected animals or laboratory samples or who received animal vaccine). Regimens include doxycycline 100 mg orally 2 times a day plus rifampin 600 mg orally once a day for 3 weeks; rifampin is not used for exposure to the vaccine with B. abortus (strain RB51), which is resistant to rifampin.
Brucellosis is acquired by direct contact with secretions and excretions of infected animals or by ingestion of contaminated food or dairy products.
Infection typically causes fever and constitutional symptoms, but specific organs (eg, brain, meninges, heart, liver, bones) are rarely affected.
Most patients recover in 2 to 3 weeks, even without treatment, but some develop subacute, intermittent, or chronic disease.
Diagnose using cultures of blood, bone marrow, or cerebrospinal fluid and acute and convalescent serologic testing.
Treat most patients with 2 antibiotics, typically doxycycline plus either rifampin, an aminoglycoside, or ciprofloxacin; monitor patients up to 1 year for relapse.