Summer sores are skin lesions in horses mostly seen during the hot summer months when flies are active. These sores can range from small, red, seemingly innocuous ulcerated lesions around the mouth or legs to large, ulcerated, weeping, cankerlike masses that are not only aesthetically displeasing but also pose a significant health risk to affected horses.

“While most lesions seen in the field are dime- or quarter-sized, I’ve seen some that have softball-sized mounds of granulation tissue on them,” says Dustin Major, DVM, Dipl. ACVS (LA), clinical assistant professor of large animal surgery at Texas A&M University, in College Station. “I’ve also seen sores on the ventral abdomen that are feet—not inches—in diameter.”

How do Summer Sores Form in Horses?

Summer sores, known to practitioners as habronematidosis or habronemiasis, are a parasitic infection caused by specific types of nematodes (roundworms) in the family Habronematidae. In horses, habronematidosis is caused by one of three parasites: Habronema muscae, H. major, and Draschia megastoma. During their normal life cycle, the adult worms live in the horse’s stomach, and eggs and larvae from adult female parasites are shed in the horse’s feces. Once in the environment, maggots of certain species of flies, such as house and stable flies, ingest the nematode larvae. When the maggots mature into flies, the adult flies deposit the infective nematode larvae onto the horse’s lips and nostrils. The horse then ingests those infective larvae, which ultimately reach the stomach and grow into adult worms, perpetuating the cycle.

Summer sores occur when flies deposit the infective larvae elsewhere on the body, typically in moist regions. This includes mucocutaneous junctions around the eyes (periocular) and genital mucosa (e.g., prepuce) or where there are skin wounds, which attract flies.

Clinical Signs of Summer Sores in Horses

When flies deposit parasite larvae in skin wounds on a horse or at mucocutaneous junctions, the larvae do not mature into adult worms like they would in the stomach. Instead, they remain in the skin, causing a local inflammatory, hypersensitivity reaction. Skin lesions can vary in appearance based on the lesion that attracted the flies in the first place.

Mild forms of the disease can be as simple as a single small, circular ulcerated region of skin around the muzzle or where there was a small wound. Worse cases can have a proliferative, exuberant, and granulomatous appearance like proud flesh. These frequently appear as weeping, moist, and even bloody. Not surprisingly, they tend to be itchy, and horses can further traumatize the area by scratching and rubbing. The center of each lesion typically harbors a region of necrotic tissue featuring a caseous (cheesy) substance and hardened granules resembling sulfur.

In some horses the lesions might grow so extensively they become nonhealing granulomatous, cankerlike masses. These lesions can attract more infected flies, potentially leading to super-infections.

“Summer sores can appear similar to a large number of other so-called ‘granulomatous’ skin lesions such as proud flesh, foreign body granulomas and phytomycosis, or even squamous cell carcinoma and equine sarcoids; however, the sulfurlike granules are considered pathognomonic for summer sores,” says Nicole Verhaar, DVM, PhD, Dipl. ECVS, senior clinician at the Equine Clinic of the University of Veterinary Medicine Hannover, in Germany.

Recognizing Summer Sores in Horses

“My rule outs for ulcerative, nonhealing wounds with varying degrees of exuberant and/or unhealthy granulation tissue include simple proud flesh, habronemiasis, squamous cell carcinoma, and pythiosis, more or less in that order,” says Major.

Pythiosis occurs in the southern U.S. and is caused by the funguslike organism Pythium insidiosum. This condition, referred to as “swamp cancer,” causes ulcerative and invasive granulomatous lesions on the skin.

While authors on the scantly available literature suggest that summer sores can be mistaken for sarcoids, Major says “sarcoids can be ulcerative, but it’s rare that I see a lesion that could be a summer sore and then have it turn out to be a sarcoid. The sulfur granules in the wound are kind of a dead giveaway that it’s not just proud flesh or squamous cell carcinoma.”

However, he did note that sulfur granules can also been seen with swamp cancer.

“Here in the South, differentiating between habronemiasis and pythiosis is really important because pythiosis is potentially life-threatening, whereas summer sores are very treatable,” Major adds.

Diagnostic Testing for Summer Sores in Horses

If a veterinarian takes biopsies of summer sores, classic microscopic findings include a high number of eosinophils, which are a specific type of white blood cell often seen with parasitic infections, as well as necrosis. Researchers have also reported that sections of larvae are only occasionally observed.

These larvae live for less than one month in skin tissue, but larval death can cause more tissue necrosis and calcification than a living parasite, says Verhaar.

Major adds, “In my experience, larvae are normally found in biopsied samples; however, I am usually looking at more severe, active lesions than what is typically seen in the field.”

“Small lesions in characteristic locations such as the oral commissures (the corners of the mouth where the lips meet), the canthi of the eye (where top and bottom lids come together), or around the prepuce (sheath) might not warrant biopsy, especially if there are sulfur granules,” notes Verhaar. That said, she still recommends taking biopsies prior to performing surgery in some scenarios.

“A small tissue sample should be taken if the diagnosis is not completely clear and squamous cell carcinoma cannot be ruled out,” she says. “For example, (do a biopsy) if the lesion is not a typical summer sore, if no granules are present, or if the patient is an older horse with a phenotype (observable characteristic) that could also fit having a squamous cell carcinoma. The biopsy results could definitely change the surgical plan and the prognosis.”

Because other conditions that present like summer sores also affect the same parts of the horse, taking a biopsy is one of the best ways for veterinarians to be sure they make the correct diagnosis. Additionally, when wounds on the distal limb become summer sores, it’s important to differentiate them from other pathology because chronic wounds can become metaplastic (converting to another type of cell) and then neoplastic (tumorous), says Major.

To help definitively diagnose summer sores, researchers developed a DNA test (polymerase chain reaction, PCR) several years ago to identify genetic material from each of the three nematode species that cause habronematidosis. Practitioners perform the test using skin samples taken from the lesion. “To the best of my knowledge, the PCR test is not commercially available,” says Verhaar.

Take-Home Message

Summer sore lesions are caused by parasitic infection of the horse’s skin and can be painful. Summer sores can vary greatly in size often appear as ulcerated wounds on the skin causing hypersensitivity and inflammation. If owners suspect their horse might have a summer sore, it is important to contact a veterinarian so they can perform appropriate diagnostics and develop an effective treatment plan.