Flies are winged insects that are usually just an annoyance, but they can transmit disease. 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 (see Skin Disorders of Dogs: Flies and Mosquitoes of Dogs).
Biting flies feed on animal blood. This group includes mosquitoes, black flies, sand flies, biting midges, mosquitoes, horse flies, and deer flies. Though the bites can be painful and may bring on allergic reactions, biting flies are usually just a nuisance 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 cats and other domestic animals.
Bot Fly Larvae Infestation (Grubs, Cuterebriasis)
This parasitic infestation of cats is caused by rodent or rabbit bot flies, which are different Cuterebra species. Most species of flies only live on one species of host animal. However, the rabbit Cuterebra fly is a common pest on cats. Rarely, cats might also be infested with warble flies, which are members of Hypoderma species.
Adult Cuterebra flies are large and bee-like and do not feed or bite. Females deposit eggs on stones or vegetation. Cats 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. Swellings with breathing pores are seen around the head, neck, and trunk. The hair is often matted and the skin is swollen. The swellings may be painful and discharge pus.
Definitive diagnosis is made when your veterinarian finds the larvae. Any suspected swellings should be explored carefully by a veterinarian. The affected area should not be squeezed because this may rupture the larva and lead to infection. Healing may be slow after the larvae are removed by your veterinarian.
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 usually prefer horses, deer, and cattle but may feed on any animal, including cats (see Skin Disorders of Dogs: Horse Flies, Deer Flies, and Other Flies of Large Animals).
Maggots (Myiasis, Fly Strike)
House flies, blow flies, bottle flies, and flesh flies lay eggs in skin wounds of any animal (including a cat) that has an infected skin wound. In newborn kittens, 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 cats. 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 cats often have raised, red sores at or near the strike site. Sores may resemble swollen pockets of pus. 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. Maggots from more than one type of fly can be present.
Promptly treating all open wounds and reducing the number of flies are 2 steps that you can take to protect your cat 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 grooming for your cat. Keep your cat's fur clean and do not allow urine or feces to collect on the skin.
Finally, if possible, keep your cat 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).
Mosquitoes belong to the family Culicidae. 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 ensuring 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 diseases such as malaria and yellow fever in people, in veterinary medicine they are also known for spreading heartworm to dogs and cats (see Heart and Blood Vessel Disorders of Cats: Heartworm Disease in Cats).
It is difficult to protect your cat from mosquitoes, especially if the cat spends much time outside. You can reduce outdoor exposure to mosquitoes by not letting your cat outdoors in the early morning or early evening hours when mosquitoes are most active. Sensitive animals, including kittens, should be housed in closed or screened buildings. 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 cats and people. They tend to be active only at night. 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 leishmaniasis, a disease caused by a parasite that infects the cells of capillaries and the spleen (see Disorders Affecting Multiple Body Systems of Cats: Leishmaniasis in Cats).
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. Often sand flies are controlled as a side effect of mosquito control programs.
Stable flies (Stomoxys calcitrans) are often called biting house flies. They are about the same size as house flies and look much like them, but they are avid blood feeders. Horses are the preferred host for stable flies; however, they sometimes feed on cats (see Skin Disorders of Horses: Stable Flies).
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Thomas R. Klei, PhD; David Stiller, MS, PhD; Stephen D. White, DVM, DACVD; Michael W. Dryden, DVM, PhD; Carol S. Foil, DVM, MS, DACVD; Paul Gibbs, BVSc, PhD, FRCVS; John E. Lloyd, BS, PhD; Bernard Mignon, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Patricia D. White, DVM, MS, DACVD