Vaccination, Early Recognition Key with Potomac Horse Fever

The name is deceiving, but Potomac horse fever (PHF) is not just a concern for horses near the Potomac River. Cases have been found across most of the United States.1 Knowing the key signs of PHF and watching carefully for them can help alert owners to protect healthy horses that might be at risk.

"Every veterinarian and horse owner worries about laminitis and colic. These could be classic signs of PHF in the right context," said Frank Hurtig, DVM, MBA, director of veterinary services with Merial. "Recognizing the signs of disease offers a cue for quick PHF testing, and examination for other disease concerns. Timely vaccination of horses in the area can help prevent against additional cases of PHF."

Some of the most commonly occurring signs of PHF are:1

  • Fever;
  • Colic;
  • Diarrhea;
  • Above signs, followed by laminitis;
  • Abortion in pregnant mares.

Up to 30% of PHF-infected horses die,1 and laminitis can continue even after other signs have stopped. If a PHF case is located, all the horses in the area may be at risk for infection, Hurtig warned.

PHF commonly occurs near bodies of water during mid- to late-summer.1 However, it has been found in nontraditional areas as remote as northern Wyoming.2 PHF has a complex life cycle in which Neorickettsia risticii infect freshwater snails containing flukes. Then, N. risticii is transferred to aquatic insects such as caddisflies and mayflies. Horses become infected after eating grass, feed or water containing these insects.1

Utilizng management practices such as draining standing water can help reduce the risk of PHF transmission.1 Hurtig recommends vaccinating horses to help prevent against the most severe complications of the disease.

"Vaccination seems like a simple investment when there is a possibility of colic, laminitis, or even death," Hurtig said. "In recent years, PHF cases appeared in several nontraditional states. That makes it a real concern for horses in all areas of the country."

In an efficacy trial, the Potomavac vaccine from Merial protected 86% of horses from clinical disease.3 Potomavac is proven safe for horses as young as three months and is demonstrated safe for pregnant mares. It is also available in combination with Imrab to help protect against both PHF and rabies.

For more information, see

1Madigan J and Pusterla N. Life Cycle of Potomac Horse Fever – Implications for Diagnosis, Treatment, and Control: A Review. 2005 AAEP Proceedings 51:158-162.

2Hamende V. Potomac horse fever cases confirmed in northern Wyoming. University of Wyoming Cooperative Extension Service. Press Release, Sept. 13, 2002. Available at Accessed Feb. 18, 2008.

3Merial Trial ER 8-88-2.

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