Fighting Flies

by Heather Smith Thomas, Salmon, ID

Flies are the most common and troublesome external parasites of horses. During the warmer months of the year no region is free of these pests. Flies cause much irritation, and can also spread disease. Large numbers can result in excessive blood loss. Fly bites can also result in skin allergies due to hypersensitivity reactions. The annoyance from flies can disrupt grazing; horses may run frantically to get away from biting flies, or seek out shady areas to swish and stomp. Flies can also distract a horse when you are riding or working with him; he may pay more attention to flies than he does to you.

Fly control can greatly help your horses, and an understanding of flies, their life cycle and habits can help you control them. Flies go through four stages in their cycle: egg, larva (maggot), pupa and adults. The females need to eat blood before they can produce eggs; blood supplies the necessary protein. Flies locate horses by sight and smell; the wind can carry the scent to them from a long distance.

Stable Flies – The stable fly is found all over the world. It is similar in size (one fourth inch long) and appearance to the house fly and is sometimes called the biting house fly. Gray in color, it can be distinguished from the house fly by the seven dark spots on its abdomen and by the long slender proboscis that sticks up in front of its head. The stable fly usually sits with its head upward. Adult stable flies often rest on vertical surfaces such as fences, walls, trees, or any structures near livestock. They go to horses only long enough to get a blood meal and only stay on a horse while drinking blood. They usually feed on the lower legs, flank, and belly, mainly in early morning or late evening.

Stable flies cause great annoyance to horses and their bites are quite painful, making the horse stomp his feet and kick to get rid of them, and lick at the bite wounds. It takes only five to ten minutes for a stable fly to drink its fill of blood, but it may puncture the skin several times in the process. The bites often bleed freely. Both the male and female stable fly take a blood meal two or three times a day. In large numbers, these flies can cause considerable blood loss and severe irritation. Their bite often results in a raised area on the skin (about 3/16 inch in diameter) with a tiny scab in the center.

The stable fly lays its eggs in rotting hay and straw (especially when contaminated by urine) and in horse manure. A complete life cycle (from egg to egg-laying adult) takes about thirty days. The larval and pupal stages take place in the rotting hay, hatching into adults about twenty-five days after the eggs have been laid. After the adult flies emerge, the females are ready to lay eggs in five to six days. Several generations can develop during the summer.

Warm, moist conditions are ideal for multiplication of stable flies, and large numbers can cause anemia in horses (from loss of blood), irritation and loss of grazing time. Occasionally, large populations of stable flies can actually cause death of a horse.

In areas with large numbers of flies, there may be more than twenty-five flies per horse. This doesn’t seem serious, until you realize that the number of flies observed on a horse at any given time may be only two to three percent of the total feeding on that horse each day.

Stable flies spread several diseases, including anthrax and equine infectious anemia. The stable fly also acts as an intermediate host for Habronema microstoma, a stomach worm of horses, which cause habronemiasis (summer sores).

In northern climates, the stable flies spend the winter in the larval or pupal stage. In warmer climates, the length of each development stage may increase during winter months, but on warm days the flies emerge, and breeding is continuous off and on throughout the winter.

Control of this fly involves removal of rotting organic matter which the flies use as breeding sites. Manure and soiled bedding should be removed daily from stalls, along with any piles of grass clippings or wet hay around the barnyard. Flies can be kept from multiplying in compost piles if the piles are sprayed with an insecticide or larvacide, or covered with black plastic. Repellents are helpful in keeping the flies away from horses. Spraying barns, stables and foliage where the flies rest will reduce their numbers.

Horse Flies and Deer Flies – These flies belong to the tabanid family – the most annoying group of biting flies that attack horses. In most geographic areas the tabanids are the number one biting fly problem. The flies may be black, brown, yellow or gray, varying in size from 3/8 inch to more than an inch in length.

They cause a great deal of worry to horses; their broad blade-like mouth parts cut a deep and painful wound. Blood flows freely from the bite, and the fly laps up blood with a fleshy lobe at the tip of its proboscis. Only the female tabanids take a blood meal; males feed on vegetable sap or juices of soft-bodied insects.

Tabanids are found in all parts of North America, especially where there are large areas of permanently wet ground. Pastures a distance from wet areas have less flies. Female flies lay masses of eggs on leaves of plants growing near water, or on objects that project over water or marshes. The eggs hatch in five to seven days and the larvae drop into the mud where they burrow in and feed on organic matter or the juices of other insect larvae or earthworms.

