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Flies and Mosquitoes of Horses

By

Charles M. Hendrix

, DVM, PhD, Department of Pathobiology, College of Veterinary Medicine, Auburn University

Last full review/revision Oct 2019 | Content last modified Nov 2019
Topic Resources

Flies are winged insects that are usually just an annoyance. However, they can transmit disease and cause problems in animals. They belong to a large, complex order of insects called Diptera. Flies vary greatly in size, food preference, development, and habits. As adults, flies may feed on blood, saliva, tears, or mucus. They also spread bacteria, viruses, and parasites.

Biting flies feed on animal blood. This group includes mosquitoes, black flies, sand flies, biting midges, horse flies, and deer flies. Though the bites can be painful and may bring on allergic reactions, biting flies are usually not dangerous and are just a nuisance unless they are extremely numerous in the horse’s environment or transmit a disease. Many of these flies, including black flies and mosquitoes, will bite both animals and humans.

Some common flying pests of horses

Some common flying pests of horses

Nonbiting flies include those that do not feed on blood and do not actually bite the host animal while feeding. Instead, these flies feed on bodily secretions. Both biting and nonbiting flies can transmit diseases to horses and other domestic animals.

Finally, some flies have larvae that may develop in the subcutaneous tissues of the skin or organs of animals, producing a condition known as myiasis (fly strike). The larvae, or maggots, may be free-living or may be parasites of the host animal.

The flies and mosquitoes that are most commonly involved in transmitting or causing disorders that affect the skin are discussed below.

Biting Midges (Gnats, No-see-ums)

These tiny insects are often called gnats and are sometimes known as “no-see-ums.” There are several species of the genus Culicoides. All are associated with habitats in and around bodies of water, such as the mud or moist soil around streams, ponds, and marshes. They can inflict painful bites and suck the blood of both humans and animals, including horses. Midges can transmit the parasite that causes onchocerciasis in horses.

Midges fly only during the warm months of the year and are most active before and during dusk. They can cause intense irritation of the skin. The preferred feeding site for biting midges is usually on the back and belly; however, this varies by species of midge. Horses often become allergic to the bites. Allergic horses scratch and rub the affected areas, causing hair loss, abrasions, and thickening of the skin. This condition is known by various names around the world: culicoid hypersensitivity (in Canada), Queensland itch (in Australia), Kasen (in Japan), sweat itch, sweet itch, summer dermatitis, and seasonal dermatitis.

Your veterinarian can provide treatment as needed for the midge bite wounds and sores caused by scratching and rubbing. Veterinarians and extension insect specialists can recommend approved midge control methods and products. Keeping your horse inside a stable during dusk will also help reduce the animal’s exposure to midges. Because they are weak fliers, air movement also reduces exposure to midges. Some owners have had success using fans in stables, fine screens on windows and doors, stable blankets, and topical applications. Although they are not approved for use in the United States, fly repellent ear tags attached to manes and tails can also be effective.

Black Flies

There are more than 1,000 species of black flies. They feed on humans and all groups of animals. Most black flies are small, tiny enough to slip through the mesh of many screens. They are most numerous in north temperate and subarctic zones, although there are tropical and subtropical species as well. Larvae are usually found in swiftly flowing, well-aerated water. In some cases, swarms of these flies will attack, inflicting large numbers of painful bites and causing whole-body reactions.

Because the female black flies have tiny, serrated mouthparts, the bites are often more painful or itchy than the bites of other flies and are more likely to become infected. There are occasional animal deaths from black fly bites, usually involving attacks by swarms of black flies. These rare deaths occur either because the animal is unusually sensitive to black fly bites or because the animal was attacked by a very large number of the flies. If the animal survives a mass black fly attack, it usually recovers quickly. Diagnosis is by appearance of bite wounds on the animal and, in some instances, the presence of the offending insect.

