Begin Main Content Area


Brucellosis is one of the most serious diseases to affect livestock, resulting in decreased milk production, weight loss in animals, loss of young, infertility, and lameness. The disease spreads rapidly and is transmissible to humans, which makes it a serious threat.

The disease is caused by a group of bacteria known scientifically as the genus Brucella. Three species of Brucella cause the most concern: B. abortus, principally affecting cattle and bison; B. suis, principally affecting swine and reindeer but also cattle and bison; and B. melitensis, principally affecting goats but not present in the United States. In cattle and bison, the disease currently localizes in the reproductive organs and/or the udder.  Bacteria are shed in milk or via the aborted fetus, afterbirth, or other reproductive tract discharges.

There is no effective way to detect infected animals by their appearance. The most obvious signs in pregnant animals are abortion or birth of weak calves. Milk production may be reduced from changes in the normal lactation period caused by abortions and delayed conceptions. Not all infected cows abort, but those that do usually abort between the fifth and seventh month of pregnancy. Infected cows usually abort once, but a percentage will abort during additional pregnancies, and calves born from later pregnancies may be weak and unhealthy. Even though their calves may appear healthy, infected cows continue to harbor and discharge infectious organisms and should be regarded as dangerous sources of the disease. Other signs of brucellosis include an apparent lowering of fertility with poor conception rates, retained afterbirths with resulting uterine infections, and (occasionally) enlarged, arthritic joints.

Brucellosis is commonly transmitted to susceptible animals by direct contact with infected animals or with an environment that has been contaminated with discharges from infected animals. Aborted fetuses, placental membranes or fluids, and other vaginal discharges present after an infected animal has aborted or calved are all highly contaminated with infectious Brucella organisms. Cows may lick those materials or the genital area of other cows or ingest the disease-causing organisms with contaminated food or water. Despite occasional exceptions, the general rule is that brucellosis is carried from one herd to another by an infected or exposed animal. This mode of transmission occurs when a herd owner buys replacement cattle or bison that are infected or have been exposed to infection prior to purchase. The disease may also be spread when wild animals or animals from an affected herd mingle with brucellosis-free herds.

Before 1934, control of brucellosis was limited mainly to individual herds. Today, there is a Cooperative State Federal Brucellosis Eradication Program to eliminate the disease from the country. Like other animal disease-eradication efforts, success of the program depends on the support and participation of livestock producers. The program's Uniform Methods and Rules set forth the minimum standards for states to achieve eradication. States are designated brucellosis Class Free when none of their cattle or bison are found to be infected for 12 consecutive months under an active surveillance program. Pennsylvania has been classified as a Class Free state since April 1, 1983.

The Division of Animal and Poultry Health develops test programs for regulating diseases such as brucellosis, and provides disease control programs to help prevent these diseases from entering Pennsylvania in an effort to protect our animal agriculture. Staff members evaluate health certificates for both interstate and intrastate movement to assure that animals in Pennsylvania are healthy and have met necessary health requirements.

Brucellosis includes the Certified Cattle and Goat Herd Programs; the Certified, Qualified and Monitored Cervid Herd programs; and the Brucellosis Validated Swine Herd Program.