The re-emergence of equine summer sores

Current weather patterns and deworming protocols are likely factors in development.
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Oct 01, 2012

For large portions of the country, the exceptionally mild winter, early and hot spring and hotter-than-normal summer have been both a blessing and a curse for the equine population.

It was a blessing because horses did not have to endure a rough, cold winter. Early spring reduced the winter hay that needed to be fed. Adequate rain in some sections of the Southeast is helping hay production, and the consistently good weather has encouraged more riding, showing and general equine activity.


The long hot summer: This year's weather may have you seeing more cases of summer sores, a type of skin lesion caused by worm larvae transported by house, stable and face flies.
These same conditions, however, have produced overweight horses that grew fatter through the "easy" winter, only to be at risk for developing laminitis on the exceptionally lush and early spring pasture growth. Hot, moist weather after a winter with mild temperatures also vastly increased the number of flies, ticks and related equine parasites.

This year's environmental changes have contributed to problems for horse owners and their veterinarians. One of the most challenging of these issues is the re-emergence of an old problem in horses: summer sores. Recent changes in equine deworming protocols may also be contributing to an increase in this particular problem.

What are summer sores?


Photo 1: The typical red, irritated appearance of a summer sore. Horses' lower legs are prone to all types of scrapes and cuts that can then be infected by Habronema species larvae.
"Summer sores" is a horseman's term for a skin condition caused by larvae of Habronema or Draschia species worms. The adult worms of these species live on the inside wall of the equine stomach and do not migrate internally. In fact, most horses with gastric habronemiasis or Draschia species infestation do not show any clinical signs, and actual damage or impact to an adult horse is thought to be minimal. Heavy infestation in some individuals, however, has been known to cause stomach irritation and can even lead to perforation and possible stomach rupture.

The occurrence of gastric habronemiasis has been reported to vary from 2 percent to 55 percent, depending on the country and study cited, but it is thought to be common enough to keep summer sores high on a differential list for season-associated skin conditions.

The normal cycle for Habronema or Draschia species worms begins when the stomach worms' eggs are excreted in the manure and into the environment. These embryonated or developing eggs are ingested by the larvae of various types of flies. House, stable and face flies are all commonly associated with Habronema and Draschia species egg development since these flies start as larvae in manure piles. Fly larvae grow and develop, and the adult flies serve as both incubators and carriers, allowing the stomach worm eggs to develop into more advanced larval stages inside the flies that then transport them to re-enter the horse. When these infected flies land near a horse's lips, the larvae are released and swallowed by the horse, completing their life cycle.


Photo 2: Moist areas of the body are sites at risk for summer sores. Habrenema species larvae were isolated from this lesion in the commisure of a horse's mouth.
The specific skin condition known as "summer sores" occurs when stomach worm larvae are deposited on injured or irritated skin tissue or mucous membranes (Photos 1 and 2). Moist areas of the body—such as the eyes (e.g., conjunctiva, medial canthus, nasolacrimal duct), commissure of the lips, ears, ventral abdomen, prepuce, penis and urethral process—are at risk. Areas on the limbs, especially from the fetlock to the coronary band, are frequently prone to mild cuts, scrapes and trauma and thus can also be susceptible to summer sores. Parasites (e.g., ticks, flies) also can irritate the horse, and the animal's subsequent rubbing and scratching can damage skin, allowing entry to Habronema or Draschia species larvae.

This is an abnormal step in the usual life cycle for these worms and where problems begin. These "out of place" larvae cannot grow into adult worms in these locations but can induce a severe local inflammatory reaction characterized by intense swelling, ulceration, redness and itching. These lesions tend to grow rapidly and usually cause horse owners to seek veterinary advice within a few days.