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Written by Dr. Tom Lenz on behalf of AQHA

 

One condition that many people do not associate with flies is habronemiasis, also called summer sores, granular dermatitis, jack sores and any number of other names. Summer sores result from a complex association between the horse, the stomach worm and its intermediate hosts, house, face and stable flies. Although stomach worms can cause inflammation in the lining of the horse’s stomach, their greatest threat is when they invade fresh wounds or moist areas on the horse’s body.

Typical signs include nonhealing skin lesions, intense itching and the formation of exuberant granulation tissue (proud flesh). The most common sites for summer sores are anywhere that a wound occurs. But they can develop in moist areas of the body like the prepuce, lower abdomen, corners of the eyes or margins of the lips because those are areas where flies commonly feed.

In the normal stomach worm life cycle, flies pick up the stomach worm larvae in horse manure, old bedding, rotten feed, etc., and deposit them near the horse’s mouth. The horse ingests the larvae that travel to the stomach and, in approximately two months, mature into adult worms that usually cause very little damage to the horse. The adults lay eggs that are passed in the horse’s manure. Flies pick up the hatched larvae and cycle starts all over again.

The problem occurs when the stomach worm larvae are deposited by house, stable or face flies that feed on fresh wounds or areas of moisture. The larvae cannot mature into adult worms, so they migrate around in the horse’s wound, causing local inflammation and severe itching. The result is the horse chews on the lesion and proud flesh begins to develop, resulting in a nonhealing lesion that can last for years and gets worse over time.

Summer sores have a “greasy” appearance with blood-tinged fluid draining from them and often contain yellow or white calcified “rice grain-like” material. Summer sores occur most commonly in the spring and summer, coinciding with fly activity. If left untreated, the lesions usually regress during winter months and appear to be healing only to flare up again in the spring.

Treatment of summer sores is often difficult and can require a number of approaches. In small lesions, deworming the horse with either an ivermectin or moxidectin paste dewormer will kill the worm larvae and allow the sore to heal. Dewormers not containing either of these two active ingredients will not be effective.

In more severe cases where a significant amount of proud flesh has formed, it might have to be removed surgically before treatment can be started. Then deworming with one of the products mentioned before should be done in concert with the topical application of a mixture of glucocorticosteroid and DMSO applied directly on the lesion to reduce inflammation and itching. It might also be necessary to wrap the lesion if it occurs on the horse’s legs to protect the wound and prevent the horse from chewing.

Antibiotics and corticosteroids can also be provided orally or injected in the case of severe lesions. Cryotherapy (freezing the lesion with liquid nitrogen) is often beneficial in some cases.

Fly control is essential to prevent additional stomach worm larvae from entering the lesions. Here are a few fly control tips:

  • Remove manure, excess feedstuffs, wet straw and other materials at least twice weekely to prevent fly breeding sites and the hatching of fly larvae (maggots).
  • Compost piles should be properly managed to maximize heat production that will kill hatching fly maggots.
  • Use parasitic wasps as biological control agents for house and stable flies.
  • Insecticides fly traps and baits, residual fly sprays to the premises, fly prevention face masks and repellents are all beneficial.
  • Feed insect growth regulators to your horses. The IGR’s pass through the horse’s intestinal tract and interfere with the growth and development of fly maggots in the horse’s manure.

 

ABOUT THE AUTHOR: Thomas R. Lenz, DVM, M.S., Diplomate of the American College of Theriogenologists, is a trustee of the American Horse Council, past chairman of AQHA’s research committee and past president of the American Association of Equine Practitioners. This article is provided courtesy of AAEP Alliance Partner, AQHA.