Flies are winged insects that are usually just an annoyance, but 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. The order Diptera includes not only the common house fly and many other insects we commonly call flies, but also mosquitoes.
The life cycles of all of the thousands of different flies are divided into 4 stages: egg, larva (flies at this stage look like worms and are commonly known as maggots), pupa (the stage in which the fly is developing inside a cocoon), and adult. Fly eggs are laid in decaying flesh, animal waste, or pools of standing water. The common characteristic of these egg locations is the presence of ample food for the maggots. Flies reproduce and grow rapidly. Depending on the season and weather conditions, a fly may take only 12 to 14 days to go from egg to adult.
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 to dogs unless they are extremely numerous or transmit a disease. Many of these flies, including black flies and mosquitoes, will bite both animals and humans.
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. They can transmit diseases to dogs and other domestic animals.
These tiny (0.04 to 0.16 inches [1 to 3 millimeters]) insects are often called gnats and are sometimes known as “no‑see-ums,” or “punkies.” There are several species. All are associated with aquatic or semiaquatic habitats, such as 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 and dogs. (For a more detailed discussion of midges, see Skin Disorders of Horses: Biting Midges (Gnats, No-see-ums).)
There are more than 1,000 species of black flies. They feed on humans and many kinds 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. In some cases, swarms of these flies will attack, inflicting large numbers of painful bites and even killing livestock.
Diagnosis is by appearance of bite wounds on the animal and, in some instances, the presence of the flies. Black flies are small and can hide in the fur coat. Adult female flies prefer to feed outdoors during daylight hours. Because black flies breed in streams, pets should be kept away from streams, especially during the day, to limit fly exposure. Over-the-counter insect repellents can help keep flies away from pets.
Individual pet owners usually have little control over the presence of black flies. Because it is difficult, expensive, and can harm the environment, area-wide control of black flies is usually best done by city, county, or other governmental agencies.
Bot Fly Larvae Infestation (Grubs, Cuterebriasis)
This parasitic infestation of dogs and cats is caused by rodent or rabbit bot flies, which are different Cuterebra species. Most species of flies only live on one species of animal. However, the rabbit Cu-terebra fly is a common pest on dogs and cats. Rarely, dogs and cats might also be infested with warble flies, which are types of Hypoderma species.
Adult Cuterebra flies are large and bee‑like and do not feed or bite. Females deposit eggs on stones or vegetation. Dogs become infested as they pass through contaminated areas. Infestations are most common in the summer and fall when the larvae enlarge and produce a swelling about 0.4 inches (1 centimeter) in diameter. Boils are seen around the head, neck, and trunk. The hair is often matted and the skin is swollen. The boils may be painful and discharge pus.
Definitive diagnosis is made when your veterinarian finds the larvae. Suspect boils should be explored carefully by a veterinarian. The boil should not be squeezed because this may rupture the larva and lead to secondary infection or an anaphylactic (severe allergic) reaction. Healing may be slow after the larvae are removed by your veterinarian.
Eye gnats or eye flies are very small, only 0.06 to 0.1 inches (1.5 to 2.5 millimeters) long. These tiny flies usually congregate around the eyes, though some species are attracted to the genital organs. They feed by sponging up mucus, pus, and blood. In the desert and foothill regions of southern California, adult flies are present throughout the year; they are most numerous from April through November. During the peak months, they are noticeable in the early morning and late afternoon. Often they are found in deep shade, among densely planted shrubs, or in the shade of a building.
Diagnosis is by appearance of signs on the animal and, in many cases, the presence of the offending insect. Eye gnats look much like miniature house flies. Insect repellents, such as those recommended for mosquitoes, can provide temporary relief from eye gnats. Be sure to follow your veterinarian's recommendations 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.
Horse Flies, Deer Flies, and Other Flies of Large Animals
Horse flies (Tabanus species) and deer flies (Chrysops species) are large (up to 1.4 inches [3.5 centimeters] long), heavy bodied, and robust. They are swift fliers with powerful wings and very large eyes. The females may feed on the blood of any vertebrate animal, including dogs, though these flies usually prefer horses, deer, and cattle. Male flies are never blood feeders; instead they feed on plant nectar and pollen. Compared to other flies, they consume larger amounts of blood at a single feeding. Like other flies, they can transmit bacteria or viruses. Control of these flies is difficult. Some insecticides may be effective but may have to be used in larger than normal doses. Your veterinarian can advise you about the most effective products and dosages for your pet.
Maggots (Myiasis, Fly Strike)
Maggot infestation is also known as myiasis, fly strike, or simply strike. House flies, blow flies, bottle flies, and flesh flies will lay eggs in skin wounds of any animal (including a dog) that has an infected skin wound. In newborn puppies, the healing stub of the umbilical cord is an attractive egg-laying site for flies. Bite wounds are often sites of initial infection in older dogs. Matted hair coats contaminated with feces also attract these flies. Eggs laid in contaminated hair coats produce maggots that move rapidly to any infected wound. Once inside a wound, the larvae quickly invade the surrounding tissue.
Affected dogs often have raised, red sores at or near the strike site. Maggots may be visible in a sore or wound. You should not try to remove the maggots yourself; wound cleaning and maggot removal by your veterinarian is required. In most cases, your pet will have to be sedated or anesthetized for removal of the larvae.
