Scientific name: Habronema muscae
Common name: Large-Mouth Stomach Worms

Description: Large-Mouth Stomach Worms are white and usually grow to about 1 in. (2.5 cm) in length. They are quite common and can affect horses externally as well as internally. Acute infestations can result in gastritis and large, tumor-like growths in the stomach that can rupture and cause internal bleeding.

Life cycle: Large-Mouth Stomach Worms have an indirect life cycle and require an intermediate host. House and stable flies fulfill the role of host and are responsible for transmitting the disease to horses. Stomach worm larvae are ingested by fly maggots, which become adult flies in about three weeks. Flies deposit the infectious larvae in skin lesions and other wounds on the horse. If the horse subsequently licks the wounds and ingests the larvae, worms will grow inside the stomach. If larvae remain in the wound, they create oozing, expanding wounds frequently referred to as “summer sores.” These sores can become chronic, non-healing wounds that can easily become infected. In addition to depositing larvae in wounds, flies will also drop the larvae on lips, nostrils, eyes and other naturally moist areas of the horse. Larvae deposited around the eyes can cause conjunctivitis. Female worms that have matured in the stomach lay eggs that are passed with manure.

Symptoms: Diarrhea and sudden weight loss often mark the presence of internal stomach worms. Summer sores, or previously clean wounds that become enlarged and covered with a reddish-yellow tissue, usually indicate larval deposits under the skin. These sores become very sensitive and may prompt the horse to relieve the itching by biting or scratching. Inflamed eyelids or conjunctivitis can be a symptom of infectious larvae near the eyes.

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