Habronemiasis is a parasitic disease of horses and other equines that is caused by the invasion of Habronema or Draschia worms into the skin or eye.

The worm larvae are transmitted by house flies, face flies, and stable flies. Summer sores are chronic lesions that develop from the Habronema worm larvae in the skin.

Larvae are ingested by the horse and develop into adults in the horse's stomach. The adults lay eggs that are hatched into the horse's manure. Flies pick these larvae up on their mouth parts. The larvae develop in the flies to a point that they can again infect the horse. Then the flies introduce these infective larvae to the horse's skin.

The lesions are usually red, raised masses with a reddened, ulcerated surface. They can be quite large - baseball sized or larger. They typically develop near wounds, or the eyes, the edges of the mouth, near the external genitalia, and sometimes around the fetlock or coronary band. Yellow granules on the surface of the wound known as "sulfur granules" have nothing to do with sulfur, but relate to the immune system's reaction to the infection.

Definite diagnosis requires microscopic examination of the tissues (biopsy), but in areas where this condition is common, the diagnosis may be assumed by your veterinarian.

The most effective treatment requires a combination of a compound to kill the worms (although there is debate about whether the worms are really still alive in the masses), and steroid treatment to dampen the inflammatory/allergic response, which is in large part responsible for the formation of the masses.