Potomac Horse Fever

    Dr. Kate Onasch

    The summer and fall months are a busy time for most horse owners. Whether competing at horse shows, racing, prepping yearlings for sales, or just enjoying watching your horses graze in your backyard, their health is still important to monitor. Potomac horse fever (PHF), caused by an intracellular rickettsial organism called Neoricketsia ristcii, can affect horses especially during the months of the summer and fall.  Many cases tend to occur near bodies of water (rivers, streams and marshes) and is most commonly seen in the Northeastern and Midwest regions of the United States but is becoming more common in Kentucky and further south. Horses can show a variety of symptoms including a decreased appetite, malaise, fever, diarrhea, and lamintis. PHF is treatable with supportive care but severe cases carry a poor prognosis. Quick recognition and action is important!

    The transmission of PHF is complex. Horses ingest mayflies and/or caddisflies, which are infected with the rickettsial organism. Ingestion of other hosts involved in the life cycle may also lead to infection. Once the organism has been ingested, it begins to infect the cells lining the gastrointestinal tract, especially the mucosal cells of the large colon. This results in colonic inflammation, “colitis”, impaired absorption by the colon, and diarrhea.  The fevers observed in most cases are due to the body’s immune response to the infection. In some cases, the horse will develop laminitis, which can be first noted by finding bounding digital pulses, hot feet, and/or discomfort.

    Potomac horse fever can look clinically similar to several other diseases so a firm diagnosis requires further testing. Tests for PHF include a blood antibody test (titer), and PCR (polymerase chain reaction) of the blood and feces.  A high titer to PHF suggests a recent exposure to N. risticii. It is important to note that if a horse has antibodies to N. risticii this means that they have been exposed to the organism but it does not confirm that this is cause of the clinical signs. PCR of the horse’s blood and feces is used to detect the DNA of N. risticii. This is a more accurate test as the presence of DNA indicates that the organism is currently in the horse’s body. These tests, combined with clinical signs, ruling out other causes, and response to treatment, are used to diagnose PHF.

    Treatment for PHF involves any number of the following therapies; antibiotics, anti‐ inflammatories, IV fluids, electrolyte supplementation, anti‐diarrhea medications, and ice boot therapy. Depending on the severity of the case the horse may require hospitalization for intensive treatment.   Severe cases may carry a poor prognosis due to severe colitis and/or laminitis resulting in sinking or rotating of the coffin bone. PHF is not contagious horse to horse but horses living in the same location may be at risk due to N. risticii being present in that particular environment.

    Prevention of PHF is mainly related to management. A PHF vaccine is available but the efficacy is questionable. Turning barn lights off at night or changing them to blue lights will reduce the number of Mayflies and Caddisflies attracted to the barn. Keeping feed covered and free of insects will also help reduce the chances of infection.  Early detection of illness will result in earlier treatment and may help reduce complications associated with PHF. If you would like to know more about Potomac Horse Fever or any other problems, please contact Hagyard Equine Medical Institute.