Potomac Horse Fever (PHF) is a bacterial infection that can result in severe colic, diarrhea, inflammation, depression and laminitis. In serious cases, it can be fatal to the horse.

PHF is caused by infection with Neorickettsia risticii and typically affects horses grazing in pastures that border rivers or creeks. This bacteria may be found in bodies of water or in aquatic insects.

PHF occurs seasonally with most cases reported between late spring and early fall. Cases are concentrated in regions of the United States and Canada including Ontario, Pennsylvania, Michigan, Illinois, Kentucky and California.

If you live in an area with high prevalence of Potomac Horse Fever, your veterinarian may recommend vaccination. However, vaccines may not fully protect against infection or symptoms. Limiting exposure to affected waterways is the best way to prevent infection.

What is Potomac Horse Fever?

Potomac Horse Fever (also known as Equine Neorickettsiosis, Equine Monocytic Ehrlichiosis, Shasta River Crud, and Equine Ehrlichial Colitis) is an acute illness that causes a fever, inflammation and gastrointestinal issues.

The predominant clinical sign of PHF is inflammation that affects the small and large intestine (enterocolitis) and leads to diarrhea.

Additional symptoms of the disease include colic and laminitis. [1][2] PHF can also cause the abortion of fetuses in pregnant mares that are infected. [3]

Symptoms of this syndrome were first documented by Dr. Frank W. Schofield, a veterinarian at the Ontario Veterinary College now located in Guelph, Ontario.

In 1924, he described a condition that was referred to by local horse owners as “horse cholera”, “dysentery” or “abdominal typhoid.” Dr. Schofield suspected this condition had a bacterial origin, but the cause remained unknown until many years later. [12]

PHF was first officially recognized in Maryland in 1979 as an illness that affected horses living near the Potomac River in the eastern United States. The disease has since been discovered in most American states, Canada, South America (Brazil and Uruguay), Europe (France and The Netherlands), and India.

In 1984, the bacterial organism that causes PHF was finally identified as Neorickettsia risticii. [14][15]

Neorickettsia risticii

This illness is caused by an infection of the intestinal tract with the bacterium Neorickettsia risticii (N. risticii). [1][2] N. risticii was previously named Ehrlichia risticii and is an obligate intracellular gram-negative bacteria.

PHF is a vector-borne disease transmitted by organisms that host a type of parasitic flatworm which N. risticii infects. N. risticii is capable of living in multiple hosts, primarily aquatic insects, that spread the disease.

PHF occurs when horses ingest parasitic flatworms or organisms carrying parasitic flatworms infected with N. risticii. They may become exposed to the bacteria through contaminated water or food sources.

The disease is not contagious and cannot be transmitted between horses via casual contact or from horses to humans. However, infections typically occur in clusters as horses drinking from the same water source or eating the same feed may all become infected.

Prevalence and Epidemiology

PHF affects horses of all ages, although foals have a lower risk of contracting the disease because they are nursing.

As a seasonal disease, PHF typically occurs in the spring, summer, and early fall. For example, 46 PHF cases were diagnosed between 2015 and 2019 in Ontario during the months of late June and early September. [4]

PHF most often occurs in temperate areas and affects animals living near creeks or rivers. If the disease has affected animals on a particular property or geographic region, the disease is likely to occur again in that same location in the future.

Aside from horses, N. risticii can also infect dogs and cats. Cattle are believed to be resistant to infection by the bacterium. [5]

PHF can be fatal if not treated promptly. The estimated mortality rate from the disease is between 17% and 36%. [6]