Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance/Mosquitoes and similar

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Mosquitoes and similar flies (Diptera)[edit]

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Characters of parasitic dipteran flies[edit]

Dipteran flies are typical insects. Most species are free-living, but the parasitic species are of great medical and veterinary importance. The wings are one on each side of the middle segment of the thorax. The hind thoracic segment has a pair of modified wings called halteres. These are small knobs on a short stalk that assist flying. At the base of the wings are various extensions of the wing surface, called squamae.The adult body of dipteran flies is divided into an obvious head, thorax and abdomen. The head bears complex mouthparts, sensory palps to assist feeding, eyes and antennae to find hosts and mates. Some types of dipterans that are highly specialized for parasitism (the hippoboscids) either lose their wings when they find a host, or never develop wings. Dipteran flies all have a complete metamorphosis.

Most ectoparasitic dipterans feed on their hosts as adults; but an important group feed on their hosts as larvae. These flesh feeding larvae cause the disease myiasis. Of such myiasis flies the ones most highly adapted for parasitism have no mouthparts in the adult stage; all feeding in the life-cycle is done by the larvae.

Classification within the Diptera is complex. This book provides a simple grouping by sub-Order and Family names. The two sub-Orders are Nematocera and Brachycera. Note that the former sub-Order Cyclorrhapha is now placed within the Brachycera (see articles on Diptera in Wikipedia)[1].

Nematoceran flies - characters of mosquitoes (Culicidae)[edit]

These flies are all typical nematocerans, with long antenna consisting of many similar segments. All mosquitoes are specialized for blood-sucking as adult females. The males feed on plant nectar. Larvae and nymphs inhabit stagnant water. Mouthparts form a long proboscis consisting of a labium as a protective sheath and within this sheath is a bundle of very fine elongated mouthparts that form a flexible piercing and sucking tube. The labium folds up when the piecing mouthparts are in use. Antennae are long. Antennae of females have short fine setae at each segment; male antennae have long fine seta at each segment, appearing like a brush. Legs are very long and thin and wings have small squamae, and small scales arranged above the wing-veins.

Anopheles minimus.jpg
Photograph shows a live female Anopheles mosquito sucking blood from a person's skin; the labium is folded away from the piercing mouthparts which show as the narrow reddish tube. Note the feeding stance of this anopheline mosquito, with abdomen held up. (Photograph by James Gathany).

Mosquito genera of medical and veterinary importance are grouped into two taxonomic types: culicine (of many genera, Culex is typical), and anopheline (mainly the genus Anopheles). The adults of these two groups have typical resting stances. Larvae and pupae are aquatic and of the two groups have different shape and behavior when suspended below the water surface to breathe. Mosquitoes populations can seasonally, or in permanently favorable larval habitats, build up to dense populations. These cause biting-stress to domestic animals and humans that is severe to intolerable, making wide areas of land uninhabitable.

Mosquito-feeding-at-skin.png
Diagram of feeding at skin represents a mosquito piercing dermal capillaries of its host using very fine flexible mouthparts (relative scales not accurate).

Mosquitoes are biological vectors of numerous pathogenic viruses, bacteria, protozoa, and worms between domestic animals and humans. Here only the most prominent pathogens and diseases are emphasized. There are many genera of mosquitoes of potential medical and veterinary importance but their identification is work for a specialist. Only six representative genera are shown here. Studies of the taxonomy and biology of mosquitoes have revealed extremely complex and varied adaptations, often making very difficult the differentiation of species of significant medical and veterinary importance using the traditional morphological criteria [2] [3] [4] [5].

Glossary[edit]

  • Halter = A paired organ in the Diptera, shaped like a knob on a short stalk and situated below and behind the forewings, used as an aid when flying; evolved by modification of the hindwings of precursor insects (see Culicoides photograph and drawing).
  • Humeral pit = A paired depression on the anterior and dorsal surface of the thorax of biting midges (2 on Culicoides).
  • Labium = A component of the mouthparts of insects that in nematoceran flies acts as a sheath for the piercing elements.
  • Myiasis = Infestation of animals with larvae of dipteran flies (see Blowflies ).
  • Palps = Paired sensory organs associated with the mouthparts of invertebrate animals; with mosquitoes their length relative to the antennae are important for identification (9, 10 on Culex).
  • Pre- and post-spiracular setae = Small groups of setae on either side of the anterior spiracle on the thorax (see 2 on Psorophora).
  • Pulvillus = An adhesive pad or feathered setae at the end of legs.
  • Scutellum = A ridge on the posterior segment of thorax of mosquitoes (8 on Culex).
  • Squamae = Flaps as extensions of the wing surface close to the insertion point on thorax (6 on Culex).
  • Radial cell = Part of the wing of midges tightly enclosed by conspicuous veins at the leading edge of wing (3 on Leptoconops).
  • Vein = Fine tubes that support the wings of insects, they inflate to expand the wings after emergence from the pupal stage (6 on Culex)

Culex (Culicidae, sub-Family Culicinae)[edit]

Culex female lateral.png

Characters: female, lateral. 1- Female in resting stance typical of culicine mosquitoes: body forms an arch with abdomen pointing downwards. 2- Scales on abdomen form a dense covering without a distinct colored pattern. 3- Abdomen is blunt ended.

