Potomac Horse Fever: An Update

Potomac horse fever is a bacterial disease that is caused by Neorickettsia risticii (formerly Ehrilichia risticii). This disease has been known by a variety of other names including: equine monocytic ehrlichiosis, equine ehrlichial colitis, and acute equine diarrhea syndrome to name a few. The disease was first recognized in the late 1970’s in the Potomac River Valley of Maryland and has since been identified in most of the contiguous United States, parts of eastern Canada, Europe, and South America.
Transmission of Potomac horse fever is complex and allows for the horse to possibly become infected in a variety of ways. Neorickettsia risticii is known to infect trematodes (also called flukes), which are flatworm parasites. The bacteria are able to persist through each of the fluke’s four life stages (egg, cercariae, metacercaria, and adult trematode). These flukes are parasites of bats, birds, and amphibians in North America. It is believed that these animals are not only the definitive hosts of the fluke parasites, but also act as a natural reservoir for Neorickettsia risticii. When infected eggs are excreted in the feces of the aforementioned hosts they contaminate the environment. These eggs may then infect freshwater snails, which is the first intermediate host of the fluke. Once inside the snail the fluke matures into a larva and is released in the water. This larva is then able to infect certain aquatic insects such as mayflies and caddisflies. The larvae are able to mature and persist into the aquatic insects. These aquatic insects, still infected with flukes, are then consumed by bats and birds. This completes the life cycle of the fluke. The horse may become infected by consuming eggs in bat or bird feces, it may consume free flukes in contaminated water, or it might ingest fluke-infected insects. At this time the only confirmed route of transmission has been by the ingestion of fluke-infected aquatic insects. Due to this fact horses are at an increased risk of contracting Potomac horse fever in wet environments, during the summer and early fall when there are large hatchings of aquatic insects. Potomac horse fever is not believed to be directly contagious from horse to horse.
Horses with Potomac horse fever typically lose their appetite and become depressed suddenly and develop a high fever (up to 107o F). Diarrhea may begin around 48 hours after the onset of clinical signs, and will range from “cow-pie” stools to very profuse, watery diarrhea. The diarrhea is typically accompanied by mild colic. Severe diarrhea will lead to dehydration, protein loss, toxemia, and possible shock. One of the most severe sequelae of Potomac horse fever is laminitis also known as founder. Laminitis will typically persist past the resolution of other clinical signs. Treatment of Potomac horse fever includes antibiotic and supportive therapy. The case fatality rate for Potomac horse fever ranges from 5-30%, with most deaths being associated with toxemia or severe laminitis.
The prevention of Potomac horse fever may be difficult, especially in areas where the disease is present on a recurrent basis. Vaccines that are currently on the market are inactivated, and horses develop only short-lived immunity to Neorickettsia risticii. This is in contrast to the immunity acquired through natural infection, which may last greater than two years. Vaccination is recommended however, as it will typically decrease the severity of disease even if it does not completely prevent infection. Vaccination should be performed twice initially, four weeks apart, and then a booster vaccine should be given approximately every six months. Current vaccines are safe to use in pregnant mares, and should be given 4-6 weeks prior to foaling. Foals should then receive their first Potomac horse fever vaccine at approximately 4-6 months of age. Other prevention methods involve minimizing contact and ingestion of aquatic insects, as this is the typical route of natural infection. This would involve keeping horses away from rivers, ponds, and other bodies of water during the late summer and early fall. Water troughs and feed buckets should also be cleaned frequently to ensure that they are free of any type of aquatic insects that may be ingested by the horse.