WEST LAFAYETTE, Ind. — Horse owners and equine-housing facilities should be on the lookout for symptoms of Potomac Horse Fever, a bacterial disease that can lead to serious health conditions or death, a Purdue University specialist says.
A horse admitted to the College of Veterinary Medicine Large Animal Hospital in mid-June has been diagnosed with the disease. Cases of Potomac Horse Fever usually don’t show up in Indiana until July or August.
“This early case could mean that the mild winter and spring will lead to more cases of the disease this summer,” said Janice E. Kritchevsky, professor of large animal internal medicine. Most insect populations are running about three weeks early, said Tom Turpin, Purdue Extension entomologist. “It doesn’t surprise me that a disease with an insect vector, or immediate host, is showing up early as well,” he said.
Freshwater snails, caddisflies, mayflies, stoneflies, damselflies and dragonflies are common carriers for Neorickettsia risticcii, the bacterium responsible for Potomac Horse Fever. The insects are attracted by nightlights at barn lots, stables and show arenas and to water troughs at these locations.
When they die, they litter the ground or fall onto feedstuffs or into water. Horses contract the disease by eating contaminated hay, grain, or pasture or drinking contaminated water. Potomac Horse Fever is not contagious. If more than one horse at the same location contracts the disease, it is because of the environmental conditions that draw the vectors.
The disease causes colitis, dehydration and diarrhea. Without treatment, which includes tetracycline-based antibiotics, intravenous fluids and anti-inflammatory agents, the disease can lead to shock, permanent lameness or even death. During the first few days, a horse “just looks off,” Kritchevsky said.
Affected horses usually stop eating, become lethargic and develop a high fever. Then they may develop watery diarrhea and become dehydrated. In severe cases, horses may founder or develop laminitis. The disease may also cause abortion in pregnant mares.
“Horses need to be diagnosed quickly,” Kritchevsky said. “Early treatment increases the likelihood of survival.” Purdue’s Large Animal Hospital treats horses with Potomac Horse Fever every year. When complications such as founder have occurred, horses have had to be euthanized. The horse admitted to the hospital in June recovered and has been discharged. To minimize the risk, Kritchevsky recommends the following:
This protects against only one strain of the more than 60 identified strains of N. risticii. But a study by Purdue’s Sandra Taylor, a researcher in equine infectious diseases, identified factors that might predict survival. Vaccination did not increase the odds of survival, but it may decrease the severity of the disease.
Call your veterinarian if a horse is off its feed, has a fever or develops diarrhea. Unless there is another explanation for the sickness, consider Potomac Horse Fever as a potential cause.
Many of the insects that carry the bacteria die soon after hatching. Their carcasses can fall into open grain bins and onto hay. Cover stored hay with tarps, and make sure grain bins are secured. Keep water buckets and troughs clean and free of debris and dead insects.
Large floodlights can provide security at night, but they also attract many flying insects. Consider turning off outside lights during summer months.
The veterinarians in Purdue’s Large Animal Internal Medicine Department are specialists in Potomac Horse Fever and other equine infectious diseases. Horse owners or veterinarians who have questions can call the hospital at 765-494-8548.