Ask Your Veterinarian Presented By Equistro: What’s The Deal With Potomac Horse Fever, Anyway?

by | 01.08.2019 | 12:25pm

QUESTION: What's the deal with Potomac Horse Fever? I thought that only happened on the East Coast, but I hear of cases here in Central Kentucky.

DR. BILL GILSENAN: Potomac Horse Fever (PHF) is a disease of horses characterized by an acute onset of diarrhea, dullness, and inappetence. As the name implies, fever typically accompanies these clinical signs. In approximately 20 to 30 percent of cases, laminitis can develop, sometimes in the absence of diarrhea. Laminitis secondary to PHF is often severe and rapidly induces dramatic changes in the feet. Needless to say, severe diarrhea and severe laminitis can have fatal implications for the horse. Therapeutic intervention with oxytetracycline in a timely manner improves the horse's potential for recovery. Thus awareness and recognition of PHF is an important means of reducing morbidity and mortality caused by the disease.

Neorickettsia risticii is an obligate intracellular bacterium and is the cause of PHF. Transmission of N. risticii starts with the bacterium infecting flatworms (trematodes). Flatworms require intermediate hosts—aquatic snails and insects—to complete their life cycle. Ultimately, aquatic insects (caddisflies and mayflies) become infected with N. risticii. Horses can inadvertently ingest these infected insects. Once this happens, N. risticii infects the horse's white blood cells and can eventually infect the large colon, causing enterocolitis and resultant diarrhea and laminitis. Not all infected horses develop enterocolitis. The mechanism by which an episode of disease is induced is not completely understood. Clinical disease is usually seen sporadically on farms, although outbreaks do occur. Importantly, PHF is not directly contagious from horse to horse.

Dr. Bill Gilsenan

The reliance of N. risticii on aquatic intermediate hosts has resulted in a definite seasonal pattern, with cases usually seen from mid-summer to late fall. The complexity of PHF transmission posed a diagnostic challenge when clusters of disease were noted. The identification of N. risticii as the cause of PHF occurred in the late 1970s and early 1980s following an outbreak of disease in Maryland and Virginia. It was noted then that the majority of cases were in close proximity (less than 5 miles) to a waterway. The term ‘Potomac Horse Fever' was reportedly coined by a television reporter, referring to the river that runs between these two states. The term is not a great descriptor of the disease but it stuck nonetheless. In fact, a seasonal syndrome affecting horses in southern Ontario was described by Dr. Frank Schofield in 1924. Horses in the report would develop dullness, diarrhea, and laminitis. It was observed then that risk of disease seemed to be associated with relative distance from a waterfront. People in the area reported that severity of the syndrome varied annually but that it had been present for at least 50 years.

Once PHF was defined as a clinical entity, written accounts of the disease increased. A scientific report in 1986 documented clusters of PHF cases in Connecticut, Idaho, New Jersey, New York, Ohio, and Pennsylvania. The disease was later chronicled in ten horses in Alberta in 1989. In 1997, the “Shasta River Crud,” a long-recognized seasonal disease causing diarrhea and laminitis in horses in northern California, was determined to be caused by N. risticii. To date, exposure to N. risticii has been documented horses in at least 43 states (including Kentucky) and in at least 4 Canadian provinces (Alberta, Manitoba, Nova Scotia, and Ontario).

The term ‘Potomac Horse Fever' suggests that this is a disease of the Northeast. However, enterocolitis caused by N. risticii was probably much more geographically widespread than anticipated when it was initially recognized nearly 40 years ago. Cases of PHF are diagnosed in central Kentucky every summer and fall. This recognition is more likely a consequence of heightened awareness of the disease rather than geographic expansion of the disease.

Bill Gilsenan received his veterinary degree from the University of Pennsylvania in 2008. Following an internship at Colorado State University, he completed a residency in large animal internal medicine at the New Bolton Center—University of Pennsylvania. He held a faculty position at the Virginia-Maryland College of Veterinary Medicine until joining the staff at Rood and Riddle Equine Hospital as an internal medicine specialist in 2015.

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