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Microscopic view of B abortus

Horses can be infected with Brucella abortus or B suis.

Equine brucellosis is caused by Brucella abortus and most commonly manifests as fistulous withers in horses, which can be a source of exposure to humans. Clinically, brucellosis may also be associated with poll evil, nonspecific lameness due to joint infection or, rarely, late abortions in mares. In­fected horses may not show signs of the disease for as long as two years post-ex­posure[1].

Clinical signs

Fistulous withers has been reported sporadically in the United State for over 70 years. The incidence of B. abortus infection in fistulous withers has progressively declined over the years. In the 1930s, 82% of the horses with fistulous withers had positive titers for B. abortus(titer> 1:50.) In the 1940s, the prevalence rate dropped to 73%. In recent re­view, the prevalence of sero-positive B. abortus reactors declined to 37.5%. Much of the decline in equine brucellosis is attributed to the Brucello­sis Eradication program for cattle. Today, the highest prevalence of equine brucello­sis is in Texas that has Class B status for brucellosis, the incidence is also higher in some Class A states, such as Florida, Alabama, and Louisi­ana, as compared to the brucel­losis-free states. Indiana is a brucellosis-free state and has not had a reported case of equine brucel­losis in the past two years. However, Brucellaabortus should be a differential diag­nosis for Indiana horses with fistulous withers, especially if they have been transported from Brucellosis positive re­gions[2].


Although no brucellosis eradication program exists specifically for equine, standards for diagnosis and treating B. abortus associated fistulous withers have been established as means to monitor possible bovine brucellosis and to help protect the public from exposure to this zoonotic path­ogen. The Animal and Plant Health Inspection Service has mandated that a sample of the infected site be obtained for culture in all cases of fistu­lous withers. Isolation should be done at a federally accred­ited facility such as the ADDL. Because B. abortus is difficult to isolate, horses with fistu­lous withers should also be serologically tested for evi­dence of antibodies to B. abor­tus. Whole blood or serum may be submitted. Titers (plate agglutination) greater than or equal to 1:50 has generally been considered positive. More recent investigators propose that a plate agglutination tit­er of 1:100 or 1:160 or less to be negative for Brucella infec­tion. In the state of Indiana, horses with confirmed B. abor­tus infection must be reported to federal authorities[3].


  1. Dawson FL, Durrant DS. (1975) Some serological reactions to “brucella” antigen in the horse. Equine Vet J 7:137–140
  2. Nicoletti PL, Mahler JR, Scarratt WK. (1982) Study of agglutinins to Brucella abortus, B canis and Actinobacillus equuli in horses. Equine Vet J '14:302–304
  3. Cohen ND, Carter GK, McMullan WC. (1992) Fistulous withers in horses: 24 cases (1984–1990). J Am Vet Med Assoc 201:121–124