Horses and other large animals worldwide are infected by Corynebacterium pseudotuberculosis. There are also reports of infection in humans resulting from exposure to infected sheep and horses. Pigeon Fever, the most common form of the disease in horses, is characterized by external abscesses, primarily in the pectoral area and ventral part of the abdomen. It is commonly diagnosed in California and other arid regions of the western United States and appears to be increasing in other, wetter areas of the country such as Kentucky and Colorado.
Treatment of this form of disease involves establishing drainage of the abscess with lavage and disposal of the abscess contents to prevent further contamination of the environment. Internal abscesses, which can be fatal if not treated appropriately with antimicrobials, are less common than external abscesses. A third form of disease, ulcerative lymphangitis, can result in lameness and lymphatic damage (big leg) unless treated aggressively with antimicrobials.
The portal of entry for Corynebacterium pseudotuberculosis, a potentially soil-borne organism, is thought to be through abrasions or wounds in the skin and mucous membranes. Insects such as the horn fly, house fly, stable fly, and biting midge have been suggested as potential vectors in horses but have not been confirmed experimentally. Using molecular techniques, a team of researchers at University of California, Davis is determining the importance of these flies in the transmission of the bacterium to horses.
Corynebacterium pseudotuberculosis is considered to survive for long periods in the soil, and it appears endemic on most California farms and ranches. The prevalence of disease is estimated at 10%, making the syndrome one of the most common infectious bacterial diseases of horses in the state. Persistence of the pathogen in the soil indicates that management techniques rather than eradication efforts will be the best first step toward reducing disease incidence. Further research is needed to better understand the epidemiology and pathogenesis of this disease.
Controlling Pigeon Fever could be accomplished with a coordinated strategy involving two complementary tactics: reducing the probability of transmission and reducing the probability of infection should transmission occur. The disease is seasonal, with peak incidence in the fall months, and the incidence fluctuates from year to year within a site. It is not known whether these patterns result from population growth of the pathogen during the warm months, from population fluctuations of the potential vectors, or both. While vaccines are commercially available for sheep and goats, none are available for horses. Administration of bacterins or toxoids offers excellent protection in sheep (more than 90% against experimental challenge). There are strain differences between bacteria infecting horses and small ruminants, so that research is needed to develop a product for use in horses. Dr. Spier has been working with autogenous bacterins/toxoids with the intention of developing a vaccine for horses. Dr. Janet Foley, a UC Davis researcher, is also investigating the strain homogeneity of C. pseudotuberculosis collected from horses in California, Kentucky, and Colorado.