Summer sores demand a careful diagnostic workup in horses
Bottom line
Summer sores, or cutaneous habronemiasis, are a seasonal parasitic skin disease in horses that can escalate quickly if they’re mistaken for routine wounds. In a new clinical explainer, The Horse outlined how these lesions develop when fly-borne Habronema or Draschia larvae are deposited into wounds or moist tissues instead of being swallowed, triggering an intense inflammatory reaction. Typical lesions are nonhealing, ulcerated, itchy, and often contain yellow “sulfur granules,” with common sites including the oral commissures, eye canthi, sheath, urethral process, and existing wounds. (thehorse.com)
Why it matters: For equine veterinarians, the key issue is diagnosis. Summer sores can resemble proud flesh, squamous cell carcinoma, sarcoids, pythiosis, and other chronic wound conditions, and experts cited by The Horse said biopsy is often warranted when lesions are atypical, lack sulfur granules, occur in older horses, or involve distal limb wounds. UC Davis notes that lesions can also sit superficially over underlying neoplasia, reinforcing the need for deep enough sampling to rule tumors in or out. (thehorse.com)
What to watch: As fly season advances, watch for more emphasis on early biopsy, aggressive fly control, and targeted deworming protocols in horses with recurrent summer sores. (ceh.vetmed.ucdavis.edu)
Key facts
- Condition
- Summer sores, or cutaneous habronemiasis
- Species
- Horses
- Cause
- Fly-borne Habronema or Draschia larvae deposited into wounds or moist tissues
- Typical lesions
- Nonhealing, ulcerated, itchy, and often with yellow sulfur granules
- Common sites
- Oral commissures, eye canthi, sheath, urethral process, and existing wounds
- Diagnostic concern
- Can resemble proud flesh, squamous cell carcinoma, sarcoids, and pythiosis
- When biopsy is often warranted
- Atypical lesions, no sulfur granules, older horses, or distal limb wounds
- Prevention
- Fly control, manure management, wound care, stabling during peak fly activity, and veterinarian-guided deworming
Summer sores are back in focus as veterinarians head into peak fly season, with The Horse publishing a clinical review on the causes, clinical signs, and diagnostics of these often frustrating equine skin lesions. Also known as cutaneous habronemiasis or habronematidosis, summer sores occur when larvae from stomach worms in the Habronema and Draschia groups are deposited by flies into wounds or moist tissues, where they can’t complete their life cycle and instead trigger a marked local inflammatory response. (thehorse.com)
The condition is closely tied to warm-weather fly activity. Adult worms live in the horse’s stomach, and eggs or larvae pass in feces, where developing fly maggots ingest them. The normal cycle resumes when flies deposit infective larvae near the horse’s mouth to be swallowed, but disease develops when those larvae are instead dropped onto damaged skin, the eyes, lips, nostrils, genital tissues, or other moist sites. UC Davis notes the disease typically appears in spring, often regresses in winter, and commonly recurs in affected horses year after year. (ceh.vetmed.ucdavis.edu)
Clinically, the lesions can range from small circular ulcers to exuberant, granulomatous, proud-flesh-like wounds that are moist, bloody, itchy, and slow to heal. The Horse highlighted expert comments from Dustin Major, DVM, and Femke Verhaar, DVM, DACVS-LA, who said sulfur granules can be a useful clue, especially in characteristic locations such as the corners of the mouth, the canthi of the eyes, and around the prepuce. Merck Veterinary Manual similarly describes nonhealing ulcerated granulomas containing yellow calcified material resembling rice grains. (thehorse.com)
The diagnostic challenge is what makes the topic especially relevant for practice. Major told The Horse that his differential list for ulcerative, nonhealing wounds includes proud flesh, habronemiasis, squamous cell carcinoma, and pythiosis. Texas A&M adds that pythiosis remains an important rule-out in parts of the southern U.S., while UC Davis warns that summer sores can occur superficially over sarcoids or squamous cell carcinoma, particularly in the genital region. Biopsy remains central: The Horse reported that histopathology often shows eosinophilic inflammation and necrosis, although larvae may only be found intermittently. A PCR test has been described in the literature, but according to Verhaar, it is not commercially available to practitioners to her knowledge. (thehorse.com)
Expert commentary across sources is consistent on prevention and case management. Texas A&M’s Dustin Major said fly control is “vital” and pointed to manure removal, barn spray systems, fly predators, fly masks, and fly sheets, along with added avermectin use such as ivermectin during fly season for susceptible horses. UC Davis likewise emphasizes interrupting the fly-parasite lifecycle through manure management, wound care, stabling during peak fly activity, and veterinarian-guided deworming plans. (ceh.vetmed.ucdavis.edu)
Why it matters: For veterinary professionals, summer sores are less about recognizing a classic lesion than about not missing what else it could be. The practical takeaway from the current guidance is that lesion location, seasonality, sulfur granules, and pruritus may strongly suggest habronemiasis, but they shouldn’t replace tissue diagnosis when the presentation is atypical or the stakes are higher, such as in older horses, distal limb lesions, or masses near the genitalia or eye. Chronic wounds in these sites can carry very different prognoses and treatment paths depending on whether the underlying problem is parasitic, neoplastic, or pythiosis-related. (thehorse.com)
That has workflow implications for ambulatory and hospital-based equine practice. Early sampling, clear client communication, and fast intervention may help reduce prolonged wound management, repeated empiric treatment, and self-trauma from pruritus. It also underscores the value of pairing treatment plans with prevention counseling for pet parents, especially for horses with a history of recurrence. (thehorse.com)
What to watch: Through summer 2026, expect continued attention on recurrence prevention, differential diagnosis in chronic equine wounds, and whether newer molecular diagnostics for Habronema species become more accessible outside research settings. (thehorse.com)