Habronemiasis  (“Summer Sores”)


Jack, a young Quarter Horse gelding, presented to North Florida Equine for this non-healing wound:

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Jack was not lame, but the wound had a stringy, yellow discharge and a bad smell.  North Florida Equine surgically removed the unhealthy tissue from the wound and found several small, firm, yellow chunks:

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“Kunker” from Jack’s wound.

North Florida Equine strongly suspected that this was a “summer sore,”  but a biopsy was taken to confirm the diagnosis, as other conditions (some quite serious) can look very similar, including: granulation tissue (“proud flesh”), cancer – including squamous cell carcinoma, mast cell tumor, or sarcoid, and pythiosis (fungal infection).

So what is a summer sore and how did Jack get it? The medical term for a summer sore is cutaneous habronemiasis.  It is a fairly common skin condition for horses living in the southeastern United States.  How it occurs is quite an amazing series of events.  It is caused by the larvae of the Habronema worm that lives in a horse’s stomach.  Worm larvae are passed in a horses’ manure where flies land and eat the larvae.  The larvae mature inside of the fly, then, about 2 weeks later, the larvae actually crawl out of the fly’s mouth when the fly lands somewhere warm and moist to feed.

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This could be on a horse’s mouth, where the larvae can head back to the stomach without causing much of an issue. Problems occur when the larvae are deposited on an open wound, the genitalia, or the conjunctiva of the eye.  In these locations, the larvae have nowhere to go and burrow into the surrounding tissue.  The horse’s body may react quite strongly to their presence with the tissue becoming inflamed and sometimes itchy and irritated, occasionally causing the horse chew or rub it.  The horse’s body will wall off the offending larvae, forming the firm yellow chunks (“kunkers”) that North Florida Equine found in Jack’s wound.

So what to do? After North Florida Equine surgically debrided the wound, Jack was placed on a treatment plan that included topical and systemic anthelmintics (to kill the worms and their larvae), corticosteroids (to calm the tissue reaction), and maybe most importantly – good fly control!  This includes topical repellent sprays and creams, physical barriers like bandages, fly masks or fly sheets, and manure removal.

On recheck the following week, North Florida Equine found that the wound had not progressed as hoped.  There was still inflammation and yellow “kunkers” present in the wound:

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With some additional questioning, it was learned that the corticosteroid treatment had been stopped prematurely – Jack and his owner got back on track, and here is the leg a few weeks later:

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Jack is doing great and his wound is almost healed!  Jack’s owners will have to continue with diligent wound care and fly control in the summers to come, as there is strong evidence that individual horses are more susceptible to summer sore formation. Other horses that are exposed to the same flies and larvae may never have a problem.