Potomac horse fever (PHF) is a seasonal gastrointestinal disease of horses, caused by infection with Neorickettsia risticii (formerly Ehrlichia risticii) bacterium. The disease has been reported in horses in living in the North and South America and Europe. Most cases of PHF occur in horses living on farms that are located near a fresh water body. PHF is not contagious and usually occurs sporadically as isolated incidents in individual horses at farms. Cases of PHF can occur anytime between May and November, however most cases occur between July and August. N. risticii lives on a type of flatworm known as flukes (also referred to as trematodes) that are found in fresh water bodies, such as rivers, creeks, ponds and lakes. Hence how the disease name came to be Potomac horse fever---as it was initially recognized in 1979 in horses living in the eastern United States nearby the Potomac River.

Disease course

Without treatment or prior vaccination, the course of the disease is usually five to 10 days. N. risticii initially invades the horse's intestinal tract upon ingestion, where it multiples in and attacks the white blood cells. This action causes colitis in affected horses; associated clinical signs may include fever (ranging between 102 to 107°F (38.9 to 41.7°C)), depression, and reduced appetite. By the next day or two, horses may have decreased gut sounds followed by higher-pitched tinkling sounds. Some horses may have moderate (cow-pie consistency) to severe (profuse, watery) diarrhea. Mild colic signs may also become apparent, along with swelling of the lower limbs and abdomen. As the disease progresses and allows N. risticii to enter the bloodstream, it causes horses to develop severe toxemia, due to the presence of the toxin within the bloodstream. This stage is often associated with onset of congested mucous membranes (with a thin red or purple band at the gum line, just above the teeth) and increased respiratory and heart rates. 15-30% of horses that develop toxemia progress to acute laminitis, usually within three days of initial presentation of diarrhea.

Early diagnosis and treatment of PHF is critical, as the disease is fatal to 17-36% of infected horses. The most effective and accurate form of diagnosis of PHF is through a blood and fecal PCR (polymerase chain reaction) test that detects the presence of N. risticii DNA in the horse's white blood cells. The disadvantage of the PCR test is that it does not provide a rapid diagnosis, and it can take up to a week for results from the diagnostic laboratory. If treatment is started early in the course of the disease, horses usually respond within 12 hours and symptoms resolving within 3 to 5 days.

Transmission

Horses become infected with N. risticii by directly ingesting flukes by drinking from water bodies or indirectly by ingesting insects or aquatic larvae that feed on the flukes (mayflies, caddiesflies, dragonflies, stoneflies, and damselflies) while grazing on pasture, drinking water, eating hay or feed. Recent research has suggested a possible connection between bats and barn swallows, which have been reported to frequently contain N. risticii in their gastrointestinal systems, which can potentially get passed within their feces.

Incubation Period
It may take more than two weeks for horses to develop clinical signs of PHF.