Before pasteurization became routine, brucellosis was one of the most common chronic infections in the United States, with roughly 50,000 confirmed cases per year in the 1940s. Federal milk and cattle eradication programs reduced US cases to under 100 per year by the 1990s. The disease never went away abroad, and the modest US trickle that remains is now sustained by two sources: travel and immigration from endemic regions, and a growing domestic raw-milk consumer market that has driven repeated outbreaks since 2017.

CDC reported 113 brucellosis cases in 2022, plus several cluster investigations tied to raw milk dairies in California, Texas, New York, and Pennsylvania. Most cases are mild but the chronic and complicated presentations (spondylitis, endocarditis, neurobrucellosis) can take a year or more to treat and leave permanent damage. This post fits inside the infection prevention guide and complements zoonotic diseases explained and the Q fever post.

Key Takeaways

What is brucellosis?

Brucellosis is a bacterial zoonosis that infects livestock primarily and humans secondarily. The bacteria are small, slow-growing, gram-negative coccobacilli that survive inside macrophages and other phagocytes, allowing them to evade much of the immune response and establish chronic infection in lymph nodes, spleen, liver, bones, and joints.

The name "Malta fever" comes from the disease's prominence among British soldiers and sailors stationed in Malta during the 19th century, where goat milk was the suspected source. The discovery of Brucella melitensis in goat milk by Themistocles Zammit in 1905 led to dairy hygiene programs that nearly eliminated the disease from Malta within a decade.

In the US, the cattle B. abortus eradication program ran from the 1930s through the 1990s. By 2007, US cattle herds were declared brucellosis-free, though the bacterium persists in elk and bison around Yellowstone National Park, occasionally spilling back into cattle in Montana and Wyoming.

How does brucellosis spread?

Human infection requires animal exposure, either through food or through direct contact.

The dominant US risk is unpasteurized dairy, often imported soft cheeses from Mexico (queso fresco from raw goat or cow milk) or directly purchased from herd-share dairies advertising "real" or "unprocessed" milk. Travelers who consume raw dairy in endemic regions are the second largest US case source.

Where is brucellosis active?

Brucellosis is endemic in:

The US, Canada, Western Europe, Australia, New Zealand, and Japan have eliminated brucellosis from commercial cattle and most small ruminant herds. Residual US risk involves Yellowstone-area wildlife spillover and imported or unpasteurized products.

What are the symptoms?

Symptoms appear 1 to 8 weeks after exposure, sometimes longer. The classic presentation is "undulant fever," in which the fever rises and falls over weeks if untreated.

The clinical picture overlaps with tuberculosis, typhoid, malaria, lymphoma, and many other chronic febrile illnesses. Most patients see several clinicians before diagnosis. Exposure history is the most useful diagnostic clue, particularly raw milk consumption, occupational livestock contact, or travel to endemic regions.

What are the complications?

Brucellosis becomes serious when it persists or invades specific tissues.

Untreated brucellosis can become chronic, with intermittent fevers and progressive fatigue lasting years. Treatment failure and relapse are common when single-agent regimens or short courses are used.

How is brucellosis diagnosed?

Diagnosis is challenging because Brucella is slow-growing and the clinical picture is nonspecific.

Many US clinical labs do not run brucellosis serology in-house. CDC's Bacterial Special Pathogens Branch and state public health labs handle confirmatory testing. The PCR vs antigen vs serology post covers test interpretation principles.

How is brucellosis treated?

Combination antibiotic therapy for prolonged duration is the standard. Single agents fail and select resistance.

Indication Regimen Duration
Uncomplicated adult Doxycycline 100 mg twice daily + rifampin 600-900 mg daily 6 weeks
Uncomplicated alternative Doxycycline 100 mg twice daily + streptomycin 1 g IM daily Doxy 6 weeks, strep 2-3 weeks
Spondylitis Doxycycline + rifampin + aminoglycoside or ciprofloxacin 3 months minimum
Endocarditis Doxycycline + rifampin + aminoglycoside + valve replacement 6 months minimum
Neurobrucellosis Doxycycline + rifampin + trimethoprim-sulfamethoxazole or ceftriaxone 4 to 6 months
Pregnancy Rifampin alone or with trimethoprim-sulfamethoxazole 6 weeks

Relapse rates run 5 to 10 percent even with adequate treatment. Patients require follow-up serology and clinical assessment for several months after completing therapy. Cure of localized complications (spondylitis, endocarditis) requires imaging and clinical resolution, not just serology.

How do you prevent brucellosis?

The single most effective prevention is avoiding unpasteurized dairy.

There is no licensed human vaccine. Livestock vaccines (B. abortus S19, RB51 for cattle; B. melitensis Rev-1 for sheep and goats) are used in eradication programs and reduce zoonotic transmission. Veterinarians have been infected by accidental needlestick exposure to these live vaccines.

FAQ

How long after raw milk consumption can brucellosis appear?

Symptoms appear 1 to 8 weeks after exposure, occasionally longer. Late presentations 3 to 6 months after exposure are documented, particularly for chronic forms with insidious onset. A history of raw milk consumption months before fever and night sweats is a useful clue that may not be recognized without specific questioning.

Is unpasteurized cheese safer than raw milk?

It depends on the cheese. Hard, aged cheeses (over 60 days at proper conditions) reduce Brucella substantially through low water activity and salt. Soft fresh cheeses (queso fresco, feta, fresh chevre) made from raw milk carry the full pathogen load and are the dominant US source of food-borne brucellosis cases. Federal regulations on aged raw-milk cheese reflect this difference.

Can dogs give people brucellosis?

Yes. Brucella canis causes a milder but real form of human brucellosis, typically in breeders, kennel workers, or owners of large breeding operations. Symptoms resemble other Brucella species infections but tend to be less severe. Routine veterinary screening of breeding dogs reduces but does not eliminate the risk. See pets and zoonotic disease.

Does brucellosis cause infertility?

Orchitis and epididymitis occur in 10 to 20 percent of men with acute brucellosis and can cause testicular atrophy if untreated. Female reproductive complications include increased miscarriage rates in untreated pregnancy, especially with B. melitensis. Long-term infertility consequences are debated but documented in some endemic-region cohorts.

Is brucellosis a bioterror concern?

Brucella is a Category B bioterrorism agent because it can be aerosolized, has low infectious dose, and produces prolonged debilitating illness. Several countries had Cold War-era brucellosis weapons programs. No deliberate-release incidents have occurred in modern times. Clinical concern remains natural exposure rather than deliberate use.