Cutaneous Habronemiasis in Horses: Summer Sores Caused by Parasites
- Cutaneous habronemiasis, often called a summer sore, is a non-healing skin lesion caused when fly-borne Habronema or Draschia larvae are deposited into a wound or moist skin area.
- These sores often look red, raised, itchy, and ulcerated, and may contain yellow-white "rice grain" material or proud-flesh-like tissue.
- Common locations include the lips, corners of the eyes, lower legs, sheath or penis, and other places where flies gather or skin stays moist.
- Your vet may diagnose it from the lesion's appearance and seasonality, but some horses need skin scrapings, cytology, biopsy, or a treatment-response approach to rule out cancer, proud flesh, or other chronic wounds.
- Treatment usually combines deworming, local wound care, inflammation control, and aggressive fly management. Earlier care often means faster healing and less recurrence.
What Is Cutaneous Habronemiasis in Horses?
Cutaneous habronemiasis is a parasitic skin disease of horses and other equids. Most pet parents know it by the more common name summer sores. It happens when larvae from stomach worms in the Habronema or Draschia group are deposited by flies onto a wound, the corner of the eye, the lips, or another moist area of skin. Instead of completing their normal life cycle, the larvae trigger a strong inflammatory reaction in the skin.
These lesions tend to become chronic, irritated, and slow to heal. They can look like proud flesh, a raw ulcer, or a raised lump with yellow-white gritty material inside. Many horses seem more itchy than painful, but some sores become very uncomfortable, especially if they are near the eye, genital area, or a place that rubs against tack.
Summer sores are most common during warm months when house flies and stable flies are active. They are not considered directly contagious from horse to horse. The problem is the combination of flies, parasite exposure, and vulnerable skin.
Because these sores can resemble other conditions, including exuberant granulation tissue, sarcoids, squamous cell carcinoma, or infected wounds, it is important to have your vet examine any lesion that is not healing normally.
Symptoms of Cutaneous Habronemiasis in Horses
- Red, raised, non-healing skin lesion
- Ulcerated sore with proud-flesh-like tissue
- Yellow or white gritty granules within the wound
- Clear, bloody, or sticky discharge
- Marked itchiness or rubbing at the area
- Swelling around the lesion
- Lesions at the lips, eye corners, lower legs, sheath, penis, or other moist areas
- Recurring sore in the same location during spring or summer
- Sensitivity when tack, flies, or handling irritate the area
- Eye irritation or tearing if the lesion is near the medial canthus
Call your vet sooner rather than later if a wound is getting larger, staying raw for more than a few days in fly season, or developing yellow-white granules or proud-flesh-like tissue. Lesions near the eye, penis, sheath, or coronary band deserve prompt attention because swelling and chronic inflammation can lead to more serious complications.
See your vet immediately if your horse has severe swelling, trouble urinating, significant eye pain, heavy discharge, lameness, or a rapidly enlarging mass. Not every summer sore is an emergency, but any non-healing wound in a horse should be treated as a medical problem until your vet says otherwise.
What Causes Cutaneous Habronemiasis in Horses?
The underlying cause is an abnormal placement of stomach worm larvae. Adult Habronema worms live in the horse's stomach and pass eggs or larvae through manure. Flies pick up the parasite during their life cycle, then later deposit infective larvae on the horse.
Normally, larvae are left near the mouth and are swallowed, returning to the stomach. In cutaneous habronemiasis, flies instead deposit larvae into a skin wound or onto moist tissues such as the lips, eyes, or genital area. The larvae cannot mature there, so they stay in the tissue and trigger a strong local inflammatory response.
Warm weather, heavy fly pressure, open wounds, and poor manure control all increase risk. Horses with a history of summer sores may also be more likely to develop them again in future fly seasons.
This condition is not caused by poor care alone. Even well-managed horses can develop summer sores if flies, parasite exposure, and a vulnerable skin site line up at the wrong time.
How Is Cutaneous Habronemiasis in Horses Diagnosed?
Your vet usually starts with a hands-on exam and a close look at the lesion's location, appearance, and seasonality. A summer sore is often suspected when a horse has a chronic, ulcerated, itchy lesion during fly season, especially if it contains yellow-white calcified or gritty material and has not responded to routine wound care.
In some horses, diagnosis is based on the combination of history and response to treatment. However, that is not always enough. If the lesion is severe, unusual, or not improving, your vet may recommend skin scrapings, cytology, or biopsy. These tests help look for larvae and, just as importantly, help rule out other causes of non-healing wounds.
That step matters because summer sores can mimic proud flesh, sarcoids, squamous cell carcinoma, fungal disease, or chronic bacterial infection. Lesions in the genital area or around the eye especially deserve a careful workup.
Fecal testing may be part of a broader parasite plan, but it does not reliably confirm a skin lesion is a summer sore. Habronema eggs can be difficult to detect, so diagnosis usually depends more on the lesion itself than on manure testing.
Treatment Options for Cutaneous Habronemiasis in Horses
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Exam by your vet
- Empiric deworming with a macrocyclic lactone if your vet feels the lesion is consistent with a summer sore
- Basic wound cleaning and protective topical care
- Fly-control plan such as fly mask, fly sheet, repellents, and manure management
- Short-term recheck if the lesion is not improving
Recommended Standard Treatment
- Full veterinary exam and lesion assessment
- Systemic deworming directed by your vet, commonly with ivermectin or moxidectin-based protocols
- Topical or local anti-inflammatory treatment when appropriate
- Targeted wound management and bandaging when feasible
- Sedation and minor debridement if proud-flesh-like tissue is interfering with healing
- Structured fly-control and environmental management plan
- Follow-up exam to confirm improvement
Advanced / Critical Care
- Everything in standard care
- Biopsy, histopathology, or additional diagnostics to rule out sarcoid, squamous cell carcinoma, or other chronic wound disease
- Sedated or surgical debridement/cauterization of excessive granulation tissue
- More intensive eye, genital, or distal-limb wound management
- Referral-level care for complicated lesions or repeat recurrences
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Cutaneous Habronemiasis in Horses
Bring these questions to your vet appointment to get the most out of your visit.
- Does this lesion look typical for a summer sore, or are there other conditions you are concerned about?
- Does my horse need a biopsy or skin sample, or is it reasonable to start with treatment first?
- Which deworming plan makes sense for this horse and this lesion?
- Is there too much granulation tissue for this sore to heal on its own?
- What kind of topical care is safest for this location, especially if the sore is near the eye or genital area?
- How should I bandage or protect this wound, and when should I leave it uncovered?
- What fly-control steps will make the biggest difference on my farm right now?
- What signs would mean the lesion is not improving as expected and needs a recheck?
How to Prevent Cutaneous Habronemiasis in Horses
Prevention focuses on fly control, wound protection, and parasite management. Clean manure regularly, reduce wet organic material where flies breed, and use barn and pasture strategies that lower fly pressure. Depending on your horse's situation, that may include fly masks, fly sheets, repellents, fans in stalls, and targeted environmental control.
Check your horse's skin often during warm months. Small cuts, rubs, and moist irritated areas are easier to protect early than after a summer sore develops. Prompt wound cleaning and covering vulnerable areas can reduce the chance that flies will deposit larvae into the tissue.
Work with your vet on a deworming plan that fits your horse's age, exposure, and fecal testing history. Broad deworming without a plan is not ideal, but horses with repeated summer sores may need a more tailored parasite-control strategy during fly season.
If your horse has had summer sores before, stay extra alert in the same season and at the same body sites. Recurrence is common, so early action can shorten healing time and may keep a small irritation from becoming a chronic lesion.