Control of black flies is difficult due to the distances (up to 11 miles [18 km]) the adult flies can travel. Some control is possible by using insecticides on the horse as directed. Stabling horses during daylight hours when the flies are active can provide some protection. Area-wide control for black flies is usually best done by city, county, or other government agencies.

Buffalo Flies

Buffalo flies are primarily a pest of cattle and water buffalo, but they occasionally infest horses as well. They are dark in color and about half the size of an ordinary house fly or stable fly. Thus far, they have not been found in the US. They are native to north Australia and New Guinea and can be found in parts of southern, southeastern, and eastern Asia as well as Oceania. Buffalo flies are not found in New Zealand. Animals infested with buffalo flies have blood loss and irritation from the bites. Because these flies stay on the infested animal, diagnosis is usually through finding the flies on the animal. The bite wounds are visible and may be infected.

Insecticides should be avoided in controlling buffalo flies. Many of these flies have developed resistance to the commonly used insecticides. To date, the most effective controls are the walk-through buffalo fly traps developed in Australia.

Eye Gnats

The eye gnats or eye flies are very small flies that congregate around the eyes. Some species are attracted to the genitals. They feed on mucous, pus, and blood. Although adult flies are present throughout the year in some regions, they only bother horses from spring through fall. During the peak months, they are noticeable in the early morning and late afternoon. They enjoy the deep shade, such as among densely planted shrubs or in the shade of a building.

These gnats do not bite. However, their mouth parts have spines that can cause small scars that open the skin to disease-causing organisms. They are persistent and, if brushed away, quickly return to continue engorging themselves. Large quantities of eye gnats can stress an animal. Diagnosis is by appearance of characteristic wounds or scars on the skin and the presence of eye gnats on the animal.

Insect repellents, such as those recommended for mosquitoes, can provide temporary relief from eye gnats. Be sure to follow veterinary instructions when using repellents. Community-wide mosquito control programs also reduce the number of eye gnats in the area. However, more adult gnats invade the area after the insecticide disperses.

Face Flies

Face flies are so named because they gather around the eyes and muzzle. Adult face flies are similar to house flies. They usually affect cattle but can also affect horses. In general, if a medium-sized fly is found feeding around the eyes and nostrils of a horse, it is most probably a face fly. They may also be found on the withers, neck, chest, and flanks. Face flies do not bite. Their mouthparts are adapted for sponging up saliva, tears, and mucus. However, they follow blood-feeding flies. After a biting fly has fed, the face flies move in and lap up the blood and body fluids on the host’s skin. Face flies are found on animals that are outdoors. They usually do not follow animals into barns.

Feeding around the host’s eyes causes irritation that stimulates the flow of tears, attracting even more flies. Because face flies have small, rough spines on their sponging mouthparts, even a few flies can cause irritation and damage to the eyes of the host. Face flies can also transmit diseases and parasites. Diagnosis is by finding the flies on the animal and finding the characteristic bite wounds especially near eyes, ears, and noses.

Control of face flies is difficult. Much research has gone into developing products and application techniques that are safe for horses but fatal to face flies; however, progress has been slow. Face flies do not like to be inside, so stabling horses can provide some protection.

Filth-breeding Flies

This group includes the common house fly (Musca domestica) and many other species, including blow flies, bottle flies, and flesh flies. Animal feces attract large numbers of filth-breeding flies because animal wastes offer an ideal location for the eggs and larvae of these flies. While these flies do not suck blood, they use their sponging mouthparts to suck semiliquid food. Thus, any liquid or semiliquid body secretions will attract these flies. They are “vomit drop” feeders and fly from feces to food, spreading bacteria on their feet and within the stomach contents they leave behind.

Good sanitation programs can go a long way toward reducing the number of filth-breeding flies. Make sure all manure is removed quickly; it should not be allowed to accumulate. If manure handling is done on site, it should be handled in a manner to reduce fly breeding. If large numbers of flies are seen, consult your veterinarian regarding the use of insecticides, fly baits, and other control measures appropriate for your situation.

