Pigeon fever is due to a bacterium named Corynebacterium pseudotuberculosis. Three forms have been described in horses: ulcerative lymphangitis or limb infection, external abscesses and internal infection.
The portal of entry is thought to be through abrasions or wounds in the skin or mucous membranes. Many insects have been incriminated as vectors for the transmission of the disease to horses, and studies have shown that Haematobia irritans, Musca domestica, Stomoxys calcitrans can act as mechanical vectors of this disease. Temporal and special analysis indicated an incubation period of 3 to 4 weeks. Within a geographic area, the disease appeared to be transmitted between 7 and 56 days throughout a 4.3 to 6.5 km distance, strongly suggesting that the disease could be transmitted through horse-to-horse contact or from infected to susceptible horses via insects, other vectors, or contaminated soil. The organism has been shown to survive for up to 2 months in hay and shavings, and more than 8 months in soil samples at environmental temperatures. The incidence of disease fluctuates considerably from year to year presumably due to herd immunity and environmental factors such rainfall and temperature. Disease incidence is seasonal, with highest number of cases occurring during the dry months of the year, although cases may be seen all year. Horses with internal infection are more frequently seen one to two months following the peak number of cases with external abscesses.
The prevention of this disease requires biosecurity measures to limit the spread of Corynebacterium pseudotuberculosis, the aim is to reduce environmental contamination and spread via insects or fomites. The bacterium is endemic in many regions of the world and survives for months in soil, particularly when contaminated with manure.
• Wearing of disposable examination gloves when working with affected horses followed by hand washing is indicated.
• Isolation of affected horses from naive herd mates
• Protecting horses from insect exposure by regular application of insect repellants to the horse including the ventral midline (prevention of ventral midline dermatitis).
• Meticulous wound care (topical fly repellants, antimicrobial ointments and bandages) to prevent infection from a contaminated environment
There is currently no licensed commercially available vaccine in Europe for control of Corynebacterium pseudotuberculosis in horses.
Once horses are recovered and there is no drainage from abscesses no precautions should be needed to reduce the risk these horses pose for spread of infection. There is no practical way at this time for eliminating the bacteria from soil.