potomac horse fever

Equine neorickettsiosis, more commonly known as Potomac horse fever (PHF), is an equine-specific bacterial disease caused by Neorickettsia risticii.

The disease was first reported in the U.S. in 1979 as a sporadic condition observed in horses pastured in proximity to the Potomac River. Current distribution is now known to extend far beyond the northeastern United States and has been reported in 43 states; three provinces in Canada; Uruguay and Brazil in South America; France and the Netherlands in Europe; and in India.

The disease is often associated with horses grazing pastures bordering creeks or rivers. Potomac horse fever is seasonal in occurrence, with the majority of outbreaks in Kentucky reported in July through September.

The disease is not contagious per se; infection is naturally acquired by horses accidentally ingesting aquatic insects harboring metacercariae (fluke larvae) infected with N. risticii. The bacterium’s life cycle involves operculate freshwater snails and aquatic insects such as mayflies and caddisflies, the latter being the source of infection for horses.

Outbreaks of PHF comprise isolated or multiple cases of the disease on a premises. Experience has shown that the disease once confirmed on a premises or in a particular area, tends to recur in subsequent years. Neorickettsia risticii can cause an acute enterocolitis in susceptible horses that is clinically manifest by fever, colic of variable severity, and profuse diarrhea. All ages and breeds of horses are at risk of developing the disease. Infections in pregnant mares can give rise to abortion immediately following infection or months after the resolution of clinical signs.

Equine neorickettsiosis has been recorded in Kentucky for a significant number of years. Incidence of the disease can be very variable with increased case numbers frequently seen in years with high rainfalls in the spring followed by above average temperatures in late spring/early summer.

potomac horse fever

In 2018, the disease was first confirmed in Kentucky on June 1. Over the period extending through the week of Aug. 25, 26 cases were diagnosed. This figure is probably under-representative of the true incidence of the disease. The case definition for PHF was based on presence of characteristic clinical signs together with a positive polymerase chain reaction test result for the causal bacterium.

A breakdown of the total number of cases revealed that the disease was confirmed in nine counties, the majority in central Kentucky (Figure 1). Affected horses ranged from one to 17 years of age. The preponderance of cases (20) was seen in mares. Although the majority of cases were in Thoroughbreds (18), the disease was recorded in six other breeds. Of the 26 reported cases in the state, six died and the remainder survived.

To minimize losses from PHF, horsemen were encouraged to review the environment in which they kept their horses and to consult with their veterinarian on strategies that might be used to mitigate disease risk. Recommendation was also given to minimize the opportunity for horses to ingest aquatic insects by turning off lighting in and around barns and other areas at nighttime.


CONTACT—E.S. Rusty Ford—Rusty.ford@ky.gov—Office of the Kentucky State Veterinarian, Frankfort

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.