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Equine Health Resource

Vaccination, Potomac Horse Fever



There is disagreement in the veterinary literature about the effectiveness of Potomac Horse Fever vaccines.

Until more is known, a reasonable approach is to use the vaccine in areas where the risk for this disease is higher. The vaccine is fairly safe, and given frequently, might help protect against disease. It is given as an intramuscular injection.

Vaccines work by stimulating the body to produce antibodies against the agent, in this case a Rickettsia. Talk to your vet to determine whether your horse needs this vaccine. If you decide to vaccinate your horses, I recommend that your vet vaccinate your horse because they can ensure that the vaccine is handled and administered properly, and address any complications.

If your horse(s) have been diagnosed with PHF, then you should talk to your vet about your vaccination program.

Related DiagnosesThis Treatment Might Be Used for these Diagnoses

Consider Potential Side Effects & Complications

Any vaccine can cause local swelling and pain. Horses may be depressed and lose their appetite. Anaphylactic reactions are possible and can cause severe depression, colic, diarrhea or hives.

Consider Reasons Not To Use This Treatment

Horses that are ill for any reason should generally not be vaccinated until they have fully recovered.

Horses that are being given systemic steroid medication or other immunosuppressive drugs may not mount an appropriate immune response following vaccination, rendering vaccination ineffective.

Do not use in foals less than 5 months if their dams have been vaccinated.

Is It working? Timeframe for effect

Antibody titers rise weeks after primary series. They rise days after booster doses. In areas in which the Potomac Horse Fever organism is endemic (occurs naturally), vaccination is thought to reduce the severity of infection in horses that do become infected.

Questions To Ask My Vet

  • What is the best time of year to vaccinate my horses, given my geographic location?

Helpful terms & topics in HSVGWritten, Reviewed or Shared by Experts in Equine Health

Merial Equine Potomavac®, Equine Vaccine
Merial Equine Potomavac® + IMRAB®, Equine Vaccine
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Author: Doug Thal DVM Dipl. ABVP


Wilson WD, Pusterla N. Current Concepts in Equine Vaccination & Infectious Disease Control in Smith BP ed., Large Animal Internal Medicine, 5th ed. St. Louis: Elsevier, 2015; 1437.American Association of Equine Practitioners website. Vaccination Guidelines. Available at: Accessed 2013.


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