Summer sores are an oozy, itchy, seasonal skin condition caused by the larvae of an equine stomach worm, typically Habronema. Flies are the intermediate host that make summer sores possible; the condition happens when the stomach worm’s life cycle is disrupted. Let’s look at how it works.
The normal life cycle. The adult Habronema worm lives in a horse’s stomach, where it may not cause too much harm unless there’s a severe infestation. The adult worm lays eggs, which pass out and hatch in the horse’s manure.
House, stable, or face flies then pick up the stomach worm larvae from the manure and carry them to the horse. If a larva is then deposited on the horse in a place (such as near the mouth) where the horse inadvertently ingests it, then that larva returns to the stomach to complete its life cycle and mature into an adult worm.
The disrupted life cycle. Larvae that for whatever reason don’t find their way into a horse’s stomach are what cause the summer-sore problem. These larvae may be deposited wherever flies feed—in open wounds or on abraded skin, or in moist places such as mucous membranes, including around the eyes, lips, and sheath.
When they don’t get ingested, these larvae cannot mature, so they may migrate around in the horse’s tissue instead, causing inflammation and intense itching. The lesions that result often have a greasy appearance or resemble proud flesh, and they resist healing.
Treatments. Strategies involve killing the parasite plus calming down the inflammation so the lesions can heal. Ivermectin and moxidectin (see box) are effective against most stomach worms. For the inflammation, your veterinarian may prescribe topical or systemic glucocorticoids, or a topical mixture of steroids and DMSO.
Excess granulated tissue may also need to be surgically removed or frozen off with liquid nitrogen. Antibiotics may be necessary if a secondary infection has set in.
Without adequate treatment, summer sores may persist or subside only to return in the next warm season.
Prevention. Fly control is paramount. Without flies as the intermediate hosts, stomach-worm larvae can’t find their way into your horse’s tissues.
Keep manure picked up and properly disposed of or composted away from where horses live. Also remove any other material—such as damp, organic trash—that can serve as breeding areas for flies.
Use other fly-control methods as well, such as fly predators that feed on fly larvae in manure, or feed-through larvicides that pass through your horse undigested and prevent fly maggots from developing.
Protective horse clothing (masks, sheets, leg coverings) can be an added defense for horses that seem prone to developing summer sores (some are more susceptible than others).
Also work with your vet to establish a deworming regimen that includes periodic use of ivermectin or moxidectin products while avoiding the overuse that can lead to resistance.
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Whack the Worms
For treatment and control of the stomach worms whose larvae can lead to summer sores, administer a dewormer containing ivermectin or moxidectin (as at right) according to your veterinarian’s advice.