Potomac Horse Fever

Learn more about this disease and the methods of prevention.

From The American Association of Equine Practitioners

Equine monocytic ehrlichiosis is caused by Neorickettsia risticii (formerly Ehrlichia risticii). Originally described in 1979 as a sporadic disease affecting horses residing in the eastern United States near the Potomac River, the disease has since been identified in other geographic locations in the United States and Canada. The disease is seasonal, occurring between late spring and early fall in temperate areas, with most cases in July, August and September at the onset of hot weather.

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Clinical signs vary but can include: fever, mild to severe diarrhea, laminitis, mild colic and decreased abdominal sounds. Uncommonly, pregnant mares infected with N. risticii (usually in the middle trimester between 90 and 120 days) can abort due to fetal infection at seven months of gestation.

If Potomac horse fever has been confirmed on a farm or in a particular geographic area, it is likely that additional cases will occur in future years. Foals appear to have a low risk of contracting the disease. The currently available commercial vaccines are killed, adjuvanted products. Two of these are also available combined with a rabies vaccine. None of the current vaccines carry a label claim for the prevention of abortion.

Vaccination Schedules

Due to the seasonal incidence of disease, vaccination should be timed to precede the anticipated peak challenge during the summer months or fall.

Adult horses, previously vaccinated: Manufacturers recommend revaccination at six- to 12-month intervals. However, veterinarians may consider an interval of three to four months for horses in endemic areas because protection following vaccination can be incomplete and short-lived.

Adult horses, previously unvaccinated or with unknown history: Administer a primary series of two doses at a three- to four-week interval. Peak protection occurs three to four weeks after the second dose.

Pregnant mares previously vaccinated against PHF: Vaccinate semiannually to annually. Schedule one dose to be administered four to six weeks before foaling. To date, no studies have been published that examine the efficacy of PHF vaccines to prevent N. risticii-induced abortion.

Pregnant mares unvaccinated or with unknown history: Administer a primary series of two doses, at a three- to four-week interval. Schedule the second dose four to six weeks before foaling.

Foals: Due to the low risk of clinical disease in young foals and the possible maternal antibody interference, primary immunization for most foals can begin after 5 months of age. The manufacturer’s recommendation is for a two-dose series administered at a three- to four-week interval. However, as with other killed products, a third dose at 12 months of age is recommended. If the primary series is initiated when foals are less than 5 months of age, additional doses should be administered at monthly intervals up to 6 months of age to ensure that an immunologic response is achieved.

Horses having been naturally infected and recovered: Administer a primary series (as described above) or booster vaccine (if previously vaccinated) 12 months following recovery from natural infection.

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