Understanding Potomac Horse Fever

Potomac horse fever often poses a diagnostic challenge because of its varied clinical presentations.
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Potomac horse fever (PHF), or equine monocytic ehrlichiosis as it is currently termed, still occurs in areas of the United States. It can often pose a diagnostic challenge because of its varied clinical presentations. Potomac horse fever was first recognized in the early 1980s in the Potomac River area of Maryland. The cause was found to be a microorganism initially named Ehrlichia risticii, which has since been renamed Neorickettsia risticii. This disease is not contagious between animals.

Much of the early research dealt with investigating how it is transmitted. It was initially discovered that transmission could occur by horses ingesting N. risticii-containing flukes that develop within aquatic snails; however, more recently researchers have found that insects, including damselflies, caddis flies, and mayflies, can also harbor the flukes containing the organism. Ingestion of these insects has resulted in PHF. This discovery is significant, since these flying insects can swarm in large numbers and be numerous on pastures. Furthermore, the insects are attracted to lights in horse stabling areas, increasing the risk of exposure. Horses do not have to be in close vicinity to a water source to contract PHF; even dead insects on pasture or in feed or water buckets can pose a risk.

Since August 2009, the University of Kentucky Veterinary Diagnostic Laboratory has diagnosed seven cases of PHF in horses presented for necropsy. The primary clinical sign was diarrhea. Additional signs reported included anorexia, toxemia, colic, edema, ataxia, and laminitis.

Apart from necropsy cases, the laboratory received blood and fecal material for PHF testing by indirect fluorescent antibody (IFA) and polymerase chain reaction (PCR) methods. Over the period of October 2010 through October 2011, 238 serum samples were tested for antibodies to the PHF agent using the IFA method. Of these samples, 89 were positive. The IFA test indicates prior exposure but does not necessarily signify active infection and cannot differentiate natural exposure from a vaccine-induced titer. Paired serum samples two to three weeks later may yield more definitive information.

Additionally, 225 whole blood and/or fecal samples were tested by PCR, and 35 samples were positive. The PCR test detects the presence of specific nucleic acid of N. rickettsia, indicating presence of the organism. There were 87 cases/samples in which both the IFA and PCR tests were performed, of which 12 were positive in both tests. Table 2 depicts the time of year that the diagnoses were made, indicating that PHF is a warm-weather disease, as would be expected with an insect-borne disease.

If horses live in an area where PHF is known to occur, it is important to limit possible exposure to the vectors. Owners should consider keeping food covered, using insect control around barns, and restricting grazing near water sources during the warmer months. A vaccine is available, but while it is safe, its ability to protect against infection is not fully established. Horse owners should consult their veterinarians for specific recommendations appropriate to their situation.

CONTACT: Neil Williams, DVM, PhD, Dipl. ACVP, 859/257-8283, nmwillia@uky.edu, Veterinary Diagnostic Laboratory, University of Kentucky, Lexington, Kentucky

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents.


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