Summer Sores

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Also Known As

Itchy or ulcerative sores caused by parasites


Summer sores, which may occur any time of the year, but become pronounced during warm months, originate when biting insects leave the larvae of stomach worms in their bite wounds. Two different types of stomach worms, Habronema muscae and Draschia megastoma, also known as Spurids are responsible for the larvae which the flies carry in their mouth parts.

When a fly feeds around a moist area, such as the mouth, the eyes, the prepuce, or a wound, the larvae are passed to the horse where they cause inflammation and open sores. The pus and moisture attract more flies, perpetuating the cycle of infection and irritation.

The larvae may also enter the gastrointestinal system and can cause gastric inflammation or ulcers if a number of larvae are present. In addition, they can migrate through the nose and end up in the lungs where they form cysts.


  • Open sores that attract flies
  • Lesions that enlarge and spread slowly
  • Hard nodules of raised, red-brown tissue
  • Nodules on moist areas of the skin, especially around eyes, mouth, and lining of sheath and urethral opening of penis
  • Lesions and nodules that bleed, ulcerate, and weep yellowish tissue fluid


Summer sores are caused by Habronema and Draschia larvae that are deposited on skin abrasions on the horse. Carried by flies, the larvae cause inflammation and skin lesions that are often moist with pus and attract more flies.


Fly control and a strict deworming program are the best prevention. Larvae in the summer sore lesions, and adult worms in the horse's stomach, can be controlled with a worming paste or liquid prescribed by a veterinarian. Fly repellents, fly masks, and other protective measures to keep flies away from the horse work well. Stable and yard hygiene is essential.


Once a horse has summer sores, the wounds should be kept clean and dry. Applications of antiseptic cream or spray will help repel flies from wounds that are in the healing process. Veterinarian treatment to remove large nodules, especially around the eyes or sheath, are usually necessary. Veterinary advice should be sought for both prevention and treatment of summer sores.

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