Summer Sores (Cutaneous Habronemiasis)

Summer sores are a common problem that we see in horses during the fly season. These sores are commonly found on the lower limb, corner of the eye, and the urethral process of geldings and stallions. They can also be found on the face neck or any other area of the body that has had an open sore or wound. They are characterized by a non healing sore with small yellow granules, and decayed tissue. They often have a foul fetid smell associated with them. Ocular habronemiasis is characterized by ocular discharge with a large amount of mucous from the medial canthus or corner of the eye. When observed closer there is often times a white or pale yellow plaque in the corner of the eye.

These sores are caused by a parasite called Habronema or the stomach worm. These parasites usually reside in the stomach of the horse, where they cause little tissue reaction. The larvae are passed in the manure where they are then ingested by the larvae of flies acting as intermediate hosts. The third stage larvae are then deposited around the horse’s mouth and swallowed to pass to the stomach where they mature to adults.

Cutaneous lesion or summer sores occur when the larvae are deposited in damaged skin or areas of natural body moisture by flies. The larvae cannot penetrate normal healthy skin. In these locations they are unable to mature to adults and the lesions or sores that result are believed to be a hypersensitivity reaction to the dead and dying larvae.

Treatment includes cleaning out the dead larvae, and dead tissue from the sore. The area should then be treated with an antibiotic and steroid ointment. The horse and all other horses on the premise should be treated with ivermectin or some other type of wormer. Old manure should be cleaned up, and fly control measures instituted.