Potomac horse fever (PHF) is a common emergency in equids in the northeastern United States, though it’s been reported in other locations. The disease is caused by the obligate intracellular bacterium Neorickettsia risticii. Other Neorickettsia species have also been isolated in PHF cases. Equids acquire the Gram-negative bacteria through a complicated transmission cycle: Neorickettsia colonize within a trematode (flatworm) host that infects aquatic snails and insects (most commonly mayflies and caddis flies). Horses typically and inadvertently consume the infected insects on pasture, near a water source. Proximity to a lake, pond, or river increases the risk of bacterial exposure. Once ingested, the bacteria colonize within cells of the horse’s gastrointestinal tract.

Potomac horse fever caseload is highest in late summer and throughout fall, when the insect population increases. Veterinarians have also reported cases in winter and spring.

Signs of disease

A fever and “pipe-stream” diarrhea are the characteristic signs of PHF. Early in the disease process, clinical signs include a lethargic attitude and decreased appetite. These findings are caused by a fever, or a temperature of at least 101.5 degrees F. Elevated body temperature can also increase heart rate and/or respiratory rate. Diarrhea then develops within several days of the initial clinical signs.

In some cases horses develop the hoof disease laminitis due to fever and endotoxemia (when Gram-negative bacteria that normally reside in a horse’s gut gain access to the bloodstream, prompting systemic inflammation). Infected pregnant mares are at risk of abortion, and severe PHF can be fatal for any horse. The clinical signs of PHF are similar to those of many infectious diseases, so implement isolation and biosecurity protocols if you note a fever or diarrhea in any horse. Potomac horse fever is not contagious, however, so you can halt these practices once a diagnosis is confirmed; just be aware that infected mayflies and caddis flies could be active on the property.

Treatment

Early treatment can reduce or eliminate PHF clinical signs. Veterinarians use polymerase chain reaction testing (on blood/manure) or serology (blood) testing to detect exposure to N. risticii. A bacterial culture of Neorickettsia spp within the manure is the gold-standard test. Unfortunately, a bacterial culture can take days to weeks. Therefore, in many cases veterinarians must begin treatment prior to reaching a definitive diagnosis.

Antibiotic therapy with a tetracycline is the only effective PHF treatment. Tetracycline is a broad-spectrum class of antibiotic that works against intracellular bacteria. Oxytetracycline provides a reliable systemic response, due to its intravenous (IV) administration, and can be given to all PHF patients. Veterinarians might recommend oral doxycycline (another tetracycline) when diarrhea is absent or mild. In severe cases the horse’s ability to absorb oral medication might be poor due to the diseased gastrointestinal tract’s hypermotility (excessive activity). Vets might need to use other therapies to manage some horses’ clinical signs. Non-steroidal ­anti-inflammatory therapy (such as flunixin meglumine, or Banamine) can reduce fever and systemic endotoxemia, while IV fluid therapy can help manage the patient’s hydration status. Nasogastric fluid therapy is not appropriate for PHF cases because this form of hydration should not be used for horses with diarrhea. Di-tri-octahedral smectite (Bio-Sponge), activated charcoal, and other gastrointestinal supplements can also help alleviate diarrhea. Ice boots might help prevent laminitis (inflammation of the tissues that suspend the coffin bone within the hoof) if used early in the disease process. As with any disease, clinical progression varies between patients, and other treatments might be warranted.

Severe PHF cases require hospitalization for intensive therapies and monitoring. Mild cases can remain on the farm, with veterinary oversight and management by a confident, capable provider.

Vaccination

While a vaccine against PHF is available, it’s not perfect, and a failed immune response at time of vaccination can leave a horse susceptible to disease. Antigenic variation of the Neorickettsia bacteria limits complete coverage by the vaccine. Nonetheless, researchers have reported a reduced number of cases in vaccinated populations. Vaccinated horses diagnosed with PHF typically recover faster and with fewer complications. Simply stated, the vaccine can prevent or reduce severity of the disease. At-risk horses over 3 months of age should be vaccinated prior to the peak seasons of exposure. Varying vaccine protocols depend on age, risk of exposure, pregnancy status, and PHF vaccination status. Consult your veterinarian for recommendations regarding your horse.