Many species of flies of the two-winged type, order Diptera, such as mosquitoes, horseflies, blowflies, and warbleflies, cause direct parasitic disease to domestic animals, and transmit organisms that cause diseases. These infestations and infections cause distress to companion animals, and in livestock industry the financial costs of these diseases are high.Wall, R. (2001) Veterinary Ectoparasites: biology, pathology & control. Oxford: Blackwell Science Ltd, .Taylor, M.A. (2007) Veterinary Parasitology. Oxford: Blackwell Publishing, .Campbell, J.B. (2001) Effects of stable flies (Diptera: Muscidae) on weight gains of grazing yearling cattle. Journal of Economic Entomology 94: 780-783. Andrews, A.H. (1978) Warble fly: the life cycle, distribution, economic losses and control. Veterinary Record, 103: 348-353. https://dx.doi.org/10.1136/vr.103.16.348 These problems occur wherever domestic animals are reared. This article provides an overview of parasitic flies from a veterinary perspective, with emphasis on the disease-causing relationships between these flies and their host animals. The article is organized following the taxonomic hierarchy of these flies in the phylum Arthropoda, order Insecta.Pittaway, A.R. (1991) Arthropods of Veterinary Importance: a Checklist of Preferred Names and Allied Terms. Wallingford, England, CABI Publishing, . Families and genera of dipteran flies are emphasized rather than many individual species. Disease caused by the feeding activity of the flies is described here under parasitic disease. Disease caused by small pathogenic organisms that pass from the flies to domestic animals is described here under transmitted organisms; prominent examples are provided from the many species.

Summary of types of disease associated with types of dipteran flies Disease caused by the feeding activity of dipteran flies is described here under parasitic disease. Disease caused by small pathogenic organisms that pass from the flies to domestic animals is described here under transmitted organisms; these organisms are often of numerous species thus only prominent examples are provided. Feeding by adult flies may cause irritation through acute stress from painful bites, resulting in loss of grazing time and reduced gain in weight.Byford, R.L. (1992) A review of ectoparasites and their effect on cattle production. Journal of Animal Science 70: 597-602. Feeding by adult flies on the blood of their hosts exposes the hosts to pathogenic organisms that are infecting the fly, this can lead to acute disease of the host's blood and other organs.Kettle, D.S. (1995) Medical and Veterinary Entomology. CABI, Wallingford, UK. .Russell, R.C. et al. (2013). The Encyclopedia of Medical and Veterinary Entomology. CABI, Wallingford, UK. . Feeding by adult flies using their sponging mouthparts can also expose the hosts to pathogenic organisms that have contaminated the mouthparts. Larvae of some flies are adapted to feed on the tissues of their host, causing direct pathological damage to organs; this is known as myiasis.Mullen, G.R. (2009) Medical and Veterinary Entomology. Burlington, Academic Press, .Hall, M. (1995) Myiasis of humans and domestic animals. Advances in Parasitology, 35: 257-334. https://doi.org/10.1016/S0065-308X(08)60073-1

Outline classification (families of main veterinary importance)

Class Insecta Order Diptera (two-winged flies) suborder Nematocera Family Culicidae (mosquitoes) Family Ceratopogonidae (biting midges) Family Psychodidae (sandflies) Family Simuliidae (blackflies) suborder Brachycera Family Tabanidae (horseflies) Family Muscidae (houseflies and others) Family Glossinidae (tsetse flies) Family Calliphoridae (blowflies) Family Oestridae (botflies and others) Family Hippoboscidae (louseflies)

(Note that the former suborder Cyclorrhapha is now usually classified as part of the Brachycera.)

Nematocera (suborder) General characteristics

Antennae are usually long, with many similar and symmetric segments. These are small to very small flies, usually of delicate morphology with relatively long legs and wings. Body and wings are often covered in fine scales. Thorax is distinctly humped, abdomen is elongated. All species of veterinary and medical importance are blood feeders, with various types of mouthparts (these variations do not relate clearly to dipteran taxonomy).Smart, J. (1948) A Handbook for the Identification of Insects of Medical Importance. London, British Museum (Natural History).

Lifecycle is a complete metamorphosis with larvae that are parasitic, living in environments such as pools of water, soil, and streams. A complete metamorphosis is illustrated by the photograph of Stomoxys eggs, larvae, and adult (Stomoxys are in suborder Brachycera). Only females feed on blood, taking a large meal to support production of several hundred eggs, followed by several more cycles of blood meal followed by egg laying.Lehane, M.J. (1991) The Biology of Blood-sucking Insects. London, Harper Collins, .Edman, J.D. (1971) Host feeding patterns of Florida mosquitoes. 1: Aedes, Anopheles, Coquillettidia, Mansonia and Psorophora. Journal of Medical Entomology, 8: 687-695. doi: 10.1093/jmedent/8.6.687. Males feed on plant nectar and similar sources of sugars.

