Potomac Horse Fever Vaccination Options for Horses

Only about 20% of horses infected with the bacterium Neorickettsia risticii ever develop clinical signs of Potomac horse fever (PHF), but those that do can suffer from fever, colic, diarrhea, and laminitis or even die. A simple, mysterious fever often shows up first, making the disease tricky to pinpoint, so two practitioners shared some diagnostic and prevention tips.

“In areas where Potomac horse fever is endemic (found regularly) and (develops) during the endemic season, I would most likely treat the horse with (the antibiotic) oxytetracycline, if there were no other causes for the fever found following a complete clinical exam,” said Tom Divers, DVM, Dipl. ACVIM, ACVECC, the Rudolph J. and Katharine L. Steffan professor of veterinary medicine at Cornell University, in Ithaca, New York, during a Partner Sunrise Session on PHF at last year’s American Association of Equine Practitioners Convention, held in San Francisco, California. “Early antibiotic treatment would likely decrease the seriousness of the disease.

“Whenever possible, it would then be appropriate to confirm the diagnosis using a combination of both blood and fecal polymerase chain reaction tests (which detect microorganism DNA), as well as saving serum in case sequential titers (which the vet uses to monitor the number of antibodies against a pathogen over time) are required,” he added.

Divers said that because fevers of unknown origin have many possible causes, including a variety of tick-borne agents, it can be very challenging to chase down the fever’s underlying cause in many cases.

Vaccinating Against PHF

One way to possibly protect at-risk horses against PHF is with vaccination.

“Efficacy of the vaccine is unproven, but the production of vaccine antibody would likely confer some protection,” Divers said.

Some of the veterinarian attendees queried whether it’s worth the expense of vaccinating in geographic areas with a low PHF incidence. In such cases on-farm control measures that potentially decrease horse exposure to aquatic insects (which can transport the infectious agent) include turning barn lights off at night or preventing horses from grazing wet areas that might serve as sites for aquatic insect concentrations.

Divers stressed that PHF is a very important disease in endemic areas because of the   seriousness and expense of treating horses with acute infectious diarrhea and the relatively high mortality rate in horses that develop secondary laminitis.

Harold C. McKenzie III, DVM, Dipl. ACVIM, equine section chief at Virginia Tech’s Virginia-Maryland College of Veterinary Medicine, in Blacksburg, Virginia, then provided data-driven recommendations for vaccinating horses against PHF in endemic areas.

The two commercially available vaccines include a killed monovalent vaccine (designed to protect against one pathogen) for PHF alone or one combined with rabies. He encouraged practitioners to remind clients that the available vaccines only provide a significant serologic response for about three months post-vaccination.

“Based on the current data regarding seasonality of the disease, I recommend vaccinating horses in the spring to catch those late spring and early summer cases and again in August to provide protection for the remainder of the season,” McKenzie said. He noted that this can be challenging, because it does not align well with most standard vaccination schedules, which typically include only spring and fall vaccinations.

Additionally, McKenzie presented a recent study in which researchers found neither the monovalent nor combination vaccine induced strong immunogenicity (able to produce an immune response) as expected; however, the monovalent vaccine was significantly more immunogenic.  The monovalent vaccine induced a significant serologic response in more horses—and to a greater degree—than the combination vaccine.

“While no data is available regarding the relative efficacy of the two vaccine formulations, these data suggest that the rabies and PHF monovalent vaccines should be used, administering them at separate injection sites,” McKenzie said.