Cutaneous Habronemiasis in Mules: Summer Sore Skin Infection Explained

Cutaneous habronemiasis is a parasitic skin disease of equids, including mules, horses, and donkeys. Pet parents may hear it called summer sores, jack sores, or bursati. It happens when larvae from stomach worms in the Habronema or Draschia group are deposited by flies onto a skin wound or a moist body site instead of being swallowed and completing their normal life cycle in the stomach.

Once the larvae enter the skin, they trigger a strong inflammatory reaction. The result is a sore that often looks raw, raised, ulcerated, and stubbornly slow to heal. Many lesions become thickened and develop excessive granulation tissue, so they can resemble proud flesh, infected wounds, or even tumors.

These sores are most common in warm months when flies are active. Typical locations include the corners of the mouth, around the eyes, the lower legs, the sheath or penis, the vulva, and any area with a preexisting scrape, cut, or drainage. In mules, the condition behaves much like it does in horses, so your vet will usually approach diagnosis and treatment using established equine guidance.

The root cause is exposure to Habronema or Draschia larvae carried by flies. Adult worms live in the stomach of equids and pass eggs or larvae in manure. Fly larvae develop in that manure, ingest the parasite, and later carry infective larvae as adult flies. Normally, those larvae are deposited around the lips and swallowed. In cutaneous habronemiasis, the fly instead drops them onto a wound or moist skin surface.

That misplaced larva cannot mature properly in the skin, but it still causes intense local inflammation. The body reacts by forming a granulomatous lesion, often with ulceration, drainage, and excessive granulation tissue. This is why the sore can look dramatic even when the original wound was small.

Risk rises in warm weather, on farms with heavy fly pressure, where manure is not removed often, or when mules have untreated wounds, eye discharge, or moist genital skin that attracts flies. Animals with a history of summer sores may also be more likely to develop them again in future fly seasons.

Your vet usually starts with the lesion’s appearance, location, seasonality, and history. A nonhealing ulcerated wound with proud-flesh-like tissue and yellow "rice-grain" or gritty material during fly season is highly suspicious. Because mules are managed similarly to horses, diagnosis generally follows equine protocols.

In some cases, your vet may collect skin scrapings, impression smears, or a biopsy. Larvae can sometimes be found in lesion scrapings, although they are not always easy to detect. A biopsy may be especially helpful if the sore is unusual, severe, recurrent, or not responding as expected.

Diagnosis also means ruling out look-alikes. Depending on the lesion, your vet may want to exclude exuberant granulation tissue, bacterial or fungal infection, pythiosis, sarcoid, squamous cell carcinoma, foreign material, or ongoing trauma. That is one reason it is smart not to assume every summer wound is a summer sore.

Prevention focuses on fly control, wound protection, and parasite management. Remove manure frequently, compost or manage it away from turnout areas, reduce wet organic debris, and use fly-control tools such as masks, sheets, repellents, and physical barriers. These steps help lower the number of flies that can carry larvae from manure to your mule’s skin.

Check your mule daily during fly season for cuts, rubs, eye discharge, and moist skin folds. Clean new wounds promptly and protect them so flies cannot feed on them. Areas around the eyes, lips, sheath, penis, and vulva deserve extra attention because flies are drawn to moisture.

Work with your vet on a sensible deworming plan rather than treating on guesswork alone. Current equine parasite-control guidance emphasizes strategic deworming and manure management, not constant routine dosing. If your mule has had summer sores before, tell your vet early in the season so you can build a prevention plan that matches your mule, your farm setup, and local fly pressure.