Culex thorax parts.png

Characters: thorax and other parts. 4- Claws on fore-legs of females are simple (claws on males are toothed). 5- Large pulvilli are present next to claws of females. 6- Scales on wing are narrow; they are without a metallic coloration; wings have small squamae at their base. 7- Setae are absent from the prespiracular area and the postspiracular area. (These areas surround the anterior spiracle of the thorax, shown as the dark ovals.) 8- The scutellum has 3 distinct lobes on its posterior margin; each lobe bears a tuft of setae. 9- Female head has short palps. 10- Male head has long palps, and bushy antennae. 11- Larva in breathing position, suspended down from surface of water.

Hosts: Cattle, sheep, horses, birds, reptiles, and humans are infested. Different species have feeding preferences for different groups of hosts, and sometimes these preferences are specific. These flies feed mainly at night-time.

Signs and disease: Irritation and avoidance behavior and dermal hypersensitivity are caused. Biting-stress varies from slight with low numbers feeding through to intolerable with the massive numbers that can occur. Various Culex species transmit West Nile virus between birds and horses, the nematode worm Dirofilaria immitis that causes heartworm to dogs, and several species of Setaria filarial nematodes. Some Culex species transmit Plasmodium protozoa that cause malaria in birds. (Note that transmission of protozoa causing various forms of malaria is not restricted to Anopheles species of mosquito. It is only the Plasmodium species of protozoa causing malaria in humans that are restricted to transmission by Anopheles mosquitoes.) Species of Culex transmit the nematode worm Wuchereria bancrofti, the cause of lymphatic filariasis in humans (which may lead to elephantiasis) [6].

Aedes (Culicidae, sub-Family Culicinae)[edit]

Aedes thorax parts.png

Characters: thorax and other parts. 1- Setae are absent from the prespiracular area but are present in the postspiracular area. 2- Scales on the abdomen give a dense covering: they may form silver colored patterns. 3- Abdomen is usually sharp ended. 4- Scales on wing are narrow. 5- Stem-vein of the wings has at its base a group of long thick setae on the ventral surface. 6- Pulvilli on females are small or like fine setae. 7- Claws on the fore-legs of females and males are toothed.

Hosts: Cattle, sheep, horses, birds, reptiles, and humans. Different species have feeding preferences for different groups of hosts, and sometimes these preferences are specific. These flies feed at night-time.

Signs and disease: Irritation and avoidance behavior and dermal hypersensitivity are caused. Biting-stress varies from slight with low numbers feeding, through to intense with the massive numbers that can occur. Species of Aedes are biological vectors of Equine encephalitis virus, the nematode Dirofilaria immitis causing heartworm in dogs, and species of Plasmodium to birds causing avian malaria. Aedes aegypti is the principal vector of the viruses causing Yellow- fever, and Dengue-fever, in humans; it also transmits Zika virus. Species of Aedes transmit the nematode worm Wuchereria bancrofti, the cause of lymphatic filariasis in humans (which may lead to elephantiasis) [7].

Distribution: Typically Aedes species occur in the tropics and sub-tropics but some species have spread more temperate regions on shipping transports.


Haemagogus (Culicidae, sub-Family Culicinae)[edit]

Haemagogus thorax parts.png

Characters: thorax and other parts. 1- Palps of males are smaller than proboscis. 2- Thorax and abdomen are widely covered with scales of metallic colors. 3- Thorax is without prespiracular or postspiracular setae. 4- Abdomen has a blunt end. 5- Scales on wing veins are narrow. 6- Females and males have fore-legs with toothed claws.

Hosts, disease and distribution: These mosquitoes feed on monkeys and also humans. They transmit the virus of Yellow Fever. They occur in tropical and sub-tropical areas of North and South America.


Mansonia (Culicidae, sub-Family Culicinae)[edit]

Mansonia thorax parts.png

Characters: thorax and other parts. 1- Thorax is without prespiracular setae, but postspiracular setae are present. 2- Thorax has a sparse covering of long setae and broad scales. 3- Abdomen is blunt ended. 4- Wings have broad scales on their veins and these scales may form speckled patterns. 5- Claws of the legs are without spurs and the pulvilli are small or absent.

Hosts and disease: Mansonia is one of the genera of culicine mosquitoes with species that transmit Wuchereria bancrofti and Brugia species of nematode worms to humans, causative agents of lymphatic filariasis in humans (which can lead to elephantiasis) [8].


Psorophora (Culicidae, sub-Family Culicinae)[edit]

Psorophora thorax parts.png

Characters: thorax and other parts. 1- Palps of male are longer than proboscis. 2- Prespiracular and postspiracular setae are present. 3- Abdomen has a pointed end. 4- Scales on body and veins of wings are narrow. 5- Females and males have fore-legs with toothed claws.

