Cutaneous habronemiasis, also known as “summer sores”, is a condition that occurs in horses secondary to skin invasion by larvae from the equine stomach worm. It causes an ulcerative granuloma in horses and is a commonly seen starting in spring/summer when the flies become active, and regressing in the winter.
Parasite life cycle
There are three species of stomach worm that affect horses, Habronema microstoma, Habronema muscae, and Draschia megastoma. The adult parasite lives in the stomach wall or on the lining of the inside of the stomach (the mucosa). Gastric infestation by the adult stomach worm rarely causes clinical signs in horses, aside from a mild gastritis (inflammation of the lining of the stomach).
The female worm lays eggs that develop into larvae during their transit through the intestines and are passed out in the horse’s manure as larvae. The larvae are then ingested by maggots of the house or stable fly which acts as an intermediate host. The fly maggots complete their development into adult flies, and the Habronema larva migrate to the mouth parts of the fly where they are deposited on the horse when the fly lands. The flies typically are attracted to warm, moist parts of the body (eyes, nose, mouth, genitalia, and open wounds). The larvae, once swallowed, travel down to the stomach where they mature into the adult stomach worm to complete their life cycle. Larvae that do not reach the stomach, but remain in the tissues, do not continue development into adult worms.
Skin lesions in horses
Typically, Habronema larvae cannot penetrate normal healthy, intact skin. However, the larvae can penetrate damaged or moist skin, and this is when skin lesions in horses may occur. The larvae irritate the tissues and cause an intense inflammatory reaction. Horses can be very pruritic (itchy), and often make the wounds worse by rubbing or biting. Skin lesions consist of ulcers and/or nodules and typically contain a large amount of granulation tissue with yellow, rice-sized calcified concretions surrounding the Habronema larvae (termed sulfur granules).
The skin lesions we see are thought to be at least partially due to a hypersensitivity reaction (allergic reaction). The reason for this thinking is because often one horse in a herd is affected, and the disease often recurs in the same horse every summer. Also, there is typically spontaneous remission in the winter. Common sites affected include the medial canthus of the eye, the third eyelid, the corners of the mouth, the male genitalia, and any open wounds.
Ocular lesions can consist of open, granulomatous skin wounds in front of the eye, or can consist of swollen, thickened conjunctiva often with mucoid ocular discharge. Cutaneous habronemiasis of the genital region can affect the sheath, urethral opening, or glans penis. Sheath swelling or thickening is commonly seen, as well as open granuloma wounds. Sheaths can often have small nodules that when opened, contain yellow sulfur granules. If the urethra is affected, geldings may be observed to “spray” when urinating or may appear to have difficulty or pain on urination.
Diagnosis is typically made based on clinical signs, history, and presence of the calcified concretions “sulfur granules”. Sometimes the lesions can be similar in appearance to that of neoplasia (ie. squamous cell carcinoma), or exuberant granulation tissue of a wound (proud flesh). In these cases, biopsy is required for accurate diagnosis.
Treatment consists of a multi-step approach and can vary depending on location and severity of lesions.
1. Topical and/or systemic corticosteroids to suppress the body’s immune response to the larvae and reduce inflammation, itching, and pain.
2. Surgical debridement of the lesions to remove excess granulation tissue and remove all visible “sulfur granules”.
3. Deworming with Ivermectin or Moxidectin – typically two doses administered 2-3 weeks apart.
4. Bandaging lesions or use of fly repellent ointment (Swat ®) to prevent reinfection.
One of the most important aspects of prevention is fly control. Although difficult, this will greatly reduce the incidence of habronemiasis. Prompt manure disposal, insect repellents and barriers (fly masks), and fly predators can all help in decreasing the fly population at a farm. Regular anthelmintic treatment (de-worming) with Ivermectin or Moxidectin are also important in breaking the parasite life cycle.