Potomac Horse Fever 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 various other geographic locations in the United States and Canada. The disease is seasonal, occurring between late spring and early fall in temperate areas.

Clinical signs are variable but may include fever, mild to severe diarrhea, laminitis, mild colic, and decreased abdominal sounds. Pregnant mares may abort infected fetuses.  Mortality rates have been reported between 5% and 30%.

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. Vaccination may not be fully protective against infection. Proposed explanations for this include lack of seroconversion and multiple field strains whereas only one strain is present in available vaccines.


The currently available commercial vaccine is a killed, adjuvanted product, which is also available combined with rabies vaccine. The current vaccine is labelled as an aid in the prevention of Potomac Horse Fever and is not labeled for 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.

Unvaccinated adult horses: Administer two doses with a 3 to 4-week interval between doses. However, veterinarians may consider an interval of 3 to 4 months for horses in endemic areas because protection following vaccination can be incomplete and short-lived.

Vaccinated Adult horses:  Annual revaccination.

Horses in endemic areas: Veterinarians should consider yearly revaccination of two doses with a 3 to 4-month interval between doses.  The first dose should be administered in the spring, prior to anticipated challenge.

Unvaccinated pregnant mares:  Administer two doses with a 3 to 4-week interval between doses. Schedule 1 dose to be administered 4 to 6 weeks before foaling. Note: Current Product label does not include administration information for pregnant mares.

Vaccinated pregnant mares: Annual revaccination with a minimum of one dose. Schedule 1 dose to be administered 4 to 6 weeks before foaling.

Foals: Administer two doses with a 3 to 4-week interval between doses. The vaccine is labelled for use in foals 3 months of age and older.  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. 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 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) 12 months following recovery from natural infection.

Reviewed and revised 2019

Resource Type

  • Guideline


  • Vaccinations

Publish Date

April 19, 2024