Corynebacterium pseudotuberculosis (Cp), also known as Pigeon Fever, is a disease characterized by abscesses in the chest muscles, axilla, inguinal area, ventral abdomen, internal organs, or legs of horses. Cp goes by many names depending on location and which form it takes: Pigeon Fever, Dryland Strangles, Equine Distemper are used. Pigeon Fever is often used because of the propensity of the organism to infect the chest, causing it swell out. It is a gram positive organism that infects and lives in the cells of organisms.The range of this disease is world wide and in the US the disease has been predominantly in the SW but has been seen throughout the south and recently entering the Midwest. It appears to be spreading.

Corynebacterium pseudotuberculosis (Cp) can also affect camelids, sheep, goats, buffalo, cattle and rarely humans. There are several subspecies that are host adapted so cross species infection is uncommon. In horses. Draining abscesses contaminate the soil where It lives well particularly in dry environments. In endemic areas cases are usually sporadic effecting single horses or outbreaks fewer than 10% of the animals in the region. In non-endemic areas exposure of susceptible horses can result in epidemics where the percentages effected are higher.

This article discusses transmission, clinical signs, diagnosis, treatment, prognosis, and prevention. It also provides links to scientific articles and discussions on this disease.

Often the horse becomes infected around the site of a recent wound. Horses with midline dermatitis, where the skin is irritated and broken, seem predisposed to the disease. We know the organism lives well in the soil. It seems likely the organism gains access to the horse through broken skin probably by flies possibly by dusty conditions. Many insects have been incriminated as vectors for the transmission of the disease to horses including both biting and nonbiting flies including horse flies and house flies (Haematobia irritans, Musca domestica, Stomoxys calcitrans). Biting flies may also play a role is spreading the disease to horses without wounds.

Transmission to horses has occured up to 5 miles away from the closest active case, presumably caused by biting flies. Following exposure to the organism there is an incubation period in the horse of 3 to 4 weeks before clincial signs develop. The organism can survive for up to 2 months in hay and bedding. And more than 8 months in dry soil samples. At this time it does not appear that the sheep and goat types infect horses. A recent survey has shown that cattle may contract the horse type therefore may be contagious to horses. Humans seem to be at low risk for this disease but the sheep form has infected some people managing infected herds.

Seasonal variation occurs with the lowest incidence in the winter and the highest incidence in the dry months often late summer and early fall. There are also yearly variations with a distinct rise in incidence following mild wet winters. This would be sensible if insects, particularly flies, are responsible for infection.