They stay in the larval stage during summer, fall and winter, then migrate in the spring to drier areas of soil, where they pupate. This pupal stage lasts two to three weeks, then the adult fly emerges. Most species complete just one generation each year.

Soon after emergence, the female fly needs a blood meal. Deer flies attack mainly around the head, neck and shoulders of the horse, while horse flies attack mainly on the legs and abdomen. Because the bite is painful, horses try to dislodge the flies with swishing tails, stamping feet, or biting at the flies. But the flies are very persistent, worrying the horse until a blood meal has been obtained. Some horses have local reactions to the bites, developing lumps in the skin of the chest, flanks and upper legs.

Large numbers of horse flies or deer flies may leave the horse’s hair crusted with blood, and there may be blood dripping from the bites. Horse flies can drink more than their own weight in blood at each feeding (up to .5 ml per fly). Daily blood loss can range from three to ten ounces per horse, not counting the blood that flows from the bite after the fly has fed. Some horses will run frantically when attacked by horse flies.

The tabanids are often mechanical vectors of diseases (carrying blood-borne disease from one horse to another) due to their habit of feeding on one animal and then immediately attacking another. They can thus spread equine infectious anemia (EIA) and equine encephalomyelitis under certain circumstances (when the sick horse is in the fever stage of encephalomyelitis and the virus is present in the bloodstream).

Control of horse flies and deer flies is difficult, unless swampy areas can be drained, or horses kept away from these areas when the flies are most active. Some kinds of fly repellent are helpful. Sprays or wipe-on repellents may protect horses for one to two days. A shed that horses can go into during the warmer part of the day when flies are active (situated as far as possible from marsh areas) can also help, since most kinds of horse flies and deer flies won’t go into a shed or barn – they prefer to be in the sunlight. These flies are most active on hot, sunny days, and less active in cool weather or when the day is overcast.

Horn Flies – The horn fly is one of the most plentiful and widespread biting flies that affect horses and cattle in the U.S. and Canada. They are primarily parasites of cattle, but these small flies can be a nuisance to people and horses when horses live near cattle. As blood suckers, horn flies practically live on the host animal, taking twenty to thirty blood meals per day. The flies may cluster around the withers, topline and underline of the horse, and the constant irritation from the bites may cause skin problems (such as abdominal midline dermatitis).

It is sometimes hard to tell the difference between horn fly skin irritation and cutaneous onchocerciasis (swelling and loss of hair – and loss of skin pigment – due to tiny threadlike worms spread by biting midges). Hypersensitivity reactions to the bites of flies or midges may also cause loss of hair and thickening of the skin along the underside of the belly.

Horn fly bites often leave the skin raw and crusted. These flies are blood suckers, but generally do not transmit disease. Their primary drawback is the worry they cause, interfering with the animal’s grazing. Heavy infestations can cause weight loss or even deaths in cattle, but horses attract fewer horn flies, and extensive blood loss from these flies is not common in horses.

Adult horn flies are only half the size of a house fly and are usually found on the withers, back, neck, shoulders, underline, and around the eyes of the host animal. When disturbed by a swish of the tail or a toss of the head, black clouds of horn flies swarm up, to immediately settle back down onto the animal again. Cattle may have hundreds of horn flies upon them at once. Horses are much more sensitive to these flies (because of thinner skin) and even as few as twenty horn flies may seriously annoy a horse. Horses may develop sores around the eyes and under the belly from horn flies.

Adult flies stay on the host most of the time, leaving only to lay eggs in fresh cattle manure. The eggs hatch within twenty-four hours and burrow into the manure, completing their development within four to five days. Then they crawl to a drier part of the manure or into the soil to pupate, taking about five more days. The adults emerge and seek a host to drink blood, then lay eggs about four days later.

Cold temperatures or lack of moisture will inhibit these flies. In cold climates the flies spend the winter as hibernating pupae. Cattle manure has the proper moisture content for development of the larvae, and although the mature flies can live equally well on cattle or horses, the eggs are generally laid in cattle manure. Horn flies can be carried long distances by strong winds, but are usually spread by movement of cattle; wherever there are cattle, there are horn flies. Insecticides applied to the host animal are effective in getting rid of these flies, because they spend almost all their time on the host. Horn flies can be controlled in cattle by use of insecticide ear tags. Some horsemen fasten these tags to horses’ manes or halters, but this is not a good idea because cattle ear tags are not recommended for horses. But there are some very effective repellents and insecticides that can be applied to the horse’s legs, belly and flanks to get rid of horn flies.

