Brucellosis: The ancient, persistent zoonotic disease hiding in your glass of raw milk

Nature · Published 30 September 2026 Swipe more like this →

In short: Known historically as Malta fever, brucellosis is a stubborn bacterial infection that jumps from animals to humans. While often linked to unpasteurized dairy, it also poses a significant occupational hazard for farmers and hunters. Because the bacteria hide inside human cells, treatment is complex, lengthy, and prone to relapse.

Brucellosis is caused by Brucella, a genus of small, rod-shaped, intracellular bacteria. Four species primarily infect humans: B. melitensis, which is the most virulent and typically found in goats; B. suis, which infects pigs; B. abortus, common in cattle; and B. canis, which affects dogs. These organisms act as facultative intracellular parasites, meaning they can survive and replicate inside the host's own cells. This survival strategy makes the infection notoriously difficult to clear, often leading to chronic, lifelong disease characterized by undulating fevers, profuse sweating, and severe joint and muscle pain.

Transmission occurs through direct contact with infected animals, inhalation of aerosolized bacteria, or the consumption of contaminated, unpasteurized dairy products. Wildlife reservoirs—including bison, elk, and wild boars—complicate control efforts, as these animals can spill the bacteria back into domestic livestock populations. Occupational groups such as veterinarians, laboratory workers, and slaughterhouse employees face the highest risk. In 2019, a notable outbreak in Lanzhou occurred when a biopharmaceutical plant accidentally released the bacteria into the atmosphere due to the use of expired disinfectant.

Diagnosing brucellosis is challenging because the bacteria grow extremely slowly in culture, sometimes taking up to two months. While serological tests like the standard agglutination test are common, they can produce false negatives or cross-react with other bacteria. Treatment requires a multi-drug antibiotic regimen—often involving doxycycline combined with streptomycin or rifampicin—administered for several weeks to ensure the drugs penetrate the cells where the bacteria reside. Even with aggressive therapy, relapses occur in 5% to 15% of patients. Because no human vaccine exists, global health strategies rely on a 'One Health' approach, integrating veterinary surveillance, mandatory milk pasteurization, and strict occupational safety protocols.

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