(Undulant Fever; Malta Fever; Mediterranean Fever; Gibraltar Fever)

ByLarry M. Bush, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University;
Maria T. Vazquez-Pertejo, MD, FACP, Wellington Regional Medical Center
Reviewed/Revised Apr 2022
View Patient Education

Brucellosis is caused by Brucella

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 and in travelers to these areas.

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.

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, neurobrucellosis (which includes acute and chronic meningitis, encephalitis, and neuritis), orchitis, cholecystitis, hepatic suppuration, and osteomyelitis (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 white blood cell 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

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.

In children <

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 people (eg, those who have unprotected exposure to infected animals or laboratory samples, who were exposed to the vaccine with B. abortus [strain RB51)B. abortus

Key Points

  • 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.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

  1. CDC: Brucellosis: Assessing Laboratory Risk Level and PEP

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