With summer just around the corner, beware of stagnant ponds and insects that may transmit Potomac horse fever (PHF). Although named after the river near Washington, D.C., cases have been found across most of the United States. Keep an eye out for signs of this deadly disease. Those include:

1. Fever
2. Colic
3. Diarrhea
4. Above signs, followed by laminitis
5. Abortion in pregnant mares

The typical infection in horses is as follows: 2 to 4 days after infection, there is a mild fever that can go undetected.  At 10 to 14 days, there is depression, fever, loss of interest in food, and a range of gastrointestinal signs from mild colic with or without soft manure, to severe and watery diarrhea.  About 25 to 30% of cases will also develop laminitis, an inflammation of the laminae in the feet.  Some of these cases can progress to founder, says Dr. Stephen Angelos of Large Animal Medical Associates, in Essex Junction, Vermont.

“Every veterinarian and horse owner worries about laminitis and colic. These could be classic signs of PHF in the right context,” says Frank Hurtig, DVM, MBA, Director, Veterinary Services for Merial, which makes the POTOMAVAC vaccine. “Timely vaccination of horses in the area can help prevent against additional cases of PHF.”

According to Merial, up to 30 percent of PHF-infected horses die, and laminitis may 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, Dr. Hurtig warns.

PHF has a complex life cycle in which the bacteria 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.

Proper pasture management and vaccination are the best defenses against Potomac Horse Fever. Early diagnosis and quick treatment will help the disease from progressing if your horse does contract it.