Surface parasites on horses, whether they are introduced directly into an open wound or make their way to the exterior following complicated internal pathways, fly-borne parasites commonly contribute to chronic surface lesions such as Warbles and other infections.
Pesky horse flies - Annoying and infectious
To protect their horses, owners should have an on-going fly control program based on minimizing the fly population by getting rid of or covering up their favorite breeding environments.
© 2012 by Carien Schippers
Blowflies (Calliphoridae) and screwworm flies (Callitrogae) lay eggs directly on wounds, and the hatching maggots invade the injured tissue and feed on it. If invaded wounds are left untreated, the severe lesions and subsequent infections can be deadly.
After warble flies lay their eggs on an animal's skin, the hatched larvae bore directly into the skin and then migrate to tissues near the backbone and into the esophagus. The larvae remain dormant for months until the onset of warm weather signals them to migrate to the skin of the horse's back.
The tissues around the larvae swell, creating "warbles" or lumps complete with minute holes through which the larvae breathe. Eventually the larvae escape through these openings, drop to the ground and burrow into the soil, but occasionally warbles mineralize instead, creating permanent nodules known as occult sarcoid tumors. Rarely, warble-fly larvae also migrate to the brain, where they cause acute neurological disease and death.
Parasites that use flies as intermediary transportation to their horse hosts include the skin-aggravating Habronema and Onchocerca and the eye-invading Thelazia species.
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 sores."
To protect their horses, owners should have an on-going fly control program based on minimizing the fly population by getting rid of or covering up their favorite breeding environments. Removing manure daily and keeping it dry by spreading it will eliminate a major source of pesky flies.
Read to learn more about Warbles and other abcesses in horses.