Brucellosis is one of the most common zoonotic diseases in the world, meaning it spreads from animals to humans. An estimated 2.1 million new human cases occur globally each year, and nearly every infection traces back to contact with infected animals or consumption of contaminated animal products. There is no routine human-to-human transmission.
How Brucellosis Spreads From Animals to People
The bacteria behind brucellosis, called Brucella, can enter your body through several routes. The most common is eating or drinking contaminated animal products, particularly unpasteurized milk, soft cheeses made from raw milk, and undercooked meat from sheep, goats, cattle, camels, or wild game like bison, elk, and wild hog. Roughly half of all human brucellosis cases worldwide are estimated to come from food.
You can also become infected by breathing in the bacteria. This happens most often in barns, slaughterhouses, and laboratories where Brucella becomes airborne in dust or aerosols. Direct contact is another route: the bacteria can enter through cuts in the skin or through the mucous membranes of your eyes, nose, or mouth when you handle animal tissues, birth products, or carcasses. Veterinarians face an additional risk from accidental needle sticks or splashes when administering live animal vaccines.
Which Animals Carry Brucella
Several Brucella species circulate in different animal populations, each with a primary host but capable of jumping to other species. Cattle, elk, and bison are the main carriers of one strain, while domestic pigs, feral swine, and caribou carry another. A third strain is most associated with sheep, goats, and camels and is considered the most virulent for humans. Dogs carry their own strain, which can also infect people. Even marine mammals, including dolphins, porpoises, and seals, have tested positive for Brucella along the Gulf of Mexico, Atlantic, and Pacific coasts since 2010. People who handle stranded or visibly ill marine mammals are at risk.
Who Is Most at Risk
Brucellosis disproportionately affects people who work closely with animals or animal products. Studies from endemic regions illustrate the scale of occupational exposure. In one survey in western Iran, 17% of butchers, 15% of slaughterhouse workers, and 8% of veterinarians tested positive for brucellosis antibodies. Similar studies in Pakistan found rates between 7% and 22% among high-risk workers, compared to around 2% in the general population.
Beyond occupational exposure, anyone who regularly consumes unpasteurized dairy is at elevated risk. This is especially relevant in regions of the Middle East, Central Asia, the Mediterranean, and parts of Latin America and Africa where raw milk cheeses and traditional dishes made from uncooked organs (like raw liver) are common. Travelers to these areas can also be caught off guard.
Symptoms and Incubation Period
After exposure, symptoms typically appear within one to three weeks, though in some cases the bacteria can remain silent for several months before causing illness. The earliest signs are nonspecific and easy to mistake for the flu: fever, drenching night sweats, fatigue, loss of appetite, headache, and difficulty sleeping. The fever often follows an undulating pattern, rising and falling over days or weeks, which historically gave brucellosis the name “undulant fever.”
Without treatment, symptoms frequently persist for months. Chronic infections can drag on for years, with recurring bouts of fever and fatigue that significantly affect quality of life.
Complications of Untreated Infection
When brucellosis goes undiagnosed or untreated for more than a month, the risk of serious complications rises. The bacteria can settle into specific organs and tissues, causing problems in several systems. Joint and spine inflammation (particularly in the lower back and hips) is among the most common complications. The infection can also affect the heart, causing inflammation of the heart lining or valves. In the nervous system, it may lead to meningitis or damage to the optic nerve. Men may develop painful swelling of the testicles. In pregnant women, brucellosis can cause miscarriage.
The good news is that complications are rare when treatment begins promptly. The infection is curable with a combination of antibiotics, typically taken over six weeks, though relapses can occur even after a full course.
How Brucellosis Is Diagnosed
Diagnosis can be tricky because the symptoms overlap with many other infections. The gold standard is a blood culture, where a lab tries to grow Brucella directly from a blood sample. This provides a definitive answer but can take days. Faster results come from blood tests that detect antibodies your immune system produces against the bacteria. A common initial screening test gives results within minutes and, if positive, is followed by more specific confirmation tests. Molecular methods that detect bacterial DNA are also increasingly used for rapid diagnosis.
Because the symptoms are so nonspecific, telling your doctor about any animal contact, travel to endemic areas, or consumption of unpasteurized dairy is critical for getting the right tests ordered.
Prevention at the Individual Level
Your most practical line of defense is avoiding unpasteurized dairy products, especially when traveling in regions where brucellosis is common. Cook meat thoroughly, particularly game meat from wild hogs, bison, elk, or caribou. If you hunt, wear gloves when field dressing animals and avoid touching your face until you’ve washed your hands.
If you work with livestock, use protective equipment: gloves, goggles, and masks when assisting with animal births or handling placentas, which carry extremely high bacterial loads. Take care when administering live animal vaccines, as accidental skin punctures or splashes to the eyes can transmit infection.
How Countries Control Brucellosis in Animals
Because every human case originates from an animal source, the most effective long-term strategy is eliminating the disease in livestock. Countries use a combination of approaches depending on how widespread the infection is. In high-prevalence regions, vaccination of young female cattle, sheep, and goats is the cornerstone. Vaccination alone, however, is not enough to eradicate the disease. It needs to be paired with regular testing of herds and removal of infected animals.
In countries where the disease has already been reduced to low levels, a test-and-slaughter approach replaces mass vaccination. Herds are routinely screened, and any animals that test positive are culled to prevent further spread. Certification of brucellosis-free herds gives farmers and consumers confidence in the safety of dairy and meat products. Eliminating other animal carriers in and around herds, including stray dogs, cats, and rodents, is also part of comprehensive control programs. Countries across Southeast Asia, the Mediterranean, and Latin America are at various stages of implementing these strategies.