Black Flies, Buffalo Gnats, Sand Flies – These are very small (3/16 inch) gray to black flies that are often present in large numbers after periods of flooding, or in areas with many streams. The larval stages are passed in flowing streams. The flies congregate in swarms and attack all animals, biting the legs, belly and head. The irritation is so great that many animals stampede or mill around nervously, often injuring one another or trampling young ones. Hypersensitivity reactions (allergic response) to the bites can be a very serious problem in some horses.

Black flies cause annoyance and itching, since they feed inside the horse’s ears, on the chest, udder, scrotum, and inside the thighs and under the belly. The bites cause swelling, oozing of blood, and bloody crusts. Swarms of these flies can fly three to five miles to seek a host.

When black flies are bothering horses, relief can be attained by putting horses in a barn or shed during the mornings and evenings when the flies are most active, since the flies don’t like to go indoors. Fly repellents, sprays, wipe-on products, etc., will help, if applied frequently (two or three times a day if necessary). If flies are bothering a horse’s ears, ear nets can help, as can clipping inside the ear and applying petroleum jelly to protect the inside of the ears; the flies will not bite through it.

Midges – Tiny biting midges (also called “no-seeums”) sometimes attack horses in swarms; thousands of these tiny flies may feed on the horse, primarily at night. Their bites cause irritation and hypersensitivity reactions, and the horse may damage his skin with constant rubbing and scratching. The itching dermatitis causes the horse to rub his mane, tail and withers – and the face, chest and belly may also be affected.

The best relief from midges can be given by putting horses indoors before dusk and keeping them in until after dawn. If stabling is not possible, frequent use of repellent sprays or wipe-ons in the late afternoons can help protect the horses. A horse exhibiting allergic reaction to bites should be treated by a vet.

Face Flies – Face flies are similar to house flies in size and color. These flies are native to Europe and Asia, and first appeared in North America in 1952. They are now present over most of the United States. Face flies are common on cattle (and spread pinkeye), and if horses are near cattle, these flies will also bother horses, feeding on the secretions from horses’ eyes and nostrils, and feeding on the blood from bites of other flies.

Control of face flies can be difficult because they stay on the horse only a short time; the rest of the day they sit on vegetation, fence posts and other nearby structures. Like horn flies, they lay their eggs in cattle manure. Insecticides applied to horses’ heads and necks can help keep them from bothering horses, as can fly masks or face screens attached to a halter.

Mosquitoes – These are not flies, but can be a serious nuisance, especially in swampy areas where pools of water or frequent flooding create breeding sites. In most species, the female must have a blood meal before she can lay eggs. Males feed on plant juices and do not bite animals. Eggs are laid on surface of water, or on low ground that will be flooded. Several batches hatch each summer. Strong winds can carry mosquitoes many miles. Snow mosquitoes appear in swarms in mountain and northern regions in the spring; they spend their winter in the egg stage and develop in pools of water when snow melts in the spring.

Mosquitoes play a major role in spread of disease, especially equine encephalomyelitis and some types of filarid worms. They can be controlled by eliminating breeding sites (filling in or draining swampy areas), destroying their larvae by treating breeding grounds with the proper larvicides, and chemically destroying adult mosquitoes. Some insecticides will kill mosquitoes on contact, but there is no preparation that can be applied directly to horses that will give long-lasting relief. Some preparations can be applied daily to give partial protection; pyrethroids can reduce their numbers by about 75 percent. Observing your horses just at dusk will give you a clue as to how many mosquitoes are bothering them.

Sidebar; Summer Sores

Summer sores, called Habronemiasis, are caused by the larvae of a stomach worm of horses, trying to develop in the skin instead of in the stomach. They are brought to the skin by flies. The adult worms lay eggs in the horse’s stomach, which are passed out with the manure. Then the larval stage of the worm is eaten by larvae of the stable fly and house flies and the tiny worms infect the fly. When infected flies feed on skin wounds or eye secretions (or other moist places on the horse such as the sheath), the worms can be transmitted to the skin.

The problem is most prevalent in warm climates. The skin lesions occur in summer after infected flies deposit the worms. But the worm larvae cannot mature and finish their life cycle in these locations. They can live for up to two years in the skin, however, causing problems for the horse.

