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By: Dr. Lydia Gray
“Summer Sores” or “Fly Sores” is a seasonal skin disease in horses referred to by veterinarians as Cutaneous Habronemiasis. It is caused by infection of the skin by the larvae of the large-mouth stomach worm Habronema (and Draschia). Summer Sores are characterized by one or more open and draining nodules and are typically found on the legs, inner corner of the eyes, prepuce (sheath) and penis, as well as moist areas especially where the skin has undergone injury or irritation such as an open wound.
Although the location and appearance of Summer Sores can be distinctive, your veterinarian may still recommend diagnosis in order to rule out other skin conditions which can look similar such as proud flesh, sarcoid, and squamous cell carcinoma. Biopsy is the best method, but deep scrapings of lesions may show actual worm larvae.
Because Summer Sores are thought to be the result of an allergic reaction to the presence of larvae in the skin, treatment is aimed at not only reducing the size of the lesions but also reducing the body’s inflammatory response. Therefore the recommendations may include a dewormer such as ivermectin to target the larval and adult stages of the stomach worm, a potent anti-inflammatory such as a corticosteroid, and an antibiotic to combat secondary bacterial infection.
Summer Sores are the result of an interruption in the normal life cycle of the stomach worm. Instead of Habronema and Draschia larvae passing into the manure, being ingested by fly larvae, then deposited on the horse’s lips to be swallowed which completes the usual cycle, flies deposit the stomach worm larvae on other parts of the horse’s body, leading to a severe local reaction that is often itchy.
In order to help prevent Summer Sores, there are three steps horse owners can take. First, use fly control measures such as fly spray, feed-through fly control, fans, manure removal, and fly predators. Second, take prompt care of any skin wounds which may attract larvae-carrying flies. Third, include ivermectin at least once in their horses’ annual parasite control program.