Potomac Horse Fever

Last Updated on February 19, 2022 by Allison Price

Neorickettsia is responsible for Potomac Horse Fever (formerly Ehrlichia). The disease was first described as a sporadic infection that affected horses living in the eastern United States, near the Potomac River. It has been recognized in other locations across the United States and Canada. It is seasonal and occurs in temperate regions between late spring to early fall.

The clinical signs can vary but include mild to severe diarrhea, mild colic and laminitis. Infected fetuses can be aborted by pregnant mares. There have been reports of mortality rates between 5% to 30%.

Potomac Horse Fever can be confirmed at a farm, or in an area. It is possible that more cases will occur in the future. The risk of infection in foals is low. The vaccine may not protect against all forms of infection. This could be due to a lack of seroconversion or multiple field strains, whereas only one strain is found in vaccines.

Potomac Horse Fever


A killed, adjuvanted vaccine is currently available. It can also be combined with rabies vaccine. The vaccine currently available is labeled as an aid to the prevention of Potomac Horse Fever, but is not intended for abortion prevention.

Vaccination Schedules:

Because of the seasonality of diseases, it is important to get vaccinations in the fall or summer before the peak challenge.

Unvaccinated adult horses – Administer two doses, with a 3- to 4-week break between them. For horses living in endemic areas, veterinarians might recommend a 3- to 4-month interval. This is because vaccinations can cause temporary and incomplete protection.

Vaccinated Adult horses: Annual revaccination.

Horses living in endemic areas should be revaccinated every year with two doses, with a 3- to 4-month break between them. Prior to any anticipated challenge, the first dose should be given in spring.

Unvaccinated pregnant horses: Give two doses, with a 3- to 4-week break between them. One dose should be given between 4 and 6 weeks before foaling. Not applicable to pregnant mares.

Vaccinated pregnant horses: An annual revaccination, with a minimum dose of one. To be given 4 to 6 weeks prior to foaling, schedule 1 dose.

Foals – Two doses should be administered with a 3- to 4-week break between each dose. Foals three months and older can use the vaccine. Primary immunization can be started after five months for most foals due to the low risk of developing clinical disease in foals under 5 years of age and possible interference from maternal antibodies. It is recommended that a third dose be administered at 12 months. To ensure an immune response, additional doses of the primary series should be given to foals younger than 5 months.

Horses that have been naturally infected, and who have recovered: After recovery from natural infection, administer a primary series (12 months).

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!