Summer sores are caused by house flies, face flies and stable flies, as they transfer parasitic nematode larvae (Habronema species) to moist areas around a horse’s open wounds, eyes, nostrils, mouth and genitalia. When the larvae create a hypersensitivity reaction, chronic, fleshy and non-healing wounds can result, known as summer sores. The condition is costly and can require months off from riding and training as the infected horse heals.