Equine brucellosis is caused by Brucellaabortus 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 (2) years post-ex­posure.

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.

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.

In fist time in India Sharma et al. (1979) reported. Brucellosis is not only a major zoonotic problem but is also linked with bioterrorism and belongs to category B (Anonymous, 2000). The severity of this disease, lack of vaccines suitable for use in man and frequent failure of clinical laboratories to correctly identify isolates led to the investigation of Brucella as an agent for bioterrorism.

Aerosol or food contamination could be the sources of dispersion. This microorganism has the advantage of being debilitating without being fatal. The infective dose for these organisms is very low, if acquired via the inhalation route. It has been estimated that 10-100 organisms are sufficient to constitute an infectious aerosol dose for humans.Brucellosis is essentially a disease of the sexually mature animals, the predilection site being the reproductive tract, especially the gravid uterus. Allantoic factors including, erythritol, possibly steroid hormones and other substances stimulate the growth of most of the Brucellae .The tropism of Brucella to the male or female reproductive tract was thought to be by erythritol, which stimulates the growth of the organism, but Brucella has also been found in the reproductive tract of animals with no detectable levels of erythritol .Erythritol, a sugar alcohol synthesized in the ungulate placenta and stimulates the growth of virulent strains of Br. abortus, has been credited with the preferential localization of this bacterium within the placenta of ruminants.