Summer sores in horses are caused by stomach worm larvae deposited on skin by flies, and treating them requires a combination of killing the larvae, reducing inflammation, protecting the wound from reinfection, and controlling flies. Left untreated, these lesions grow into ugly, ulcerated masses that won’t heal on their own. The good news is that most summer sores respond well to treatment when you address all the contributing factors at once.
What Causes Summer Sores
Summer sores, technically called cutaneous habronemiasis, happen when flies deposit the larvae of stomach worms (species called Habronema muscae, Habronema majus, and Draschia megastoma) onto broken skin, moist areas, or the mucous membranes around a horse’s eyes, lips, or sheath. The three fly species most commonly responsible are house flies, face flies, and stable flies. These flies pick up the larvae from manure, where the worms’ eggs hatch, and carry them to the horse.
When larvae land on normal, intact skin, nothing much happens. But when they’re deposited on a wound, a scratch, or a naturally moist area, they burrow in and trigger a severe hypersensitivity reaction. The horse’s immune system overreacts to the dying larvae, producing angry, granulomatous tissue that ulcerates and refuses to heal. The result is a rapidly growing, raw lesion that itches intensely and often contains small yellow, calcified granules the size of rice grains, sometimes called “kunkers.” These lesions tend to appear in warm months when fly populations peak and can recur year after year in the same spots.
Kill the Larvae First
The foundation of treatment is an oral dewormer to kill the larvae driving the reaction. Ivermectin, given at the standard equine dose of 200 micrograms per kilogram of body weight, is the most widely used option. A single dose of oral paste is typically the starting point, though your vet may recommend a second dose a few weeks later depending on the severity. Moxidectin, another common equine dewormer, works through a similar mechanism and is sometimes used as an alternative.
Killing the larvae doesn’t produce instant results. Because the lesion is driven by an immune overreaction to the parasites rather than by live, feeding organisms, the inflammation persists even after the larvae are dead. That’s why deworming alone is never enough. It stops the source of the problem, but additional treatment is needed to calm the tissue and allow healing.
Reduce Inflammation With Corticosteroids
Because summer sores are fundamentally an allergic reaction, corticosteroids are one of the most effective treatments. They can be applied topically as an ointment directly on the lesion, injected into the lesion itself, or given systemically by mouth, depending on the size and location of the sore. Many veterinarians prescribe a compounded topical preparation that combines a corticosteroid with an antibiotic and a larvicidal ingredient, addressing inflammation, secondary infection, and any remaining larvae all at once.
Topical treatment needs to be applied consistently, often daily, and the wound should be gently cleaned before each application. Expect the lesion to look worse before it looks better in some cases, especially if there’s a lot of granulation tissue. The itching usually improves within the first week of corticosteroid treatment, which is a reliable early sign that things are heading in the right direction.
When Surgery Is Needed
Some summer sores develop so much excessive granulation tissue that topical treatment alone can’t break through. In these cases, surgical debridement or cauterization may be necessary to remove the proud flesh and those characteristic yellow, calcified kunkers embedded in the tissue. This is especially common with lesions around the eyes or on the sheath, where swelling and tissue overgrowth can become severe quickly.
Surgery isn’t a standalone fix. It reduces the bulk of abnormal tissue so that medical treatment can work more effectively. After debridement, the wound still needs ongoing topical care, fly protection, and often another round of deworming. Without those follow-up steps, the sore will almost certainly return.
Protect the Wound From Flies
Treating a summer sore while flies continue landing on it is like bailing water without plugging the hole. Physical barriers are critical. Fly masks protect lesions near the eyes. Leg wraps and bandages shield sores on the lower limbs. Fly sheets cover larger body areas. Strong fans in stalls and run-in sheds help keep flies from landing.
Thick barrier ointments applied around the wound edges can deter flies from the immediate area, though they can’t be used on all locations. Products designed to repel flies from wounds work reasonably well in the short term, but most topical fly sprays lose effectiveness within 10 to 20 minutes, so they shouldn’t be your primary defense. Physical barriers and environmental management are far more reliable.
Fly Control at the Barn Level
Reducing the overall fly population on your property is just as important as protecting the individual wound. Flies breed in manure, so picking stalls and paddocks regularly is the single most impactful thing you can do. In warm, humid climates or high-density boarding facilities, manure management is even more critical because fly populations can explode quickly.
Fly predators, which are tiny parasitic wasps that destroy fly pupae in manure, offer a biological control option that many farms use throughout fly season. Feed-through fly control products work by passing through the horse’s digestive system and preventing fly larvae from developing in the manure. Barn misting systems provide another layer of control, though they work best as part of a broader strategy rather than as a sole solution. The goal is to hit flies at multiple stages of their life cycle: in the manure, in the air, and on the horse.
What Healing Looks Like
Summer sores are notoriously slow to heal. Even with aggressive treatment, expect weeks to months before a lesion fully resolves, depending on its size and location. Early signs of progress include reduced itching, flattening of the granulation tissue, and the wound edges beginning to contract inward. The yellow calcified granules should stop appearing as the larvae are eliminated.
One of the frustrating characteristics of summer sores is seasonal recurrence. A lesion that heals in the fall as fly populations drop may reappear the following spring in the same spot. Horses that have had summer sores once are more susceptible to getting them again, likely because the hypersensitivity response is already primed. This is why year-round deworming protocols, early-season fly control, and prompt attention to any wound during warm months are so important for horses with a history of the condition.
Conditions That Look Similar
Not every non-healing wound on a horse is a summer sore. Equine sarcoids, which are skin tumors caused by a papillomavirus, can look strikingly similar, especially the more aggressive types that form ulcerated, proliferative masses. Proud flesh from a poorly healing wound, fungal infections, and certain cancers can also mimic summer sores. A clinical diagnosis based on appearance alone is considered unreliable, so if a lesion isn’t responding to treatment as expected, a biopsy or skin scraping can help confirm what you’re actually dealing with. The presence of those distinctive rice-grain-sized yellow granules in the tissue is one of the strongest visual clues pointing toward habronemiasis rather than another condition.