Merck Manual

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Professional Version

Horse Flies and Deer Flies

By

Jan Šlapeta

, MVDr, PhD, GradCertEd (Higher Ed), Sydney School of Veterinary Science, The University of Sydney

Last full review/revision Aug 2022 | Content last modified Sep 2022

Tabanus spp (horse flies) and Chrysops spp (deer flies) are large (up to 3.5 cm long), heavy bodied, robust dipterans with powerful wings and very large eyes. They are swift fliers. These flies are the largest in the dipteran group, in which only the females feed on vertebrate blood. Horse flies are larger than deer flies; many horse flies are highly colored. Deer flies are medium sized; they have a dark band passing from the cranial to the caudal margin of the wings and a yellow to brown abdomen with black patches and longitudinal bands.

Adult horse flies and deer flies lay eggs in the vicinity of open water. Larval stages are found in aquatic to semiaquatic environments, often buried deep in mud at the bottom of lakes and ponds. The developing larval stages have sharp mandibles and are predatory on smaller vertebrates and invertebrates; they will even lightly nip at the toes of humans wading in shallow water. This is an unusual dipteran fly in that both larval stages and adult flies are capable of inflicting pain as they bite. Their development takes 4–5 months, even longer in low temperatures, and larvae can hibernate.

Pathology of Horse Flies and Deer Flies

Adults are seen in summer, particularly in sunlight. Adult females of both species feed in the vicinity of open water and have reciprocating, scissor-like mouthparts, which they use to lacerate tissues and lap up the oozing blood. They consume 0.1–0.3 mL of blood at a single feeding. Bites are painful and irritating, giving rise to weals. These flies feed primarily on large animals, such as cattle and horses, which become restless when the flies are present. If animals are in harness they become unmanageable in the presence of horse flies. Site preferences include the underside of the abdomen around the navel, the legs, or the neck and withers. Horse flies and deer flies feed a number of times in multiple feeding sites before they become replete. When disturbed by the animal’s swatting tail or by the panniculus reflex, the flies leave the host, yet blood continues to ooze from the open wound. These flies may act as mechanical transmitters of anthrax Anthrax in Animals Anthrax is a noncontagious zoonotic disease caused by the spore-forming bacterium Bacillus anthracis. Anthrax primarily affects domestic and wild herbivores. Clinical features, characterized... read more Anthrax in Animals , anaplasmosis Anaplasmosis in Ruminants Anaplasmosis is a tickborne disease of ruminants caused by intracellular bacteria that infect red blood cells, causing fever and anemia. Diagnosis relies upon Giemsa-stained blood smears and... read more Anaplasmosis in Ruminants , tularemia Tularemia in Animals Tularemia is a bacterial septicemia that infects >250 species of wild and domestic mammals, birds, reptiles, fish, and people. It is listed as a category A bioterrorism agent because of the... read more Tularemia in Animals , and the virus of equine infectious anemia Equine Infectious Anemia Equine infectious anemia (EIA) is a noncontagious infectious disease of equids caused by a virus of the same name. Clinical outcomes range from subclinical to a range of signs of variable severity... read more Equine Infectious Anemia . Oozing blood attracts other flies that become mechanical vectors of these infectious diseases as well.

Diagnosis of Horse Flies and Deer Flies

These flies can be identified by their large size, powerful wings, large compound eyes, and lacerating scissor-like mouthparts. Species identification of intact adult and larval horse and deer flies is best left to an entomologist.

Treatment and Control of Horse Flies and Deer Flies

  • Horse flies and deer flies are the most difficult of all bloodsucking flies to control

  • Animals should be moved away from horse fly and deer fly habitats

Horse flies and deer flies are the most challenging of all the bloodsucking flies to control. Many of the adulticide compounds used for other biting flies can kill both horse and deer flies. However, because these flies are intermittent feeders that alight on the host for only a short time, they may not be exposed to these compounds long enough to be affected. Thus, larger doses of the compounds may be required.

Horse fly traps have been effective when used around cattle confined to manageable areas. For production animals, pyrethroid pour-ons function as limited repellents. Self-application techniques are usually not effective for horse and deer flies.

Manipulation of these flies’ aquatic habitat has been attempted, such as by removing unnecessary woody plants from residential areas or draining wet areas. Application of insecticides in the water may have detrimental environmental effects.

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