Brucellosis is a bacterial infection in cattle caused by Brucella abortus, best known for triggering late-term abortions, typically between the fifth and seventh month of pregnancy. It spreads through contaminated birth materials and can also infect humans, making it both a livestock health crisis and a public health concern. In the United States, decades of aggressive testing and vaccination have brought the disease to the point where all 50 states are now classified as brucellosis-free, but the infection remains a serious problem in many parts of the world.
How Cattle Get Infected
The primary route of transmission is contact with contaminated birth materials. When an infected cow aborts or delivers, the placenta, fetus, and surrounding fluids are loaded with bacteria. Other cattle in the herd pick up the infection by ingesting these materials or through contact with mucous membranes, such as the eyes, nose, or mouth. The bacteria can also enter through broken skin.
Brucella abortus is shed in milk, urine, and semen. Natural mating isn’t considered a major transmission route, but artificial insemination with contaminated semen can deposit bacteria directly into the uterus, making venereal spread more efficient in that context. Contaminated feed troughs, water sources, and even shared syringes can also carry the organism from animal to animal.
Calves can become infected by nursing from an infected dam, and a small percentage are born already carrying the bacteria. This vertical transmission means the disease can quietly persist in a herd across generations if it goes undetected.
Signs and Symptoms
Clinical signs typically appear two to four weeks after exposure, though the disease can remain silent for much longer. The hallmark sign in pregnant cows is abortion during the last trimester. Calves that survive to birth are often weak and unhealthy. Other signs include:
- Retained afterbirth, often followed by uterine infections
- Decreased milk production
- Poor conception rates or infertility
- Weight loss
- Enlarged, arthritic joints (less common)
One of the challenges with brucellosis is that many infected animals don’t look sick. A cow may carry the bacteria and shed it at calving without ever showing obvious illness, which is why routine testing matters far more than visual inspection for catching this disease early.
How Brucellosis Is Diagnosed
Diagnosis relies on a combination of clinical signs, blood tests, and bacterial culture. Serologic tests, which detect antibodies in blood, milk, whey, or semen, have long been the standard method. In dairy herds, a screening tool called the Brucella milk ring test is used at three- to six-month intervals to flag infected herds.
When individual animals need screening, such as at livestock markets, a rapid card test (also called the rose bengal test) can identify presumptively infected cattle. If results are positive, supplemental tests like complement fixation provide a more specific confirmation by targeting the antibodies associated with actual Brucella infection rather than vaccine responses.
For definitive proof, laboratories can culture the bacteria directly from placental tissue, udder secretions, or from the stomach contents and lung tissue of aborted fetuses.
Economic Toll on Herds
The financial damage from brucellosis goes well beyond the aborted calf. Lost milk production, infertility, weight loss, and the costs of testing and quarantine all compound. A modeling study published in PLOS Neglected Tropical Diseases estimated that in pastoral farming systems in Tanzania, each infected cow cost roughly $74 per year in lost production. For households relying on livestock as their primary income, annual losses from brucellosis amounted to about 4.4% of all livestock-derived income. That may sound modest in percentage terms, but for subsistence farmers it can mean the difference between food security and hardship.
Vaccination and Prevention
The primary vaccine used in the U.S. is a live strain called RB51, given as a single subcutaneous injection. Heifer calves should be vaccinated between four and 12 months of age, though some states set narrower windows. Adult cattle can only be vaccinated by state or federal veterinary officials, reflecting how tightly the program is regulated.
The vaccine itself requires careful handling. It must be mixed just before use, kept cool and out of sunlight, and used within one hour of reconstitution. Vaccinated heifers receive a tattoo in the right ear that includes an “R” for the RB51 strain and the last digit of the vaccination year, creating a permanent record on the animal.
Beyond vaccination, protecting a herd means controlling what comes in. Testing new animals before introducing them, isolating pregnant cows during calving, and promptly cleaning up birth materials all reduce the chance of an outbreak. Shared equipment like syringes and feed troughs are potential carriers, so basic hygiene practices matter.
Eradication Status in the U.S.
The cooperative state-federal brucellosis eradication program is one of the longest-running animal disease campaigns in U.S. history. As of March 2026, every U.S. state and territory is classified as “Free” for cattle brucellosis, meaning the herd infection rate is effectively zero. Reaching this milestone required decades of mandatory testing, quarantines, slaughter of infected animals, and widespread vaccination.
Maintaining that status requires ongoing vigilance. The Greater Yellowstone Area, where wild bison and elk carry Brucella abortus, remains a persistent source of potential spillover to cattle grazing nearby. Reintroduction from imported animals is another risk, which is why interstate movement rules still require testing in many cases.
Risk to Humans
Brucellosis is zoonotic, meaning it can jump from cattle to people. The bacteria enter the body through the mouth, nose, eyes, or breaks in the skin. The most common routes are drinking unpasteurized (raw) milk, eating cheese or ice cream made from raw milk, and direct contact with infected animals or their birth tissues.
People at highest risk include veterinarians, dairy farm and ranch workers, slaughterhouse employees, butchers, and hunters who field dress game. In humans, brucellosis causes recurring fevers, sweating (sometimes with a distinctive moldy smell), and fatigue that can wax and wane for months or even years. The bacteria can spread to the heart, brain, spine, and bones, causing inflammation and potentially lasting damage. Pregnant women face a risk of miscarriage.
Pasteurization eliminates Brucella from milk, which is the single most effective measure protecting the general public. For anyone who works directly with livestock, wearing gloves during calving and avoiding contact with aborted materials are practical steps that significantly reduce exposure.