Brucellosis in horses is caused by Brucella abortus or Brucella suis bacteria. The infection is rare, especially in the USA and other countries that have control programs for the infection in food animals. It can cause abortion, infection of the sexual organs in stallions, and infertility. The disease is present in most countries of the world. The most common abnormality associated with brucellosis in horses is suppurative bursitis, a pus-filled inflammation of connective tissue over the shoulders or poll. It is commonly called fistulous withers. Infection is diagnosed through laboratory tests that isolate the bacteria in the blood, semen, or milk of the infected horse or blood tests that detect antibodies to the bacteria. Antibiotics are the usual treatment, although elimination of the infection is difficult to accomplish. It is unlikely that infected horses are a source of the disease for other horses, other animals, or people.