Summer sores return to focus as fly season raises equine risk
Bottom line
Summer sores in horses are a seasonal but clinically important form of cutaneous habronemiasis, and a new explainer from The Horse highlights how easily these lesions can be mistaken for other nonhealing skin conditions. In the April 19, 2024 article, Stacey Oke, DVM, MSc, outlines that the lesions are caused when fly-borne larvae of Habronema or Draschia are deposited into wounds or moist mucocutaneous areas, triggering an inflammatory, granulomatous reaction. Common sites include the lips, eyes, distal limbs, ventral abdomen, and genital region, and lesions can range from small ulcerated sores to large, exuberant masses with characteristic yellow “sulfur granules.” The article also emphasizes that diagnosis often requires more than appearance alone, because summer sores can resemble proud flesh, pythiosis, squamous cell carcinoma, or sarcoids. (thehorse.com)
Why it matters: For veterinary professionals, the diagnostic message is the key takeaway. Merck Veterinary Manual and UC Davis both note that confirmation can involve biopsy, deep scrapings, and identification of larvae or calcified concretions, while UC Davis cautions that lesions can overlie an underlying tumor, especially in the genital region, making a sufficiently deep biopsy important. Updated AAEP parasite control guidance also underscores that cutaneous habronemosis remains a well-described manifestation of stomach worm infection and may be especially relevant in some arid U.S. regions, where it can be more common than other clinically important parasitic syndromes. (merckvetmanual.com)
What to watch: As fly season advances, expect renewed emphasis on differentiating summer sores from other granulomatous or neoplastic lesions, and on pairing lesion workups with fly control and parasite management strategies. (ceh.vetmed.ucdavis.edu)
Key facts
- Condition
- Summer sores in horses, also called cutaneous habronemiasis or habronematidosis
- Cause
- Fly-borne larvae of Habronema or Draschia deposited in wounds or moist tissues
- Lesion sites
- Lips, eyes, distal limbs, ventral abdomen, and genital region
- Typical appearance
- Small ulcerated sores to large, exuberant masses with yellow sulfur granules
- Common differentials
- Proud flesh, pythiosis, squamous cell carcinoma, and sarcoids
- Seasonality
- Typically appears in spring and regresses in winter
- Diagnostic note
- Diagnosis often requires biopsy, deep scrapings, or identification of larvae or calcified concretions
- Regional note
- May be especially relevant in some arid U.S. regions, including the Southwest
A spring 2024 clinical explainer from The Horse is putting summer sores back on the radar for equine practitioners as fly season drives another round of cases. The condition, also called cutaneous habronemiasis or habronematidosis, develops when larvae of stomach worms in the genera Habronema and Draschia are deposited by flies onto wounds or moist tissues, where they trigger chronic, ulcerative, granulomatous lesions rather than completing their normal life cycle. The Horse reports that lesions may be small and subtle early on, but can also become extensive, painful, pruritic, and difficult to heal. (thehorse.com)
The timing of the article is notable because summer sores are strongly seasonal. UC Davis describes the disease as typically appearing in spring and regressing in winter, with recurrence common once a horse has been affected. That seasonality, combined with the role of houseflies and stable flies in transmission, helps explain why these lesions can seem to flare quickly as insect pressure rises. The AAEP’s updated internal parasite control guidelines, published in 2024, also reinforce that the adult stomach worms themselves are rarely associated with obvious gastric disease, while the cutaneous form is the better-recognized clinical problem in practice. (ceh.vetmed.ucdavis.edu)
In The Horse article, equine surgeons Dustin Major, DVM, Dipl. ACVS (LA), at Texas A&M University, and Nicole Verhaar, DVM, PhD, Dipl. ECVS, at the University of Veterinary Medicine Hannover, describe the wide range of presentations. Lesions are often found at the oral commissures, periocular tissues, distal limbs, ventral abdomen, and genitalia, and may appear moist, bloody, itchy, and packed with necrotic debris. Verhaar notes that sulfur-like granules are considered especially suggestive of summer sores, but both experts stress that the differential list is broad, including proud flesh, foreign body granulomas, pythiosis, squamous cell carcinoma, and sarcoids. (thehorse.com)
That overlap is why diagnostics matter. Merck states that diagnosis is based on finding nonhealing, ulcerated granulomas that may contain yellow calcified material, with larvae sometimes identifiable in lesion scrapings. UC Davis goes further, saying diagnosis can be challenging and may require clinical history, biopsy-confirmed concretions, and recovery of larvae from biopsy or scrapings. Importantly, UC Davis warns that summer sore lesions can occur superficially over an underlying cancer, particularly sarcoid or squamous cell carcinoma in the genital region, so biopsy depth is not a minor technical point, it can change the case assessment entirely. (merckvetmanual.com)
Industry and expert guidance outside the article points in the same direction. The AAEP’s 2024 parasite control guidelines describe cutaneous habronemosis as a persistent granulomatous lesion caused by aberrant larval deposition by fly vectors and note that these cases are often challenging to treat. The guidelines also suggest the syndrome may be comparatively common in some arid parts of the U.S., particularly the Southwest, which is a useful reminder that regional parasite ecology still shapes equine case mix. (aaep.org)
Why it matters: For equine veterinarians, this is less a “new disease” story than a reminder about diagnostic discipline during peak fly months. A nonhealing summer wound, periocular lesion, or genital mass may look straightforward at first glance, but missing habronemiasis can delay appropriate parasite-focused management, while assuming habronemiasis too quickly can obscure neoplasia or other serious disease. The case-level challenge is that summer sores sit at the intersection of dermatology, parasitology, wound management, and preventive medicine. They also create a practical communication issue with pet parents, who may interpret them as simple wounds unless clinicians explain the parasite and fly-vector component clearly. (thehorse.com)
What to watch: Through the 2026 fly season, expect continued emphasis on early lesion recognition, deeper biopsy when the presentation is atypical or severe, and prevention strategies that combine wound protection, fly control, and evidence-based parasite management rather than relying on lesion appearance alone. (merckvetmanual.com)