Flies and Mosquitoes of Dogs

The short version

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.

Biting flies

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, deer flies, and others. 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. Examples include face flies, head flies, filth-breeding flies (such as the common house fly), and eye gnats.

Biting Midges

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.

Black Flies

There are more than 1,000 species of black flies. They feed on humans and many kinds of animals. Most black flies are small (1–6 millimeters long)—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 Cuterebra 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) or the human bot fly (Dermatobia hominis).

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 skin 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. The larvae can also develop in the brain, nose, throat, and eyelids.

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, rarely, an anaphylactic (severe allergic) reaction. Healing may be slow after the larvae are removed by your veterinarian.

Eye Gnats

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.

Frequently asked questions

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