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Many people do not associate flies with habronemiasis (summer sores), granular dermatitis, or jack sores. Summer sores result from a complex association between the horse, the stomach worm, and its intermediate hosts–house, face, and stable flies. Adult stomach worms shed larvae into the environment through the manure of infected horses. These larvae are then ingested by maggots, where they develop into adult flies. They reside in the mouthparts of the flies, where they are deposited on the wounds or mucocutaneous junctions of horses, causing an infection.
The classic presentations of summer sores are one or more open, draining, ulcerative nodules with swelling, redness, and itching.Generally located on the legs, inner corners of the eyes, lips, sheath, penis, and any moist area or where skin has undergone injury or irritation, such as an open wound.
Some horses are shedders, but even a wormed horse can get summer sores. This is because the larvae that cause the massive inflammatory reaction are actually already dead. It is the body’s immune response to the dead larvae that causes the intense itchiness and summer sore formation.
The key to therapy is early aggressive treatment! De-bulking is often necessary to remove the dead larvae from the sores. After de-bulking edges of the wound, some need to be injected with corticosteroids subcutaneously. Usually, the infection is aggressive, and therapy has to be repeated. The area needs to be covered at all times when possible with a medicated ointment. An oral de-worming product, preferably an avermectin product, needs to be administered once a week for 4 weeks. Fecal counts do NOT detect Habronema larvae.
Prevention is the most effective way of controlling summer sore outbreaks. Fly control is an important step. There are products on the market that are an oral feed additive that acts as an insect growth regulator containing cyromazine. This prevents the larvae from developing into adults. Remove manure and excess feedstuffs, twice weekly to prevent fly breeding sites and the hatching of fly larvae. Insecticides, fly traps and baits, residual fly sprays, fly prevention face masks, and repellants are all beneficial.
If you are suspicious your horse may have summer sores, contact your veterinarian as soon as possible to hopefully stop a small problem from becoming a large problem.