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Medical Index
First Posted: May 23, 2010
Jun 19, 2012

Summer Sores, Jack sores, Bursatti, Habroniasis and Draschia megastoma in Horses

by Debora Johnson

Image from "Equus Caballus Magazine", however, website is no longer found.
Habronema infection (known as habronemiasis) commonly manifests as "summer sores," ulcerations of moist areas of horse's skin. Photo by Cookie Originals.

Flies, flies, flies! Summer has arrived and so have those nasty flies. So, what are summer sores and what causes them? Yes, it is the common house fly and stable flies that are the intermediate hosts for the parasites that cause summer sores. Summer sores or habroniasis/Merck Vet Manual is a condition that affects horses. A parasitic larvae known as nematodes is the cause of summer sores.

Summer Sores
The adult habronema lives in the stomach of the horse.
Photo by Andrzej Polozowski

"...Parasites that use flies as intermediary transportation to their horse hosts include the skin-aggravating Habronema and Onchocerca - Scroll down page to Neck Threadworms and the eye-invading Thelazia species (added: A genus of spiruroid nematodes that inhabit the lacrimal ducts and surface of the eyes of various domestic and wild animals but rarely of humans). Habronema hitchhike on biting stable flies and houseflies to gain entry through wounds or while flies are imbibing fluids from the horse's lips and nostrils. The larvae that mature in the horse's stomach after being swallowed aren't particularly troublesome, but those infecting wounds inhibit healing and encourage excessive tissue growth in the open wound. This reaction may in part be allergic, for the lesions tend to recur on those infected parts of the body every summer, hence their common name of 'summer sore.'

Midges pass along the thread-like filarial worms called Onchocercae. Though most horses tolerate these skin invaders without incident, some sensitive hosts develop skin eruptions on the head, neck, belly and chest, which, in turn, invite further fly attacks. Though the lesions heal rapidly, they frequently recur; dry, scaly skin shedding dandruff-like particles, particularly along the mane and withers, belly and chest, is the most common sign of Onchocercae infection and is easily confused with Culicoides itch. Eye worm probably afflicts a third of the world's horses, yet it does surprisingly little damage. House-and face flies act as vectors for this parasite, which normally remains in the inner corner of the eye but can sometimes be seen crawling over the eyeball itself. The parasite causes tearing, and the discharge may attract even more flies and subsequent eye infections...." Insect-borne Equine Diseases

Life Cycle

Flies ingest the immature larvae and act as hosts for these parasites. The flies deposit these larvae on the horse's muzzle, skin, etc., while biting the horse. The horse eats the larvae and the adult nematode lives in the stomach of the horse, produces more larvae, the passes the larvae out in manure. Maggots (immature flies) feed on the manure thus ingesting the larvae, and the cycle repeats.


  • Skin Ulcers/lesions on the legs, penis, sheath, eyes, around the muzzle and on the skin or any open skin wound.
  • Ulcers/lesions appear in the spring and summer/temperature remains above 70 degrees F.
  • Lesions may develop at mucous membranes of body openings like the eye or the urethra
  • Ulcers go into remission in the winter
  • Horse is itchy
  • Horse bites or rubs the sores
  • Sores have a hard nodule/bump like feel
  • Proud flesh or granulated tissue is present on sores
  • Tiny yellow granules are often visible/
  • Tearing in the eyes
  • Squinting in the eyes
  • Difficulty urinating if on the penis or sheath


  • Visual appearance of ulcer or lesion
  • Biopsy
  • Systemic steroids such as Azium
  • Topical steroid ointments
  • Ivermectin
  • Fly control on property
  • The use of fly sheets on horse
  • Fly masks
  • Fly strips
  • Use Thuja Ointment on lesions to prevent larvae infestation
  • Fans in stalls
  • Use feed supplements containing organophosphates. This prevents fly larvae from surviving in the manure.
  • Formulations of trichlorfon, dexamethasone, and DMSO.
    Always consult your vet first.

Any horse can get summer sores. None are immune, however, some are more predisposed than others. It is also important to know that the stomach worms (Habroniasis) can be passed along to other horses by the life cycle of the worm. As the cycle of the parasite recycles the flies can carry the larvae and vector it to other horses as the fly bites and draws blood, passing on the larvae. Therefore, it is really important to control this parasite and do your best to eradicate it.

For More Information:

Veterinary Corner 08/01: Summer Sores (Habronemiasis)

Medical Index