Summer sores, or cutaneous habronemiasis, have traditionally been a condition that occurs in horses in tropical or sub-tropical parts of the world. In the United States they occur mostly in more temperate areas like Florida and the Southeast.
However, as the summers appear to be getting warmer and longer in the Midwest summer sores, called "jack sores" by some, may become an issue.
A UCC Davis For Equine Health article "Summer Sores" by Amy Young and Dr. Stephen White, published April 14, 2022, suggests that climate change may cause an increase in the range and active season of biting flies (horse flies and stable flies), which serve as the host for screwworms (the parasite that causes the sores).
Posts by Horse Review on Facebook have also been warning horse owners that the range is increasing.
Summer sores can be traced to any of the three species of Habronema worms that parasitize the horse. Adult worms live in the stomach where they typically cause little reaction. Females lay eggs and the larvae are passed in the feces where they are ingested by the maggots of either the housefly or the stable fly, which serve as intermediate hosts. The normal life cycle is complete when flies deposit the infective larvae around the horse's lips, where they are subsequently swallowed and the larvae develop into adults in the stomach.
Summer sores, however, develop when the larvae are deposited in previously damaged skin or mucous membranes such as the lips, nostrils or genitalia. Once in these areas they cannot complete the life cycle and instead cause a local inflammatory reaction. The lesions themselves consist of areas of ulceration that usually contain small, gritty, yellow nodules (sulfur granules).
Clinical signs of summer sores range from mild to severe, and include skin lesions that do not heal, itching and possibly the formation of proud flesh. The area around the lesion may have a "greasy" appearance. Blood-tinged fluid might also drain from the lesions. The lesions may contain yellow or white calcified material that looks like grains of rice.
Vet attention is needed to completely treat summer sores, and getting the lesions to clear up can be difficult.
The sore must be cleaned "by debriding" and then treated with Ivermectin and a topical cortisteriod. The wound should be covered, if possible, and it is important to follow stringent fly management practices.
Lesions may resolve on their own during cool weather, or with reduced fly activity. Unfortunately, once a horse has developed a summer sore, even if it heals, they are likely to re-occur.
The best way to prevent summer sores is to practice good fly control. This includes keeping stalls and dry lots clean and free from urine and manure. Using protective fly gear for horses—including sheets, leg boots, and face masks, also help as does using fly repellent. Another fly deterrent is to keep a horse clean by regular grooming and bathing.