Summer sores in horses are lesions caused by the parasite Habronema after it lays eggs on damaged skin or mucosa in a horse. The disease is seasonal and typically only seen in warmer months; however, summer sores can cause horses pain and can be challenging for veterinarians to treat.

Veterinarians might choose to biopsy lesions to rule out other causes and guide treatment protocol. Researchers say treatment options vary depending on the severity and location of summer sores. Some spontaneously regress in the winter, whereas others resolve with deworming and/or anti-inflammatory drugs. Surgical debulking might also be necessary in more severe cases.

Treating Summer Sores in Horses

Even though summer sores have the potential to self-resolve, both Dustin Major, DVM, Dipl. ACVS (LA), clinical assistant professor of large animal surgery at Texas A&M University, in College Station, and Nicole Verhaar, DVM, PhD, Dipl. ECVS, senior clinician at the Equine Clinic of the University of Veterinary Medicine Hannover, in Germany, still advocate for some form of treatment.

“I recommend treating cases with clear signs because you do not want to wait until things get out of control,” says Verhaar. “In my opinion, first-line treatment should be deworming and anti-inflammatory medication. Depending on the size and location of the sore and the time of year, surgical debulking needs to be considered if there is concern for the surrounding structures being affected if waited much longer.”

If you and your veterinarian recognize small lesions early, a topical steroid/antibiotic ointment might be an effective treatment option, adds Major. With lesions that occur at the oral commissure (the corners of the mouth where the lips meet), you must apply the ointment frequently because horses often quickly lick it off.

“The problem with just sitting on them (summer sore lesions) until fall is that they’ll start forming in late spring and then fester for months, becoming larger and more pruritic (itchy),” Major explains. For example, he says he has diagnosed lesions on the prepuce (sheath) or tip of the penis that became problematic: “They are common on the urethral process, and these can lead to partial urinary obstruction and discomfort as well as result in bloody urine on the stall floor. I’ve partially amputated multiple penises due to severe habronemiasis.”

In general, summer sore treatment involves:

• Debulking;
• Providing topical and systemic anti-inflammatory drugs;
• Deworming with moxidectin;
• Protecting the wound from flies; and
• Providing environmental fly control.

Treating Summer Sores Around Horses’ Eyes

Verhaar and colleagues recently described how they managed five cases of periocular habronemiasis in a 2018 study. In these cases they identified a granulomatous mass in horses presenting with profuse discharge from the affected eyes, as well as soft tissue swelling and squinting. In each case they also observed sulfur granules on the lesions.

1. They managed one case medically with a dewormer (moxidectin), an antibiotic eye ointment, and daily flushing of the nasolacrimal (tear) duct in the eye with the immunomodulator levamisole. The veterinarians also prescribed a custom eye drop containing levamisole.
2. In the second case researchers completely removed the granulomatous tissue. They dewormed the horse, administered a custom eyedrop with the immunomodulator levamisole, and gave an antibiotic eye ointment. They also administered a steroid injection in this horse’s eye tissue (conjunctiva).
3. The third case also underwent surgical excision (removal) of the mass, followed by treatment with an antibiotic eye ointment.
4. The fourth case had a large granulomatous mass that could only be partially excised. Medical management included treatment with an oral anti-inflammatory drug (meloxicam), deworming, and an injection of a steroid into the conjunctiva.
5. In the fifth case it was not possible to surgically remove the granulomatous mass. The researchers treated the horse with an eye ointment containing an antibiotic and steroid anti-inflammatory. They also dewormed the horse and, in this case, elected to “wait and see” because these lesions reportedly can spontaneously regress in the winter. Indeed, the lesion completely resolved within three months; however, the horse was lost to follow-up, so it is unknown if the lesion recurred. “This was the worst case of habronematidosis I have seen, with one side of the face of the horse completely ulcerated,” says Verhaar.

How to Prevent Summer Sores in Horses

You can minimize all forms of habronematidosis by controlling fly populations in your horse’s environment. “There are all kinds of environmental controls: fly predators, feed-through fly control, fly spray systems for barns, etc. A good fly mask and fly sheet go a long way towards preventing summer sores,” says Major.

Take-Home Message

Our experts recommend having a veterinarian examine even small ulcerated sores before they get too big, especially if they are in areas where surgical excision options are limited, such as the periocular tissues, prepuce, and distal limbs.

“It is so much easier, economical, and faster to treat these early,” says Major. “It’s also important for owners to understand that because this is essentially a hypersensitivity reaction, this will likely become a recurrent problem they will need to deal with every year. A comprehensive fly control system and deworming program based on fecal egg counts is the best way to prevent these from becoming a problem in the first place.”