Potomac Horse Fever – Know the Signs of this Potentially Deadly Disease

You notice your horse isn’t eating, or maybe it’s running a fever. Those are just some signs of what could be Potomac horse fever (PHF)–a potentially deadly disease.

“PHF is a serious disease, especially this time of year, and if you haven’t vaccinated your horse for it, it’s important to understand the causes and signs as well as how to diagnose it,” says Hoyt Cheramie, DVM, equine specialist, Merial Large Animal Veterinary Services.

He offers these pointers:

PHF is a disease caused by the bacteria Neorickettsia risticii.2 The bacteria can be found in flukes (flatworms) that develop in aquatic snails and are released into bodies of water.2 Aquatic insects such as damselflies, caddisflies and mayflies might pick up infected immature flukes, and horses grazing near freshwater creeks, rivers or on irrigated pasture can ingest the insects carrying PHF.2


In the beginning, the signs of PHF may be subtle. The horse may have:

  • loss of appetite
  • fever
  • depression
  • decreased intestinal sounds
  • diarrhea
  • mild colic2

Any combination, though not necessarily all, of the signs may be present.2 Some horses may develop laminitis, and pregnant mares are at risk for abortion.2 Other complications include toxins in the blood, decreased intestinal motility, more severe colic signs and dehydration.2

Polymerase chain reaction (PCR) tests are considered the most sensitive tests for PHF, as they look for and identify the bacteria DNA in the white blood cells or manure.2 Because other diseases can have similar clinical signs as PHF (such as colitis, primary colic or peritonitis), veterinarians recommend testing for other potential causes of illness in addition to PHF.2

“Ultimately, vaccinating with POTOMAVAC can offer protection against PHF,” said Cheramie.

The goal of an immunization program should be to time vaccination so the horse has the highest level of immunity before and during highest exposure to disease. For PHF, this is just before peak insect hatch–usually mid- to late summer into early fall.

In one trial, 86% of horses vaccinated with POTOMAVAC were protected from disease. On the other hand, 90% of unvaccinated horses became sick with PHF and 20% died.3 POTOMAVAC is safe for horses as young as three months of age.4

Outbreak Alerts
You can stay informed about certain outbreaks by signing up to receive free notification emails or texts when they are confirmed in specific areas. Outbreak Alert, a free online resource created by Merial, tracks confirmed cases of reportable diseases.

Always consult your equine veterinarian if you suspect any disease, and ask about using POTOMAVAC to help protect your horse against PHF.

Merial is a world-leading, innovation-driven animal health company, providing a comprehensive range of products to enhance the health and well-being of a wide range of animals. Merial employs 6,100 people and operates in more than 150 countries worldwide with over €2 billion of sales in 2014. Merial is a Sanofi company. For more information, please see

POTOMAVAC is a trademark of Merial, Inc. ©Merial, Inc., Duluth, GA. All rights reserved. EQUIBPM1502 (09/15)

1. Based on Market Dynamics, Inc. AHS study data for period from Q12004 through Q42013. Ranking represents cumulative dollar sales volume over the period.
2. Potomac Horse Fever. Merck Veterinary Manual. Accessed October 2, 2015 from
3. Data on file at Merial. Research Report ER 8-88-2.
4. POTOMAVAC product label.






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