In 2018, 26 PHF cases were diagnosed in Kentucky horses. However, this figure is probably under-representative of the true incidence of the disease. | Photo: iStock

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