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 risticiicauses 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.

Specimen

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 container—feces 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.

Author: Jenny Evans