Summer Sores

What are summer sores?

Summer sores, also known as cutaneous habronemiasis, habronematidosis, granular dermatitis, and jack sores, are a parasitic disease of equids linked to the life cycles of stomach worms (nematodes).

The disease is attributed to any of the three species of Habronema worms that parasitize the horse: H. muscae, H. microstoma, and H. megastoma. Adult worms live in the stomach where they typically cause little reaction. Females lay eggs and the larvae are passed in the feces where they are ingested by the maggots of either the housefly or the stable fly, which serve as intermediate hosts. The normal life cycle is complete when flies deposit the infective larvae around the horse’s lips, where they are subsequently swallowed and the larvae develop into adults in the stomach.

Summer sores develop when the larvae are deposited in previously damaged skin or mucous membranes such as the lips, nostrils or genitalia where they cannot complete the life cycle and instead cause a local inflammatory reaction. The lesions themselves consist of areas of ulceration that usually contain small, gritty, yellow nodules (‘sulfur granules’).

The disease is seasonal, first appearing in the spring and in most cases spontaneously regressing in the winter months.

What are the clinical signs of summer sores?

Clinical signs of summer sores range from mild to severe, and include skin lesions that do not heal, itching, and formation of proud flesh (exuberant granulation tissue). Lesions often occur at common wound sites, but can also develop in moist areas where flies often feed such as the prepuce, lower abdomen, and corners of the eyes and lips. The area around the lesion may have a “greasy” appearance and blood-tinged fluid may drain from the lesions. The lesions may contain yellow or white calcified material that looks like grains of rice.

How are summer sores diagnosed?

Diagnosis of habronematidosis can be challenging as lesions can also be caused by other skin infections and conditions. Clinical signs and a detailed history, along with the presence of calcified concretions confirmed by biopsy, and recovery of larvae from biopsy or scrapings are used to confirm diagnosis. It is important to remember that summer sores can occur superficially to an underlying cancer such as sarcoid or squamous cell carcinoma, especially in the genital area. Such lesions can often be very large. Thus, a biopsy should always be deep enough to rule an underlying tumor in or out.

How are summer sores treated?

Treatment of summer sores can be difficult. Effective treatment involves a veterinarian cleaning the wound (debriding), often along with administration of ivermectin, corticosteroids, and topical treatments. The wound should be covered, if possible, and it is important to follow stringent fly management practices. Lesions may resolve on their own during cool weather or with reduced fly activity.

What is the prognosis for summer sores?

The prognosis for horses with summer sores varies based on a number of factors, including the location and size of the lesions and response to treatment. In some cases, lesions respond well to treatment and/or do not interfere with performance. Large wounds that are slow to heal can negatively impact training and showing. In some horses, the disease may recur again the same year or in subsequent years.

How can summer sores be prevented?

Once habronematidosis develops in a horse, it will usually recur every summer. A genetic susceptibility has been proposed in some horses, especially those affected annually, but further research is needed. Adhering to strict fly preventative measures can minimize recurrence, but it is difficult to completely prevent recurrence of summer sores.

Preventative measures primarily involve disrupting the Habronema life cycle. The best way to prevent summer sores is to practice effective fly control. Remove and dispose of manure, soiled bedding, and wet feed. Stable horses during times of peak fly activity, and consult with your veterinarian regarding regular deworming protocols. Regularly inspect your horse for new wounds or abrasions, particularly in the summer months. Keep wounds clean and dry.

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu.