Biting Flies

A number of different flies will attack horses. The flies breed in the hot summer months, and this is the time when fly attacks occur. They are more prominent in tropical and subtropical countries than in countries with temperate climates, although even in temperate climates fly attack can be severe. Some flies, the nuisance flies, have mouthparts for lapping fluids and feed on the secretions around the eyes, nose, etc. Other flies, the biting flies, have mouthparts which can penetrate through the skin of the animal for the fly to feed on blood. The bites of the flies can be painful, but also, while feeding, the fly injects into the skin saliva and other substances which, for example, stop the blood from clotting as the fly feeds. Some horses can develop allergic or hypersensitivity reactions to the injected materials manifested by skin lesions, although why only a few and not all horses react in this way is not known. Flies also will transmit diseases either mechanically, simply by physically carrying the organism on the mouthparts from horse to horse, or as an intermediate host in the life cycle of the transmitted parasites.

There are four stages in the life cycle of the flies. The adult female flies lay hundreds of eggs. From these hatch larvae. The fly larvae are maggots, usually white, segmented and somewhat circular, increasing in breadth from front to back. They grow from about 1 mm to 10 mm in length. The larvae then pupate, frequently inside a hardened, protective pupal case. During the pupal stage the tissues of the larva dissolve and are re-formed to produce the adult fly. Once developed, the adult fly emerges from the pupa and is on the wing to attack horses.

[Dappled horse]

Different types of flies have preferred places (manure, water, etc.) in which they breed for the development of the larval stages. Therefore, to control flies adequately in a stable, it is important to know which types of flies are present. Most flies can be readily differentiated by a veterinarian from their size, color, patterns on their wings and structure of their mouthparts.

House Flies

These are yellow-gray nuisance flies, which feed on the secretions around horses' eyes, nose, vulva and prepuce, and on wounds. They are common in stables as they prefer to breed in horse manure. The flies irritate the horse when they are present in any numbers, causing it to shake its head and swish its tail. Their irritation to the eye can cause excess tear formation, attracting other flies, which further damage the eye. In addition to the general control measures, fly veils attached to the browband can help to protect horses' eyes. The flies can transmit internal stomach parasite Habronema.

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Stable Flies

These grayish-colored flies also are common in stables as they prefer to breed in wet bedding contaminated with horse urine and manure. During the day they can be seen resting on sunny walls and windows. Their bites can be quite painful and they will bite a horse several times or switch between horses to complete their feeding. They can be extremely annoying to the horse, which will stamp, kick and swish its tail. The bite often leaves a small nodule and scab and may bleed after the fly has fed, attracting other flies such as the house fly. Some horses can develop crusts on the back, chest and neck when bitten by many flies. Stable flies also will bite humans and dogs. Stable flies transmit a number of bacterial pathogens and parasites such as Trypanosoma and Habronema.

Horn Flies

These small, dark gray flies are normally found biting cattle as they prefer to breed in fresh cow manure, but they can be very irritating to horses grazing with or near cattle. The flies often cluster on the horse's ventral abdomen. Their bites are painful, and they often stay in the same area for quite some time after feeding, causing the horse to become very agitated and repeatedly stamp its feet and kick at its belly. In some horses the flies cause discrete areas of crusts, perhaps ulcers, on the belly and hair loss. Apart from the general measures for fly control, often a thick coat of vaseline over the lesion will provide a barrier to prevent more fly biting.

Horse, Deer and Buffalo Flies

These flies are recognized easily by their painful bite, their large size (up to 1 inch), brownish color and brightly colored eyes. They breed in mud and water and are most commonly seen flying near water on hot, sultry days, although some species can be active in woodlands. Their bites make horses restless, and they rub and bite at themselves to remove the flies, which will then repeatedly return to the same horse or to one very close by to finish feeding. The horses will try to move away from the flies or even stampede. They will stop feeding and lose weight if continually attacked.

The horse flies transmit equine infectious anemia (EIA) between horses, carrying the virus mechanically on the mouthparts. Since the flies will not move far to complete their feeding, separation of horses and separation of paddocks to prevent the flies feeding in quick succession on different horses will prevent transmission of EIA.

Black Flies

These are small, black, hump-backed flies which breed in fairly fast-running water in streams and rivers. They emerge to attack animals, particularly those grazing near water on warm days. Their bite is painful and they also inject a toxic substance, leaving behind a small, painful, fluid-filled blister or nodule. If the flies swarm and hundreds of them bite a horse, they can inject enough toxin to kill the animal.

Fly Control

Application of residual insecticides to premises are frequently used to control both house and stable flies. Longer-lasting residual insecticides provide control for an extended period when sprayed onto sites where adult flies congregate. Flies contact the insecticide when they land on the treated surfaces. Sides of buildings, inside and outside surfaces of stalls, and fences may be potential day or night resting sites for these flies. Observation of the barnyard situation will quickly indicate the favored testing sites for flies.

Knockdown sprays are effective for killing adult flies present at the time of application. The chemicals used for these applications are usually short-residual insecticides that have a quick knockdown and high-contact toxicity. Several types of spray or fogging apparatus may be used. Wind velocities should be low at the time of application and the droplet or particle size should be small (50 to 75 microns) to ensure drift through the corral area. This method requires less application time, but the disadvantage is that it will only kill those flies present at the time of application and thus provides only short-term relief.

Direct application of sprays and dusts to animals may be used in some situations to protect them. Materials used for direct animal applications usually have short-residual activity and this type of application is labor intensive. Other methods of fly control, such as baits, electric grids, and traps, have some limited use for house fly control but are ineffective for the blood-feeding stable fly. Baits may be used effectively for house fly control in enclosed areas. Fly papers, cords, and strips may also help alleviate fly problems in these areas. Such methods are usually ineffective in open areas. Control of immature flies (larvae) is sometimes possible. Usually, the best approach is to remove the potential source of fly production with sanitation practices. When this is not possible, a larvicide can kill the developing flies. A larvicidal insecticide may be applied directly to places where eggs are laid and larvae develop.


  1. Veterinary Notes for Horse Owners. Captain M. Horace Hayes
  2. Equine Science. R. O. Parker

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