Potomac Horse Fever Cases Seen at UKVDL

Between May and July, the University of Kentucky Veterinary Diagnostic Laboratory (UKVDL) confirmed three deaths and 14 positive cases of Potomac horse fever (PHF) in Kentucky.

In June a Thoroughbred mare from a Lexington clinic and a Saddlehorse mare from a West Liberty clinic both died from PHF. In July a 9-year-old Thoroughbred mare from a Lexington clinic also died. These three animals were submitted to the UKVDL for necropsy after showing clinical signs and/or lesions consistent with PHF and testing positive on polymerase chain reaction (PCR), a technique used to amplify a single or a few copies of a piece of DNA.

This acute disease is seen in spring, summer, and fall in Kentucky and can cause high fever, anorexia, colic, laminitis, abortion, diarrhea, and death in horses of all ages. It is caused by Neorickettsia risticii, a Gram-negative, intracellular bacterium with a liking for white blood cells. This organism is closely related to N. helminthoeca, the agent of salmon poisoning in dogs. N. risticii's vector is a fluke that develops in freshwater snails, then is released as the parasite metacercaria, which are ingested by aquatic insects. A horse can become infected if he ingests an infected insect inadvertently. Outbreaks are often associated with horses turned out in pastures bordering creeks or rivers.

If PHF is detected early, veterinarians have reported treatment with oxytetracycline to be successful. If you suspect your horse has PHF, send 10 ml EDTA (ethylenediaminetetraacetate) blood and 5 g feces to UKVDL for PCR testing. Results will be available the same day or next day, thereby facilitating rapid, appropriate treatment. The charge for testing is $35 in-state and $52.50 out-of-state plus a $10 accession fee. Also, please consider submitting to UKVDL for necropsy horses that have died after showing clinical signs consistent with PHF (costs $90 plus a $10 accession fee).

Several inactivated PHF vaccines are on the market, and vaccination trials have demonstrated protection as high as 78%. Treating low-lying areas inhabited by snails can reduce the vector's impact. Any and all efforts to reduce horses' insect exposure can also be helpful as a preventive measure.

To stay abreast of what the laboratories are seeing in submitted cases, visit www.vdl.uky.edu and click on Animal Health Risk Outlook. An interactive Kentucky map will appear. Place your mouse cursor over counties in your practice or living area to see a 30-day moving window of diagnoses rendered at UKVDL and the Breathitt Veterinary Center in Hopkinsville, Ky.

For more information, contact Craig Carter, DVM, PhD, UKVDL director and professor of epidemiology, at craig.carter@uky.edu or Jackie Smith, MS, UKVDL section chief, scientist II-epidemiology, at jsmit8@uky.edu.

Craig Carter, DVM, PhD, Dipl. ACVPM, UKVDL director and professor of epidemiology provided this information.

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