Understanding The Season Of Summer Sores In Horses

Bay horse eating grass in summer yellow field

With the notorious summer heat creeping back in, horse owners may begin to worry about the sores their equine friends can develop during the scorching season. 

Dr. Dustin Major, a clinical assistant professor of large animal surgery at the Texas A&M School of Veterinary Medicine & Biomedical Sciences, offers insight for horse owners on summer sores and the potential parasitic harm they can bring to horses during the warmer months.

The Site Of The Crime

Cutaneous habronemiasis, or summer sores, are the result of a parasitic infection that occurs when the life cycle of stomach worms — such as Habronema muscae, Habronema major, or Draschia megastoma — is disrupted. 

In the typical life cycle, these worms harmlessly pass through the horse’s digestive tract, and their eggs are excreted by the horse. After hatching into larvae, they are ingested by maggots within the manure. Once the maggot has matured into a fly, the parasite then uses the fly as a means of transportation, continuing the cycle as the larvae are dropped off into the mouth of the horse, making their way into the stomach where their life initially began.

Where things go awry is when the fly carrying the parasitic larvae is drawn to a moist membrane such as the eyes or an open wound; if a larva is dropped in one of these vulnerable areas, it can burrow into the horse in search of their intended home — the horse’s stomach — leaving the horse with a painful, itchy sore.

“It’s important to understand that horses that develop summer sores do so because they have an increased inflammatory response to these larvae, so horses that are prone to making summer sores are more likely to experience this reaction,” Major explained.

What To Look For

Sores that appear on a horse’s eye, mouth corners, lips, sheath, urethra, or open wound are easily noticeable and unable to heal without veterinary care. 

Major said it is easy to mistake summer sores for other infections, but they have one distinction — yellow spots known as sulfur granules. Sulfur granules appear as small, yellow, and hard particles within the granulation tissue of the ulcerated area. 

“These wounds typically appear as round-to-irregular ulcerative sores with unhealthy-looking granulation tissue within them,” Major said. “They tend to be mildly painful and can be extremely itchy, to the point that some horses will self-traumatize by rubbing the sore.”

Major encourages owners to contact their veterinarian as soon as a sore appears to help determine if it is a summer sore or a symptom of a separate infection.

“Here in the Southern U.S., another rule-out for lesions like this is pythiosis — more commonly known as Swamp Cancer — which is an infection involving an aggressive fungal-like organism that has a poor prognosis,” Major explained. “A biopsy of the lesion is often indicated to rule out this disease as well as cancers such as squamous cell carcinoma or sarcoids.”

Prevention And Treatment

Summer sore prevention begins at the core of the issue — flies. 

“Fly control is vital in the summer and key to preventing summer sores,” Major said. “This can include barn sprayer systems, fly predators, feed-through products, picking feces out of stalls and paddocks, and using fly masks and fly sheets.

“Additionally, horses that are susceptible to the development of summer sores may benefit from additional deworming with an avermectin product (e.g., ivermectin) during fly season to limit the larvae within the horse and environment,” he said. 

Treatment typically includes the prescription of a steroid and triple antibiotic ointment for topical application, which can be used on early lesions. 

“However, debridement, or removal of the infected tissue, is often necessary, in which case veterinary intervention would be indicated,” Major explained. 

Major also said it is crucial to keep your veterinarian updated on the horse’s recovery, or lack thereof, to avoid prolonged treatment and further debilitation of the horse’s health.

“When in doubt, it’s much better to have the vet out early, because more complicated lesions can be difficult to clear up and often require multiple debridements, if not more significant surgical intervention,” Major added.

With ample knowledge and the ability to correctly identify summer sores, horses can be treated at home when the festering sore is spotted early on by their vigilant owner. Ensuring your horse remains free of summer sores this season is key to a healthy and happy summer for all.

Pet Talk is a service of the School of Veterinary Medicine & Biomedical Sciences, Texas A&M University. Stories can be viewed on the web at vetmed.tamu.edu/news/pet-talk. Suggestions for future topics may be directed to vmbs-editor@tamu.edu.

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