Disease relations

Nematocerans are very important as transmitters of viruses, protozoa, and nematode worms. Also, they are often important for biting stress when in large numbers, and may cause allergic reactions at their feeding sites. A typical pathway of transmission of organisms by blood-feeding flies is shown below as a virus transmitted by an adult fly that feeds repeatedly on several hosts (see diagram for Biological transmission, and photograph of Culicoides). Insects that transmit pathogenic organisms are commonly known as vectors. The diagram shows what is known as biological transmission (= developmental transmission) because it is necessary for the transmitted organism to develop in the fly to the state where the organism is at an infective stage of its lifecycle.Mellor, P.S. (1995) The transmission and geographical spread of African horse sickness and bluetongue viruses Annals of Tropical Medicine and Parasitology 89: 1-15. The repeated cycles of egg laying and feeding of the female fly (= gonotrophic cycle) provides the opportunity for the transmitted organisms to develop, multiply, and be exposed to further vertebrate hosts where they can complete their lifecycle. Only the adult flies are involved in this biological transmission, in contrast to biological transmission by other arthropods such as lice or ticks in which all active stages of the lifecycle feed on blood.

Mosquitoes (family Culicidae) Typical genera are Culex, Aedes, and Anopheles

Morphology, lifecycle, hosts, and feeding The basic structure of dipteran flies is illustrated in the diagram. Veterinary parasitology also covers arthropods in the class Acari, the ticks of domestic animals, and mites of livestock, which have distinctly different structure from arthropods in the class Insecta. However, flies in the order Diptera show clear division of the body into head, thorax, and abdomen with distinct segmentation of the thorax and abdomen. The thorax contains large blocks of muscle that power the single pair of wings. Digestive and reproductive organs fill the abdomen. Also unique to the Diptera is a pair of halteres (derived from wings during evolution) that aid agile flight by these flies.Snow, K.R. (1990) Mosquitoes. Slough, Richmond Publishing Co. Ltd. .Lane, R.P. & Crosskey, R.W. (1993) Medical Insects and Arachnids. Chapman & Hall, London. Mosquitoes have veined and scaled wings, long legs, and long hypodermic mouthparts sheathed in a protective labium (see photograph of Aedes female engorged with blood). Adult females lay eggs in batches on surfaces of stagnant water. Larvae feed on microorganisms and organic detritus in water. Pupation occurs at the surface of the water. Feeding by females is on a wide variety of mammals and birds, whilst males feed on plant sugars.

Parasitic diseases and transmitted organisms

Biting stress can be severe in varied climatic regions (cold northern or tropics) or conditions of much surface water available for breeding where populations of mosquitoes can become dense.Lancaster, J.L. & Meisch, M.V. (1986) Arthropods in Livestock and Poultry Production. Chichester: Ellis Horwood Ltd. . Species of genera Aedes, Anopheles and Psorophora transmit equine encephalitis viruses to horses. Culex, Aedes, and Anopheles species of mosquitoes transmit Plasmodium protozoa that cause types of malaria in birds. Culex mosquitoes transmit West Nile virus between birds and horses; they transmit Rift Valley fever virus to livestock species and humans. The nematode worm Dirofilaria immitis that causes heartworm disease in dogs is transmitted by species of Culex and Aedes.Simon, F. (2009). What is new about animal and human dirofilariosis? Trends in Parasitology 25: 404-409. See photograph of microfilaria stage of Dirofilaria immitus in blood of dog; adult worms in dog's heart grow to several centimeters long. (Mosquitoes are of major importance as transmitters of many types of pathogenic microorganisms to humans causing diseases such as malaria and yellow fever. Plasmodium species causing malaria in humans are exclusively transmitted by Anopheles mosquitoes.)

Biting midges (family Ceratopogonidae) Typical genera are Culicoides and Leptoconops (the term "midge" is also used for dipteran flies that are harmless to domestic animals such as those also known as lake-flies (Chironomidae).

Morphology, lifecycle, hosts and feeding These are small to minute flies (a typical vernacular name is "no-see-ums"). Wings are veined, short, and rounded, usually with distinctive patterns of dark brown on clear backgrounds. Mouthparts are relatively short and complex with three pairs of cutting or slashing elements that create a superficial wound from which blood is imbibed. Lifecycle is similar to that of mosquitoes: female feeds on blood, matures, and lays batch of eggs repeatedly. Males are not blood feeders. The site for larval development is within wet soil and bogs. Hosts of biting midges are wide variety of mammals and birds.

Parasitic diseases and transmitted organisms Severe biting stress to cattle, sheep and horses is caused. Horses suffer from a cutaneous hypersensitivity reaction called sweet-itch, or Queensland-itch that is caused by antigenic components of saliva of biting midges.Schaffartzik, A. (2012). Equine insect bite hypersensitivity: What do we know? Veterinary Immunology and Immunopathology 147: 113-126. Anderson, G.S., et al. (1991) Culicoides obsoletus as a causal agent of Culicoides hypersensitivity (sweet itch) in British Columbia. Journal of Medical Entomology, 28: 685-693. doi.10.1093/jmedent/28.5.685. Poultry may be severely afflicted with biting stress. Species such as Culicoides imicola and Culicoides variipennis transmit bluetongue virus between sheep and cattle (see diagram and photograph above), and they transmit African horse sickness virus between horses and other equids. Culicoides midges transmit Leucocytozoon protozoa to poultry birds.Du Toit, R.M. (1944) The transmission of Blue-tongue and Horse-sickness by Culicoides. Onderstepoort Journal of Veterinary Science and Agricultural Industry. 19: 7-16.

Sandflies (family Psychodidae) The important genera are Phlebotomus (distributed in Africa and Eurasia) and Lutzomyia (distributed in the Americas).Freeman, P. & De Meillon, B. (1953) Simuliidae of the Ethiopian Region. London, British Museum (Natural History). The term "sandfly" is often used to refer to various other types of flies.

Morphology, lifecycle, hosts, and feeding