This is the time of year (late summer and early fall) when we see a lot of "summer sores" around horses' eyes, on their lips, sheath, or developing wherever a horse has cuts or abrasions. They typically start out looking pretty innocuous, but will later become large, open sores.
Cutaneous Habronemiasis, commonly known as Summer Sores, is an inflammatory reaction in the skin caused by the presence of larvae from equine stomach worms. The adult worm lives in the stomach and usually does not cause clinical problems for horses, but as eggs are shed in the feces they can be picked up by flies. The egg grows into a larvae within the fly and is deposited on moist areas of the horses skin when the fly lands. Some horses will have a severe reaction to the presence of these larvae, and the resulting inflammation creates the summer sore.
How to Recognize a Summer Sore
The classic summer sore is usually a firm, sometimes round reaction in the skin that produces granulation tissue (proud flesh) and will often contain hard, small, yellow granules. The most common location for summer sores is the corners of the eyes, the lips, the external genitalia of geldings and stallions and anywhere an open wound is present, especially on the legs and around the heel bulbs and coronary band. The sores can be very small in the early stages, or can create very large proud flesh reactions.
How to Treat Summer Sores
The primary goals of treatment are to reduce inflammation in the skin to allow it to heal, and to stop the life cycle of the Habronema stomach worms. Habronema is susceptible to ivermectin, so in areas where summer sores are present it is best to administer ivermectin to all the horses in that area. A repeat administration of ivermectin in one month is ideal, as the flies that carry the larvae can remain in the environment for several weeks.
To treat the skin lesions, any granules and proud flesh should be debrided and removed, and a topical lotion (available from your veterinarian) to kill the larvae and reduce inflammation should be applied. Fly control is paramount, as flies deposit the inflammatory larvae into the wounds. SWATT fly repellent ointment works well for wounds, and for lesions near the eyes a fly mask should be worn. For horses with very large lesions that do not seem to heal, systemic steroids may be warranted.
If you see lesions like these on your horse, please call your Veterinarian for evaluation and treatment.