Watch for Summer Sores in Horses

Many horse owners and veterinarians have reported a bad fly season this year and, for those residing in Southern states, there’s no end in sight. Flies are aggravating and distressing and bring along an inherent risk for summer sores.

“Summer sores are the product of a stomach worm, or Habronema (muscae) infection,” explained Nathan Voris, DVM, senior equine technical Services veterinarian for Zoetis. “As part of the Habronema life cycle, infective larvae are shed in horse feces and later picked up by houseflies. Infected flies deposit Habronema larva in open wounds, or into tissues around the horse’s mouth, lips, eyes, groin area damaged during feeding activities. The infection causes a significant inflammatory reaction, or summer sore. If Habronema and houseflies are both present, summer sores can be a problem for any horse.”

Persistent summer sores are extremely bothersome and slow to heal and, in many cases, remove the horse from training or showing. It’s a situation all too familiar to Carolyn Hornbeck of Raeford, N.C., and her 10-year-old American Quarter Horse gelding.

“He was a healthy horse that I’ve had for several years,” Hornbeck said. “It started out as a small sore on his ankle and, in no time at all, grew into a large wound comparable to the size of a giant filet (of steak) that would not heal. After a costly surgical procedure and eight months of (treatment), it scarred down to the size of a quarter and he was able to go back to riding.”

It was a mystery to Hornbeck why her horse had encountered such a big problem. Like most horse owners, she was doing everything in her power to ensure he had proper care. Nearly four years after that awful summer sore experience, the problem returned, this time for a different horse. Hornbeck soon realized the importance of treatment and control of Habronema and the critical need to disrupt the parasite’s life cycle.

“Horse owners naturally tend to focus on treating the horse with the summer sore infection, but infective Habronema larvae are likely being shed by other outwardly healthy horses in the same environment,” Voris said. “Aggressively controlling the Habronema stomach worm in every horse on the property is extremely important to prevent summer sores. Working with your veterinarian to develop a properly managed parasite-control program that includes effective fly control and dewormer, along with adequate sanitation, is the best way to protect horses from future infections.”

Voris recommends using moxidectin to treat and control Habronema to reduce the summer sore risk. Also, including feed-through products containing 2.12% cyromazine as part of a daily grain ration could help reduce fly burdens, he said.

“Cyromazine, an insect growth regulator, interrupts the life cycle of Habronema larvae by inhibiting development of houseflies with 100% efficacy in freshly treated manure,” Voris explained. “When fly larvae develop in manure infected with Habronema larvae, any horse within the fly’s quarter-mile migration radius becomes at risk for infection. But (cyroazine products) help to stop the vicious cycle. It is a valuable addition to any equine fly management program to increase comfort and decrease the spread of disease caused by flies.”

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