Fly Strike (Myiasis)

In addition to spreading disease, filth-breeding flies may lay eggs in skin wounds that have become contaminated with bacteria or in a matted hair coat contaminated with feces. The larvae (maggots) develop in and around the wound and can tear the skin to obtain more food. This type of infestation is known as myiasis or fly strike. Finding maggots in a sore or wound is the normal method of diagnosis. If precise identification of the fly is needed or desired, removed maggots can be sent to a laboratory for identification. Because the first maggots in a wound often create favorable conditions for other flies, the strike site may contain maggots from more than one type of fly.

Certain species of filth-breeding flies are known for their larvae, which are called screwworms (so named because their shape resembles a wood screw). There are several types of screwworms that can affect horses, including primary or New World screwworms (found in Central and South America and the Caribbean), and Old World screwworms (found in Africa, India, and Southeast Asia). None are currently found in the United States.

Treatment of fly strike involves removal of the maggots, cleansing and removal of dead tissue from the strike site, and medication to control infection and reduce the horse’s discomfort. If your horse develops a screwworm infestation, it must be reported to appropriate state and federal authorities in the USA.

Horn Flies

Horn flies (Haematobia irritans) are a common pest of cattle in Europe, North Africa, Asia Minor, and the Americas. However, they will also feed on horses. The flies themselves are about half the size of a stable fly and dark in color. They have a bayonet-like mouthpart that juts out from the head.

The common name for these flies comes from the fact that they often cluster around the base of the horns of cattle. They also cluster around the shoulders, back, and sides of an animal, all areas where tail switching is usually ineffective. Horn flies spend their entire lives on their host except for the period when females leave to deposit their eggs in fresh feces. In some warmer climates (such as Florida or far southern Texas), these flies reproduce actively throughout the year.

Horn flies feed frequently, up to 20 times a day. They suck blood and other body fluids. Horn fly bites and wounds are painful and cause blood loss and irritation in afflicted horses. Infested horses may lose weight and show painful lesions wherever the flies cluster. Diagnosis is by finding the bite wounds and the flies on the horse.

Treatment of the bite wounds may require antibiotics to control bacterial or other infections. Relief from horn fly infestation will encourage normal food consumption and allow the animal to recover from any blood loss.

Controlling horn flies often involves the use of chemical liquids and sprays. Because horn flies generally prefer to feed on cattle, keeping horses separated from cattle and cow manure may help reduce their exposure.

Horse Flies

Horse flies are large, heavy bodied, robust insects with powerful wings and very large eyes. They may exceed 1 inch (3 centimeters) in length. Many are highly colored. The females feed on animal blood. These flies can transmit such diseases as anthrax, anaplasmosis, tularemia, and the virus of equine infectious anemia.

Adult horse flies lay eggs in the vicinity of open water. Larval stages develop in aquatic to semiaquatic environments, often buried deep in mud at the bottom of lakes and ponds. Adults are seen near water in summer, particularly in sunlight.

Adult females have scissor-like mouthparts that they use to slice into the skin and lap up the oozing blood. Mass horse fly attacks can cause significant blood loss in a horse. Bites are painful and irritating. Horses become restless when the flies are present. Flies usually attack the underside of the abdomen around the navel, the legs, or the neck and withers. When disturbed by the animal’s swatting tail or by the muscle twitch reflex, the flies leave the host, yet blood continues to ooze from the open wound. Secondary infection from bacteria or other parasites is possible. Horse fly bites are diagnosed by the presence of the flies near the horse and the characteristic bite wounds.

Horse flies are the most difficult to control of all of the blood-sucking flies. A number of insecticides will kill horse flies. However, because these flies are occasional feeders that land on the horse for only a short time, the flies may not be exposed to ordinary doses of fly control products for long enough to kill them. Larger doses of the insecticide may be required. To protect your horse, follow the advice of your veterinarian for the use of fly control insecticides. Horse fly traps are effective when used around horses confined to manageable areas. Insect repellants are only slightly effective in discouraging horse flies around individual animals.

