Summer sores, or, more correctly, cutaneous habronemiasis or Draschia species infestation, can occur in all equine species (horses, donkeys, mules and zebras) and are most prominent in parts
of the world with a tropical or temperate climate. It's tough to know exactly how common this disease is because it can be
difficult to definitively diagnose; many other skin infections can be similar in appearance.
Lesions associated with proud flesh, or proliferative granulation tissue, can look similar to a summer sore, and both problems
tend to occur after skin trauma or injury. Various skin tumors, such as sarcoids or squamous cell carcinomas, can also look
like summer sores. Also included on the differential diagnosis list would be various fungal skin infections, foreign body
granulomatous reactions and pythiosis (Photo 3). In addition, larvae can be deposited in a horse's eye, and ophthalmic lesions
may mimic types of ocular tumors or neoplasia, foreign bodies, phycomycosis or onchocercosis.
Photo 3: This excessively irritated granular reaction is a pythiosis lesion, but it shares many physical characteristics with
a severe summer sore. Both diseases should be on a differential diagnosis list for aggressive skin lesions.
The best diagnostic methodology requires deep tissue biopsy of the lesion for histopathologic confirmation of the larvae.
This is sometimes difficult because of the location of lesions and because larvae are not always easily recovered from even
active lesions (Photo 4). Histopathologic examination will commonly reveal extensive ulceration and a diffuse eosinophilic
infiltrate. This type of report, coupled with the case history, environmental information and elimination of other causes,
usually results in a diagnosis of summer sores. In addition, molecular diagnosis is evolving, and a PCR assay is available,
though it is not extensively used at this point.
Photo 4: This lower leg lesion began as a simple scrape and rapidly became more reactive and irritated. Deep biopsies of this
site failed to reveal Habronema species larvae, but treatment with ivermectin, anti-inflammatories and corticosteroids resulted
in resolution of the skin reaction.
Treatment and prevention
Treatment usually includes larvicidal, anti-inflammatory and antimicrobial components. Ivermectin has commonly been used to
kill both the adult worms in the stomach and the larval forms in the skin tissue. A single dose of ivermectin is generally
sufficient to kill Habronema species larvae. But some horses don't respond or they become reinfected, necessitating multiple doses.
Corticosteroids reduce the inflammatory hypersensitivity reaction, and antimicrobials treat secondary infections, usually
a result of self-inflicted biting, rubbing and itching. Refractory lesions may require extensive surgical débridement to adequately
remove degenerative larvae within the tissue.
In terms of prevention, fly control, manure removal and the quick and appropriate treatment of skin wounds and infections
can greatly reduce the incidence and severity of summer sores.