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Shoo Fly, Don’t Bother Me: Summer Sores

Written by Ross University veterinary student, Kirsten Shade

Cutaneous Habronemiasis, more commonly known as “summer sores,” are of significant importance in the equine world. The term summer sore originated from the appearance of a non-healing wound that is more common in the summer months.

Life Cycle:

While it is a common misconception that fly eggs are the cause for the non-healing wound, it is the equine stomach worm that is the actual culprit. The flies are intermediate hosts that deposit the larvae of the equine stomach worm into wounds or breaks within the skin. This allows for the larvae to invade into the tissue creating an intensely itchy area of inflammation and small yellow granules.

Common areas for summer sores include the extremities, corners of the eyes and mouth, sheath in males and the lower abdomen. Sometimes a biopsy is needed to confirm the diagnosis. Fecal flotation to diagnose a stomach worm infection is often unreliable as the eggs are difficult to locate under the microscope.

Although the causative agent of the summer sore is the larvae of the stomach worm, there are rarely clinical signs associated with the gastric infection. However, deworming with an ivermectin based product will prevent continued shedding of the larvae into poop. Fly control is also very important in reducing the risk of larvae entering any skin lesions. Regular cleaning of the stables in combination with insect repellents, fly masks and/or sheets can be helpful in reducing the number of flies carrying larvae.

Treatment of summer sores can require extensive care with various topical, oral, and injectable medications. Summer sores with larger amounts of proud flesh may have to be removed or debrided surgically so that topical treatments penetrate the tissues more effectively. Wrapping of the affected area, if possible, may also be used to prevent reinfection of the wound.


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