Mosquitoes

Mosquitoes belong to the family Culicidae. They are tiny and fragile but possibly the most voracious of the blood-feeding insects. About 300 species have been described worldwide; about 150 species of mosquitoes are found in the temperate regions of North America. Only adult female mosquitoes are blood feeders.

Although they are known for spreading such diseases as malaria, yellow fever, dengue, and elephantiasis, mosquitoes are also a host for canine heartworm, and they pose a concern for horse owners because they spread viral encephalitides, including Western and Eastern equine encephalomyelitis and the West Nile virus.

Adult mosquitoes are rarely found on the animals; however, the bite locations are usually raised and itchy. Horses may scratch, lick, or rub the location. The bite location can then be further injured and infected with bacteria or other disease-causing organisms. Your veterinarian can guide you in the selection of the most appropriate medication(s) for mosquito bite wounds.

It is difficult to protect your horse from mosquitoes. Insect repellents and insecticides are not widely effective. You can reduce outdoor exposure to mosquitoes by keeping your horse stabled in the early morning or early evening hours, when mosquitoes are most active.

Mosquitoes often lay their eggs on the surface of standing water. Even small amounts of standing water, including that found in old tires, bird baths, watering troughs, and flower pots, can attract mosquitoes. Eliminating sources of standing water or applying appropriate insecticides may help limit the mosquito population.

Stable Flies

The stable fly (Stomoxys calcitrans) is often called the biting house fly. It is about the same in size and general appearance as the common house fly. It has a bayonet-like, needle-sharp mouth. Stable flies are found throughout the world, and in the USA they are found in the midwestern and southeastern states. Adults may live 3 to 4 weeks.

The larvae develop in decaying organic matter, including grass clippings, the edges of hay stacks, and seaweed along beaches. Breeding can occur where hay has become mixed with urine and feces.

Both male and female stable flies are blood feeders, and horses are the preferred hosts. The fly usually lands on the legs, abdomen, or ears, with its head pointed upward and inflicts painful bites that puncture the skin and bleed freely. Secondary infection from bacteria or other parasites is possible. The painful bite, blood loss, and irritation results in stress to the animal. Stable flies stay on the host for only short periods of time. This is an outdoor fly; however, in the late fall and during rainy weather, it may enter barns. Diagnosis is by the presence of stable flies and the appearance of the characteristic bite wounds on the animal.

The best way to control stable flies is by good sanitation practices. Areas along fence rows, under feed bunks, or wherever manure and straw or decaying matter can build up should be kept clean. These provide the conditions in which the larval flies develop. If good sanitation is maintained, chemical control is less likely to be needed. Various insecticides can be sprayed where flies may rest in barns or on fence rows. However, insecticides on animals are not usually effective. Stable flies usually feed only once or twice daily for short periods, thus minimizing exposure to any insecticides.

In cases where stable fly bites have become infected or inflamed, your veterinarian can recommend appropriate medications to make your horse more comfortable and speed healing. Stable flies are mechanical vectors of anthrax, surra, and equine infectious anemia, and they are the intermediate host for Habronema muscae, a nematode found in the stomach of horses. Blood tests may be recommended to check for diseases that can be transmitted by stable flies.

Tsetse Flies

Tsetse flies (Glossina species) are important blood-feeding flies found in parts of Africa. Tsetse flies are the intermediate hosts for trypanosomes that cause fatal diseases in both humans (African sleeping sickness) and domestic animals (nagana). Horses can die from trypanosome infection. The disease brings on a profound lethargy that ends in death.

Control of tsetse flies is critical to the control of nagana and African sleeping sickness. Tsetse fly traps, bush clearing, fly screens, insect repellents, and insecticides are the traditional control techniques. Recently, programs involving the release of sterile male tsetse flies have offered hope for an environmentally friendly and effective control procedure for these flies.

For More Information

Also see professional content regarding flies in horses.

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