Is Brucellosis Zoonotic How It Spreads To Humans?

Brucellosis is a bacterial infection that passes from animals to people — it is zoonotic, and that is not in dispute. The bacteria behind it, from the genus Brucella, live in cattle, goats, sheep, pigs, and some other animals, and they can cross into humans through direct contact with infected animals, through contaminated animal products, or by breathing in the bacteria in certain settings. Person-to-person spread is rare and is not the main way this disease moves.

Is Brucellosis Zoonotic and How It Spreads To Humans?

Yes. Brucellosis is one of the classic zoonotic diseases, meaning animals are the source and humans are almost always the end of the line. People do not typically pass it to each other.

The bacteria enter the body through a few well-understood routes. Each route reflects where the bacteria are found in the infected animal — in milk, in birth tissues, in urine, and in blood.

• Unpasteurized dairy: Milk, soft cheese, butter, and ice cream made from raw milk from infected goats, sheep, or cattle. This is the most common route worldwide.
• Direct contact: Touching infected animals, especially during birth, slaughter, or handling of placentas and aborted fetuses. The bacteria can enter through cuts, scrapes, or the eyes.
• Inhalation: Breathing in bacteria, mainly a risk in laboratories, slaughterhouses, and some farming settings.
• Needle or wound exposure: Accidental inoculation, mostly a lab or veterinary workplace concern.

What brucellosis is not is a disease that spreads easily through casual contact, crowds, or shared air in everyday life. That distinction matters, because it shapes who is actually at risk.

Which Animals Carry Brucella Bacteria?

Cattle, goats, and sheep are the main reservoirs, and each tends to be linked with a particular Brucella species. Pigs, dogs, and some marine mammals can also carry their own strains.

In infected animals, the bacteria concentrate in the reproductive organs and in milk. When an animal gives birth or miscarries, the placenta and fluids can contain enormous numbers of bacteria. That is why birthing and slaughtering are the riskiest times for people who work with animals.

The disease in animals often goes unnoticed. An infected goat or cow may look healthy while still shedding bacteria in its milk. This silent shedding is a key reason raw dairy is such an important route of human infection.

Countries that have eliminated brucellosis in their livestock herds — through testing, culling, and vaccination programs — see far fewer human cases. In the United States, cattle brucellosis has been largely eradicated, though it persists in wildlife such as bison and elk in certain regions, and it remains a problem in some other parts of the world.

How Do People Get Brucellosis From Raw Milk and Cheese?

Drinking raw milk or eating soft cheese made from it is the single most common way people get brucellosis globally. Pasteurization — heating milk to a specific temperature for a set time — kills Brucella bacteria.

This is not a theoretical risk. Outbreaks have been traced to unpasteurized goat cheese and raw milk from infected herds. The bacteria survive in refrigerated milk and in some cheeses for extended periods, so cold storage does not make raw dairy safe.

Hard, aged cheeses are generally considered lower risk than soft, fresh cheeses because the aging process and lower moisture make it harder for bacteria to survive — but the reliable protection comes from pasteurization, not from aging alone.

People who drink raw milk for perceived health benefits are taking on a real infection risk. The claimed benefits of raw milk are not supported by strong evidence, while the infectious risks are well documented.

Who Is Most at Risk of Brucellosis?

Occupational exposure drives a large share of cases. The people most at risk are those whose work or daily life brings them into close contact with animals or animal products.

• Farmers, shepherds, and ranchers
• Veterinarians and veterinary staff
• Slaughterhouse and meat-processing workers
• Laboratory workers who handle Brucella cultures
• Anyone who regularly consumes unpasteurized dairy
• Hunters who field-dress wild animals such as feral pigs

In the United States, human brucellosis is uncommon, and most cases are linked to travel to or consumption of dairy from regions where the disease is still common — parts of the Mediterranean, the Middle East, Central Asia, Africa, and Latin America. A clinician who does not ask about travel or raw dairy can easily miss it.

There is no strong evidence that brucellosis is more severe in one sex or the other, but pregnancy is a genuine concern. Infection during pregnancy is associated with a risk of miscarriage, so pregnant women with possible exposure should seek medical evaluation promptly.

What Are the Symptoms of Brucellosis in Humans?

Brucellosis typically produces flu-like symptoms that come on gradually, often one to four weeks after exposure. The onset is usually slow rather than sudden.

Common symptoms include fever, heavy sweating (sometimes with a distinctive musty odor), fatigue, muscle and joint pain, headache, and loss of appetite. The fever can be intermittent, which is why the disease was historically called undulant fever — the temperature rises and falls in waves.

Because these symptoms overlap with many other illnesses, brucellosis is frequently missed at first. The key clues are exposure history: raw dairy, animal contact, travel to an endemic region, or laboratory work.

Without treatment, the infection can persist for weeks to months and can settle into a chronic form. Complications can include infection of the bones and joints (particularly the spine), the heart valves (endocarditis), and, less often, the nervous system. These complications are more likely when diagnosis and treatment are delayed.

How Is Brucellosis Diagnosed and Treated?

Diagnosis rests on blood tests that detect antibodies against Brucella or, in some cases, by growing the bacteria from a blood sample. Because the bacteria grow slowly in culture, antibody testing is often the more practical first step.

Treatment requires a combination of antibiotics taken for an extended period — typically several weeks to a few months. Combination therapy is used rather than a single drug because it lowers the chance of relapse, which is a well-recognized problem with this infection. Relapse can occur even after appropriate treatment, so follow-up matters.

This is not an infection to treat at home or to wait out. Anyone with suspected brucellosis should be evaluated by a physician, and treatment decisions — including which antibiotics and for how long — belong with a clinician who can weigh the specifics of the case. No clinical guidelines recommend self-treatment for this disease.

There is no widely available vaccine for humans. A vaccine exists for livestock, and animal vaccination programs are a major part of how countries reduce human cases.

How Can Brucellosis Be Prevented?

Prevention centers on two things: keeping the bacteria out of your body and keeping them out of your food. Both are well established.

• Avoid raw dairy. Choose pasteurized milk and cheese. This is the most effective single step for most people.
• Use protective equipment. Gloves, goggles, and proper clothing when handling animals, birthing, or slaughtering — especially if you have open cuts.
• Handle high-risk materials carefully. Placentas and aborted fetuses from livestock should be treated as heavily contaminated.
• Follow lab safety rules. Brucella is a recognized laboratory hazard and requires appropriate containment.
• Cook meat thoroughly. While less common than dairy, undercooked meat from infected animals can be a source.

For people in high-risk occupations, awareness and protective gear do most of the work. For everyone else, the simplest protection is to skip unpasteurized dairy products.

One point worth clarifying: brucellosis is not spread by casual contact with an infected person, and it is not a disease you pick up from a crowded room. The fear of person-to-person spread is largely misplaced. The real risks are specific and avoidable.