Summer Sores at a Glance
Summer Sore Location Frequency
Comparison
| Feature | Summer Sore (Habronema) | Normal Wound | Key Difference |
|---|---|---|---|
| Cause | Fly-deposited larvae | Physical trauma | Parasitic vs mechanical |
| Appearance | Ulcerated, raised, yellow/gray granules | Clean or scabbed | "Coral bead" granules pathognomonic |
| Itching | Intense — horse rubs constantly | Minimal after initial | Self-trauma worsens lesions |
| Treatment | Ivermectin + steroids + fly control | Cleaning + bandaging | Parasite-specific therapy needed |
Summer Sore Treatment Plan
Summer Sore Prevention Checklist
| Implement comprehensive fly control (traps, sprays, predators) | |
| Clean stalls daily to reduce fly breeding | |
| Cover wounds with fly-proof bandages | |
| Use fly masks on horses with eye/face wounds | |
| Apply wound fly repellent daily during fly season | |
| Treat with moxidectin or ivermectin as vet directs | |
| Inspect all wounds daily for granulation changes | |
| Consult vet if wound not healing in 7–10 days |
Summer sores (habronemiasis) are skin lesions caused by the aberrant migration of Habronema or Draschia nematode larvae through equine skin, most commonly around wounds, the eyes, prepuce, or sheath. The larvae are deposited by house flies and stable flies feeding near moist skin areas; instead of completing their lifecycle in the stomach, they migrate through the skin and trigger an intense hypersensitivity reaction. Lesions are proliferative, ulcerative, and intensely itchy, and they do not heal on their own -- the combination of larval presence plus hypersensitivity reaction causes self-trauma that enlarges lesions over the summer fly season.
| Affected Site | Presentation | Key Treatment |
|---|---|---|
| Skin wounds / moist areas | Granulomatous, itchy, reddish-brown proliferative mass | Ivermectin 0.2 mg/kg + dexamethasone 5–7 days; debride large lesions |
| Ocular (conjunctival) | Granuloma, excessive tearing, caseous medial canthus discharge | Fly mask + systemic ivermectin + topical adjunct |
| Genital (prepuce / sheath) | Swelling, ulceration, self-mutilation behavior | Full treatment course; second dose at 4–6 wks if lesions persist |
Why Summer Sores Form Where They Do
Habronema muscae, H. majus, and Draschia megastoma adult worms live in the horse's stomach without causing significant disease in most cases. Eggs pass in feces and develop into larvae in fly maggots. When mature flies feed on moist ocular discharge, skin wounds, genital mucosa, or sweat-dampened skin, they deposit larvae. In wounds, nasal discharge areas, and genital tissues, the larvae cannot complete their lifecycle and migrate aimlessly through tissue, triggering a granulomatous hypersensitivity reaction.
Lesions are seasonal, worsening through summer and often regressing spontaneously in cold weather when fly populations crash -- a key diagnostic clue that differentiates summer sores from other wound complications. However, lesions that partially regress often return worse the following summer without treatment, and calcified yellow or sulfur granules ("kunkers") develop within the granuloma over time.
Recognizing Habronemiasis
Classic summer sore: a moist, reddish-brown, irregular granulomatous mass at a wound or moist skin site, covered with a soft, necrotic surface, intensely pruritic (the horse rubs, bites, or kicks at the lesion). Lesions around the eye (ocular habronemiasis) appear as conjunctival granulomas, excessive tearing, and yellow caseous material at the medial canthus. Genital lesions involve the prepuce and smegma area in geldings and stallions, causing swelling and self-mutilation.
Diagnosis is clinical based on appearance and seasonality plus geographic location with high fly populations. Definitive confirmation is by biopsy and histopathology showing eosinophilic inflammation with nematode larvae; not all biopsies find intact larvae, but the inflammatory pattern is characteristic.
Treatment
Treatment targets both the parasitic component and the hypersensitivity reaction. Ivermectin (0.2 mg/kg orally, standard deworming dose) or moxidectin kills Habronema larvae and is the foundation of treatment. A single dose often produces rapid improvement within 1 to 2 weeks; severe or recurrent cases warrant a second dose 4 to 6 weeks later. Concurrent corticosteroids (dexamethasone 0.05 to 0.1 mg/kg orally daily for 5 to 7 days, tapering) reduce the hypersensitivity component that drives lesion proliferation. Large lesions may require surgical debridement combined with medical therapy.
Topical treatment of lesions with combination preparations (ivermectin paste plus DMSO, or veterinarian-compounded corticosteroid/antiparasitic creams) is used as an adjunct. Protect lesions from flies with wound dressings or fly masks for ocular lesions during treatment.
Prevention
The most effective prevention is fly control: manure management (remove feces every 1 to 3 days to interrupt fly breeding cycles), fly traps, barn fans that reduce fly landing, fly sheets and masks, and prompt wound treatment to prevent fresh wounds from becoming habronemiasis entry points. Annual anthelmintic treatment targeting Habronema (ivermectin or moxidectin) reduces stomach worm populations and larval shedding.
Sources: Reinemeyer CR & Nielsen MK, "Parasite Control in Horses" (2013); AAEP Internal Parasite Control Guidelines (2019); Merck Veterinary Manual, "Habronemiasis" chapter.