New Real-Time PCR Assay for Diagnosing Potomac Horse Fever

Neorickettsia risticii, the causative agent of Potomac horse fever (PHF), causes fever, anorexia, leukopenia (reduced white blood cell numbers), and occasional diarrhea in horses and can be fatal in up to 30% of cases. Exposure is 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. The University of Kentucky Veterinary Diagnostic Laboratory (UKVDL) has detected a high number of positive cases this year, possibly due to the increased summer precipitation.

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 evidence of an active infection. On 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 presence of the organism in blood and feces might not temporally coincide.

The UKVDL has developed and implemented a new real-time PCR assay, which is quicker and more sensitive than existing tests.

To confirm suspect cases, the UKVDL recommends providing 10 mL of anticoagulated blood in EDTA tubes (purple top) and either feces (at least 5 grams) or a fecal swab for each animal tested using real-time PCR analysis. Please use an appropriate specimen containerfeces in gloves can no longer be accepted. Screw-cap tubes are preferred.

Fee and schedule
The fee is $35 in-state and $52.50 out-of-state total for both samples. Turn-around is one to two working days. The test is run Mondays through Fridays.

Please call 859/257-8283 for further information.

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