Watch for Potomac Horse Fever

Staff at the UK Veterinary Diagnostic Laboratory has seen a recent trend in positive results for Potomac horse fever.
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Staff at the University of Kentucky Veterinary Diagnostic Laboratory has seen a recent trend in positive results for Potomac horse fever (PHF), which is caused by Neorickettsia risticii. Results are detected using a real-time PCR assay test.

Neorickettsia risticii causes fever, anorexia, leukopenia (reduced white blood cell numbers), and occasional diarrhea in horses and can be fatal in up to 30% of cases. Neorickettsia risticii can also sometimes result in abortion in pregnant mares. Veterinarians have diagnosed PHF across North America, usually in horses one year and older.

Horses become exposed through accidental ingestion of the metacercarial (encysted) stage of a trematode (parasite) within its insect host (such as mayflies). Veterinarians believe horses are infected through inadvertently ingesting insects that land in drinking water. Risk factors include association with rivers, streams, and other aquatic habitats and grazing pastures next to waterways.

The veterinary diagnostic laboratory can help diagnose PHF. A complete blood count might reveal a transient leukopenia in the early stages. A single positive indirect fluorescent antibody (IFA) test for PHF indicates exposure to the agent. Paired samples collected two weeks apart with a fourfold rise in titer is evident of an active infection. In live animals, a PHF PCR assay should be performed on EDTA blood (i.e., blood collected in a tube containing a chelating agent and anticoagulant) as well as a fecal sample, as the organism’s presence in blood and feces might not temporally coincide

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