The skin lesions may appear on any part of the body, but are found most often on the lower legs (where wounds are common), at the corner of the eye, and on the sheath of male horses. At first they look like infected wounds, but they do not heal. The affected area enlarges with granulation tissue protruding outward from the skin surface, reddish brown in color (bloody at times), and often covered with coagulated wound drainage.

After the wound is two or three weeks old it becomes circular in shape, and may have areas of calcification and dead tissue within it. There is usually some drainage, and severe itching. Often the first clue is that the wound is growing larger instead of healing. Some horses develop a hypersensitivity reaction.

If a wound is left untreated in summer, it may develop into a summer sore, and may not heal until cold weather. The skin lesions usually heal in the fall but often reappear the next spring because the worm larvae are still there.

Summer sores may appear on the sheath, because this area attracts flies, due to moisture from urine. Besides the obvious lesion on the sheath, the horse may also spray urine instead of producing a normal stream (due to a lesion on the tip of the penis, obstructing the normal flow).

Summer sores on the legs may grow quite large, interfering with use of the horse until the wound is treated or the lesion regresses. This problem can be prevented by proper care of wounds to keep them clean and keep flies away from them. Ivermectin given to the horse in two doses three to six weeks apart will kill most of the worm larvae in the lesions. Fly control will help reduce the population of house flies and stable flies that spread the worms.

Sidebar; Fly Control

Horses can be helped in their fight against flies by diligent management, and by careful use of insecticides. Shelter can be helpful to reduce the attacks by several types of flies during the peak of the fly season. Screens on stalls, especially if the screens are coated with insecticide, can be beneficial. A fan in the stall may also help. Fringe attached to the halter or a bridle browband can help a horse dislodge flies around his head when he shakes his head. Mesh fly masks can keep flies from landing around the horse’s eyes. Summer fly sheets (open weave cool sheets) can help keep flies from biting on the body.

Manure management and control of wet areas are key ways to limit fly breeding. Remove manure and any wasted feed daily from stalls and pens; spread it thinly for quick drying, or compost it in a covered pile. Make sure there is good drainage in your barnyard, and no moist areas (no leaking faucets or waterers).

One way to break the flies’ life cycle is with fly predator wasps. These tiny nocturnal wasps lay eggs in the pupae of house flies, stable flies, and a number of other flies. The wasp eggs use the fly pupae as nourishment, killing the fly before it can fully develop. The tiny wasps are harmless to people and animals. If you use this kind of “biological” fly control, you must be very careful using insecticides or they will also kill the wasp population along with the flies.

Insecticides (for killing flies) and repellents (for keeping flies away) can be purchased in several forms. Always be sure to use only those preparations that are meant for horses; other types may be harmful. Always follow label directions. If a horse sweats very much after an insecticide or repellent is applied (as he will do if ridden on a warm day), the effects won’t last long, and you’ll need to apply more when you are done with the ride.

Topical spray repellents are used by many horsemen. Most of these contain a small amount of insecticide and may also contain sunscreens, coat conditioners, or ingredients to help keep the repellent on the hair longer. Read labels and find out if the product is meant to be used full strength or diluted. Besides the standard sprays, wipes, and stick (rub on) repellents, there are also strips impregnated with repellents or insecticides that can be attached to a horse’s halter.

In a fly control program, you must become familiar with the life cycle and habits of the fly you are trying to eradicate, so you can attack it at a stage it is most susceptible to the control measure you are using. There are some forms of insecticide that have long-lasting action (residual insecticides) up to six weeks, designed to apply on fly resting areas such as bushes or barn rafters. Fogs and mists can be used daily; they can be expelled into the barn air, or sprayed onto a horse’s body with a hand sprayer. Fly-killing strips can be used in enclosed areas like tack rooms and feed rooms. Sticky baits and fly traps can be used in areas with large fly numbers, but must be cleaned or emptied periodically. Flies can also be killed with electric fly attractors. There are also some chemical larvicides that can be fed to horses; the chemical goes through the horse (with no effect on the horse) and out with the manure, where it kills any larvae that hatch in the manure from flies laying their eggs there.

If a certain type of fly will be a problem all summer, the best control will be obtained if you make an attempt to eradicate that pest early in the season, before it appears in large numbers – rather than waiting until its population is large and widespread. This is especially true when trying to control stable flies. If using insecticide, repeat the treatment before the flies become numerous again. Then there won’t be a chance for a large breeding population to develop. For specific information on any insecticides or fly control method, contact your veterinarian or county extension agent. With proper care, you can keep your horses’ fly problems to a minimum this summer.