Finding maggots in a sore or wound is the normal method of diagnosis. If precise identification of the fly is needed or desired, 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.
Treating all open wounds and controlling the presence of flies are 2 steps that you can take to protect your pet from strike. To treat open wounds, gently wash the wound with mild soap, rinse well, and then apply a veterinarian-recommended medicated salve. You should carefully trim the fur around the wound to reduce the chance of infection. Check the wound several times a day to be sure it does not become inflamed or infected. Equally important is routine bathing and grooming for your pet. Keep your dog clean and do not allow urine or feces to collect on the animal.
Finally, if possible, keep your pet in a fly-free area protected by screens. To control flies in the area, be sure all garbage and decaying animal matter are removed. All garbage and trash containers should be securely covered. Remove standing water, especially places that accumulate any organic matter (including yard waste). In kennels and yards, feces should be removed and urination areas washed down daily, especially during warm months.
Pseudomyiasis (false strike) occurs when fly maggots have been consumed and are found within a dog's digestive tract. Dogs, cats, and other animals consume the maggots while grooming or when eating flesh infested with the maggots. In most cases, these maggots pass through the animal undigested.
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, but they rarely affect dogs. The screwworms include the primary or New World screwworm (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 of the wound, and medication to control infection and reduce the dog's discomfort. If your pet develops a screwworm infestation, it must be reported by your veterinarian to appropriate state and federal authorities.
Mosquitoes belong to the family Culici-dae. They are tiny and fragile but possibly the most voracious of the blood-feeding flies. About 300 species have been described worldwide, but only about 150 species of mosquitoes are found in the temperate regions of North America.
Mosquitoes often lay their eggs on the surface of standing water. Even small amounts of standing water can attract mosquitoes. You can reduce the number of mosquitoes near your home by being sure that there is no standing water. Eliminate or turn over any container that can hold water and check your gutters to be sure that they run freely; standing water in gutters is an ideal location for mosquito eggs.
Only female mosquitoes feed on blood. They annoy animals and humans, cause blood loss, and transmit diseases. Although they are known for spreading such diseases as malaria, yellow fever, dengue, and elephantiasis in people, in veterinary medicine they are also known for spreading heartworm to dogs and cats (see Heart and Blood Vessel Disorders of Dogs: Heartworm Disease in Dogs).
It is difficult to protect your dog from mosquitoes, especially if the dog spends much time outside. You can reduce outdoor exposure to mosquitoes by not walking your dog in the early morning or early evening hours. Those are the hours when mosquitoes are most active. Sensitive animals, including puppies, should be housed in closed or screened buildings, and the mosquitoes inside killed with safe insecticide. Imidacloprid is a topical drug that can be used on dogs to repel adult female mosquitoes for up to 4 weeks. Mosquitoes are not attracted to light, so “bug zappers” do not help control mosquitoes; they may actually be harmful because they destroy insects that prey on mosquitoes.
Sand flies are most numerous in tropical and subtropical regions. They are tiny (0.06 to 0.16 inches [1.5 to 4 millimeters] long) and have moth-like, hairy wings. Female sand flies have piercing mouthparts and feed on the blood of a variety of warm-blooded animals, including dogs and humans. They tend to be active only at night and, in contrast to black flies, are only weak fliers. During daylight hours, sand flies seek protection in crevices and caves, in vegetation, and within dark locations, including buildings. They breed in dark, humid environments that have a supply of organic matter that serves as food for the maggots.
Evidence of small bite wounds is the usual sign. The flies are rarely found on animals. Sand flies are an intermediate host for visceral leishmaniasis, a disease caused by a parasite that infects the cells of capillaries and the spleen and other organs of humans, dogs, cats, and horses (see Disorders Affecting Multiple Body Systems of Dogs: Leishmaniasis (Visceral Leishmaniasis) in Dogs).
Successful sand fly control is not usually possible with ordinary insecticide spraying because the breeding locations are hard to reach. Removal of dense vegetation helps control sand flies in outdoor environments. Often sand flies are controlled as a side effect of mosquito control programs.
Stable flies (Stomoxys calcitrans) are often called biting house flies. These flies are about the same size as house flies and look much like them, but are avid blood feeders. Both male and female stable flies feed on blood. They are found throughout the world. In the US, they are most commonly found in midwestern and southeastern states. Horses are the preferred host for stable flies, though they are known to feed on the tips of the ears of dogs with pointed ears, especially German Shepherds. Stable flies are known to transmit anthrax, surra, and equine infectious anemia. Good sanitation practices can reduce the stable fly population by up to 90%. Areas along fence rows, under feed bunks, or wherever manure and straw or decaying matter can accumulate should be kept clean.
The tsetse flies (Glossina species) are important blood-feeding flies found in portions of Africa. Tsetse flies are the intermediate hosts for trypanosomes that cause the fatal diseases African sleeping sickness in humans and nagana in domestic animals. Both horses and dogs can die from trypanosome infection. The disease brings on a profound lethargy that ends in death. Dogs show progressive nervous system signs. Infected dogs often lose their sight, howl involuntarily, and die in what appears to be great pain.
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 dangerous flies.
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Patricia D. White, DVM, MS, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; William W. Hawkins, BS, DVM; Thomas R. Klei, PhD; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; David Stiller, MS, PhD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice E. Villalobos, DVM, DPNAP; Stephen D. White, DVM, DACVD