Habronemiasis, otherwise known as “summer sores”, is fairly common among horses in the southeast US. It occurs as a result of nematode infestation of the skin and typically manifests as granulomatous erosions and/or masses. Habronema muscae, Habronema majus (H. microstoma) and Draschia megastoma are the usual invaders.
There are THREE forms of habronemiasis in the horse:
1) Conjunctival Habronemiasis produces granulomatous lesions caused by invasion by Habronema spp. larvae occurring on the third eyelid, the eyelid proper, or on the conjunctiva of the medial canthus. Excessive tearing and squinting often accompany these lesions.
2) Cutaneous Habronemiasis is manifested by granulomatous lesions caused by the invasion of skin wounds or excoriations by the larvae of Habronema spp. and Draschia megastoma. This form is also known as swamp cancer, summer sores, bursati, granular dermatitis and jack sores. Cutaneous habronemiasis presents as ulcerative skin nodules in the spring and summer, with regression in winter months. There is no breed or sex predilection (i.e. any horse can be affected). However, some horses seem to be more prone to the condition, developing lesions year after year. Commonly affected areas include legs, penis, preputial sheath, eyes, and any open skin wounds. Geldings and stallions that have infestation of the penis or sheath may exhibit dysuria (difficulty urinating).
3) Gastric Habronemiasis causes large granulomatous masses in the stomach mucosa due to the invasion of Draschia megastoma larvae. These lesions usually go unnoticed (i.e. are aclinical) until stomach wall perforation occurs. The larvae of Habronema majus and Habronema muscae usually don't cause clinical signs more severe than mild gastritis.
How do Horses get infected with Habronema Species?
The common house and stable flies are intermediate hosts for these parasites, meaning that they ingest and harbor the immature larval stages and then deposit them into the skin while biting. The larvae are occasionally deposited around the muzzle, thereby facilitating ingestion by the horse. Adult stages of the nematode live in the stomach and produce larvae which are subsequently passed in manure. The life cycle is repeated when the larvae in the manure are ingested by maggots (immature flies).
Diagnosis is generally based on the clinical appearance and behavior of the lesions. Pruritis (itchiness), swelling and excessive granulation are typically evident and occur secondary to the horse’s own immune hypersensitivity to the larvae. In many cases, tiny yellow granules are visible within the granulomatous reaction(s).
Biopsy is required to make a definitive diagnosis.
At The Atlanta Equine Clinic, we implement FOUR strategies for addressing habronemiasis:
1) Ivermectin Deworming. Administer one full dose orally (by mouth), then administer a second full dose orally 2 weeks later.
2) Topical and/or Systemic Steroid Therapy. Steroids effectively suppress the horses immune hypersensitivity to the larvae, thereby reducing swelling, granulation, and pain.
3) Fly Control. We recommend using a combination of fly repellents, fly strips and fly masks/sheets, particularly those that cover the underside of the belly. The use of fans on stall doors will also help to decrease the number of flies in the environment.
4) Topical Wound Care. Using a thick ointment (such as Vet's Complete® Wound Creme) to cover open wounds may help to discourage flies from landing and depositing larvae into the tissue.