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Also known as

Bangs disease, contagious abortion, undulant fever


Brucellosis is the most common cause of abortion in cattle around the world, except where it has been controlled or eradicated by vaccination. Brucellosis affected about 25% of cattle in the U.S. before control programs and vaccination. Due to the threat to human health, a rigorous program to eliminate this disease in cattle was begun as soon as a vaccine was developed. Vaccination is not 100% effective, however, so a test and slaughter program was also used.

Herd testing, slaughter of all infected animals, and vaccination of all healthy heifers eventually eradicated this disease in most parts of the U.S. except in areas around Yellowstone Park where wildlife like elk and bison can pass it to cattle. Cattle, hogs, goats, bison and elk are hosts for this disease. 

The disease mainly affects sexually mature animals, causing abortion, retained placenta, and ill thrift. Bacteria take up residence in the udder and lymph glands and invade the uterus if the cow becomes pregnant. In the uterus, bacteria attack the fetal membranes that supply blood to the fetus, eventually killing it and causing abortion or resulting in a weak calf. A few calves are born normal but may retain the infection throughout their lives, with heifers possibly spreading the disease at their first calving. Infected bulls may develop arthritis or inflammation of the testicles (and subsequent infertility). Brucellosis causes abortion in the last trimester of pregnancy and results in a subsequent period of infertility for the cow. Longstanding infections may result in arthritic joints.

In an unvaccinated herd, the infection spreads rapidly and causes many abortions. An infected cow usually aborts only once after exposure; subsequent gestations are often normal. After exposure, cattle develop antibodies.

Incubation period may be variable and is often related to stage of gestation at time of exposure. These bacteria are shed in milk and uterine discharges, and the cow may become temporarily infertile. Bacteria may be found in the uterus during pregnancy and in the fluids expelled after calving, and can be shed in milk for a variable length of time—often for the rest of the cow’s life.


  • Abortion or birth of premature or weak calves,
  • Retained placenta,
  • Reduced milk yield,
  • Fever,
  • Poor-doing


This disease in cattle, water buffalo, and bison is caused mainly by a bacterium called Brucella abortus.  A related bacterium called B. suis is occasionally found in seropositive cows but does not cause clinical signs and is not contagious from cow to cow. In some countries, the disease in cattle is caused by B. melitensis but this bacterium is not present in the U.S.

Bison and elk in Yellowstone Park carry brucellosis, and pose a threat to livestock whenever they come out of the Park and mingle with cattle or contaminate pastures or bedgrounds and feeding areas.  

In some countries a common source of exposure is through purchase of infected cattle. At calving, the disease is spread to susceptible (unvaccinated) cows via aborted fetuses, fetal membranes, and discharges from infected females. Most cows that abort continue to shed the bacteria for 2 to 4 weeks afterward, contaminating pastures and pens. Other cattle that lick an aborted fetus or contaminated spots or discharges become infected. Dogs or predators dragging a fetus or placenta across a pasture can spread the bacteria. Infected cows may also excrete bacteria in their milk and might infect their nursing calves.  Bulls can become infected, but rarely transmit the disease.

Transmission may sometimes occur by artificial insemination when contaminated semen is deposited in the uterus, but not when deposited in the cervix. Brucella bacteria may also enter the body through mucous membranes, conjunctivae (eyes), wounds, or even intact skin in both humans and animals.

Brucella bacteria can remain viable in fetuses and manure in a cool environment for more than 2 months, but exposure to direct sunlight kills the organisms within a few hours.


 Any cattle suspected of having the disease, or any new additions to a herd that have no evidence of calfhood vaccination (metal tag in the ear and an ear tattoo bearing the same number) can be tested. In a region or on a farm that might be infected, use sterile gloves for any type of reproductive work. Don’t keep horses with cattle, since horses may carry the infection, and keep dogs out of livestock areas.

If a cow aborts or calves prematurely, consult your vet, and wear protective gloves when handling the animals. Aborted fetuses and placental material should be sent to a diagnostic lab for diagnosis. Scrub well afterward, and don’t touch your eyes, nose or mouth after touching the premature calf or aborted fetus. Use plastic garbage cans or tubs to pick up an aborted calf and placenta; don’t drag it through a pen.  

Your vet can take samples from the fetus and dam to test for brucellosis, and the cow should be isolated from other cattle until you know the results. The area where the abortion occurred should be thoroughly disinfected, and any remaining fetal or placental tissues burned or buried. Keep any cow that retains her placenta (or any other suspect cow with vulva discharge) separate from other cows for 3 to 4 weeks after calving.

Brucellosis can be avoided if you buy only females that had calfhood vaccinations (or from a herd that is known to be free of this disease). For best insurance--if you have any doubts--keep the purchased animal separate from others until it can be tested. If you live in a state that requires vaccination, vaccinate all heifer calves within the proper age limit.  

Vaccination of calves with B. abortus Strain 19 or Strain RB51 provides fair immunity and resistance to infection, though some vaccinated calves may become infected, depending on severity of exposure. A small percentage of vaccinated calves develop antibodies to Strain 19 that may persist for years and might confuse diagnostic test results. Today calves in the U.S. are vaccinated with Strain RB51. It does not cause production of antibodies that would be detected by most serologic tests.

Most states are now free of brucellosis, but in some states no cows or heifers can be shipped out of state unless they’ve been vaccinated (the destination state requires proof of vaccination). In some states all heifer calves must be vaccinated between 2 and 10 months old. Consult your vet regarding the regulations and requirements for your state.


There is no effective treatment.

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EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.