What is an ectoparasite? An ectoparasite is a parasite that lives on the skin surface of an animal–in this case, the horse. The primary ectoparasites of horses are houseflies, stable flies, mosquitoes, and, to a lesser extent, horse and deer flies.


The common housefly (Musca domestica) is by far the most common pest for horses. Houseflies are a nuisance as well as carriers of disease to humans and horses. A housefly in two weeks time may lay more than 1,000 eggs in any number of conducive breeding areas. Horse manure is a favorite larval food of houseflies. Mucous secretions and wounds on horses are a protein sources for the adult housefly. Houseflies feed around the eyes, nose, genital openings, and wounds on a horse. In high fly-populated areas, this feeding can be very annoying and potentially dangerous for the horse.

Housefly problems around horses are usually the direct result of on-site fly breeding in manure, split feed, and wet bedding. Environmental control through good sanitation is essential to minimize housefly populations. Manure and wet bedding should be removed from stalls at least once a week. Leaky waterers should be fixed, and muck holes in stalls should be filled in and kept dry. Be sure to clean around the edges of feed bunks and under fence lines and other protected places that may provide a fertile larval breeding ground.

Stable Fly

The stable fly is the most common blood-feeding fly pest for horses. The stable fly (Stomoxys calcitrans) resembles the housefly but has a slender tubular beak that it uses to pierce the skin and suck blood. Controlling stable flies involves weekly disposal of manure and other breeding sources, such as urine-soaked straw, moist decaying vegetation, and compost piles.

Stable flies usually feed on the horse’s lower front legs. Horses will stomp their feet in reaction to the fly’s painful bite. Foundered or lame horses that can’t stomp are especially prone to attack. Proper composting or removal of old bedding, wet hay, manure and other moist organic debris is vital in reducing stable fly breeding areas. Adult stable flies can travel considerable distances, resulting in large numbers of flies appearing at horse facilities that have minimal stable fly breeding areas. On-site treatments can be applied by spraying potential fly nesting sites with a residual insecticide and sticky traps.


In addition to their annoying bites, mosquitoes are serious disease vectors. Mosquitoes can carry West Nile virus, malaria, and encephalitis. All mosquitoes must have water in which to complete their life cycle, so breeding sites should be eliminated or treated with environmentally sensitive pesticides to control the mosquito larvae. Only the female mosquito bites to obtain a blood meal. She may live as long as three weeks during the summer or many months over the winter in order to lay her eggs the following spring.

Horse Fly and Deer Fly

Both the horse fly and deer fly are biting flies that are annoying to horse. Horse flies have a bite that is relatively painful, and most horses will attempt to get away from a horse fly if it lands on them. Horse flies are about the size of your thumb and make a loud buzzing noise when flying.

Craig Wood, University of Kentucky