You may have heard the term summer sores before but may not know much about these lesions. Summer sores, technically called habronemiasis, are lesions on the skin or around the eyes. They can be caused by any one of three parasite larvae transmitted by house and stable flies. Larvae are deposited by flies on the skin, and larvae causes ulcerated and bleeding non-healing lesions. These lesions contain small yellow granules that generally make the horse itchy. A biopsy is required for the diagnosis because sarcoids and squamous cell carcinoma lesions appear similar to summer sores.

Treatment of habronemiasis involves surgical removal of the lesion, if possible, or at least decreasing the size of the lesion with surgery. Cortisone has been effective in many cases of habronemiasis as the condition is believed to be a hypersensitivity reaction to the parasite. The cortisone treatment can be given in the feed or orally, injected directly into the lesions, or used topically on the lesion’s surface. Many of the topical medications also contain antibiotics as well as medciation to kill the larvae. All horses should also be treated with ivermectin orally as it has been shown to kill the larvae as well as adult worms of the parasites in the horse’s stomach. Ivermectin is also one the products that is routinely used for deworming and this is one reason we see many less summer sore cases today than 40 years ago as when deworming for regular parasites, you are also killing some larvae that lead to summer sores before a problem develops. Another key to decreasing summer sores in horses is fly control, which is very important in treating summer sores.