Hosts and disease: Horses are one of the hosts of this genus of mosquitoes. Psorophora ferox is one of the species that transmits to horses the virus causing Venezuelan equine encephalitis.


Anopheles (Culicidae, sub-Family Anophelinae)[edit]

Anopheles female lateral.png

Characters: female, lateral. 1- Female in resting stance that is typical of Anopheles, whole body forms a straight line with abdomen held higher than head. 2- Scales on the abdomen are absent. 3- Abdomen is blunt or sharp ended, depending on the species.

Anopheles thorax parts.png

Characters: thorax and other parts. 4- Claws on the legs of females and males are simple. Pulvilli are absent. 5- Scales on wing are narrow and vary in color to form distinct patterns. 6- Setae are absent from the prespiracular area and the postspiracular area. 7- The scutellum has a posterior margin with a slightly wavy or evenly rounded margin and setae are distributed evenly around this margin. 8- Female head has long palps. 9- Male head has long palps with club shaped outermost segment; antennae are bushy. 10- Larva in typical breathing position, suspended parallel to the surface of water.

Hosts: Cattle, sheep, horses, birds, reptiles, and humans are used as hosts. These flies feed at night-time.

Signs and disease: These mosquitoes cause irritation and avoidance behavior, also dermal hypersensitivity. Biting-stress varies from slight with low numbers feeding through to intense with the massive numbers that can occur. Various species of Anopheles are biological vectors of the viruses causing the various forms of equine encephalitis. They also transmit the nematode Dirofilaria immitis causing heartworm in dogs. Species of Anopheles transmit the nematode worm Wuchereria bancrofti, the cause of lymphatic filariasis in humans (which may lead to elephantiasis). The Anopheles gambiae complex of species is an example of various anopheline mosquito species that are adapted to feeding on humans and are the principal biological vectors of those species of Plasmodium protozoa that cause human malaria [9] [10]. (Other genera of mosquitoes may transmit protozoa causing forms of malaria in domestic and wild animals.)

Distribution: This is mainly tropical and sub-tropical, but some anopheline species inhabit cool temperate regions.

Characters of blood-sucking midges (Ceratopogonidae)[edit]

These flies are all typical nematoceran flies, with long antennae consisting of many similar segments. Female midges feed on blood using complex mouthparts with pairs of cutting blades that make a wound down to the capillary blood. Their mouthparts are similar to the much larger tabanid Horse-flies, which is why their feeding is so irritating. Larvae and nymphs inhabit wet soil and bogs.

Culicoides female biting midge.jpg
Photograph shows a female Culicoides biting midge, with its prominent piercing mouthparts and conspicuously patterned wings. A halter protrudes from the posterior profile of the thorax. Lateral view of specimen mounted on a microscope slide.

Wings of blood-sucking midges have a reduced number of veins compared to mosquitoes and there are one or two distinct cells within the pattern of veins at center of the leading edge. These midges are all small, and often called names that reflect that: No-see-ums, Punkies, and so on. However, they are also sometimes called Sand-flies, but to entomologists this name means flies in the family Psychodidae. The genera Culicoides and Leptoconops are the most important to health. Forcipomyia species are also of minor importance [11].


Culicoides (Ceratopogonidae)[edit]

Culicoides female lateral.png

Characters: female, lateral. 1- Antennae of females have 14 or 15 segments. 2- Thorax has a pair of indentations called humeral pits. 3- Wings usually have distinctive patterns of dark grey/brown areas and clear areas. 4- Two radial cells are present. 5- Between the radial vein and the m vein, the r-m cross vein is present. 6- Palps are short and simple. 7- Mouthparts of females have complex cutting and piercing elements.

Hosts: Cattle, sheep, horses, birds, and humans are used as hosts. Numerous species of Culicoides have a fairly narrow range of preferred hosts in tropical regions but some species in cool temperate climates will feed on a wide range of vertebrate animals, as available. These flies feed mainly at night-time.

Signs, symptoms and disease: Irritation, biting-stress and avoidance behavior are caused to livestock and humans alike. Agricultural and forestry workers suffer reduced productivity. Dermal hypersensitivity develops after repeated exposure to feeding Culicoides. This causes much trouble to horses, and is known as Sweet-itch, Queensland-itch, and similar. Numbers of midges can build up to dense swarms around cattle and sheep, reducing their productivity through stress and reduced grazing. [12]. Culicoides imicola is a biological vector of Bluetongue virus between sheep in Africa and Mediterranean countries, C. variipennis transmits this virus in North America. African horse sickness virus is transmitted between equids by Culicoides species in Africa and southern Europe, through to Pakistan [13].


Leptoconops (Ceratopogonidae)[edit]

Leptoconops female lateral.png

Characters: female, lateral. 1- Antennae have 12 to 14 segments. 2- Wings have no r-m cross vein. 3- Wings have a large second radial cell and often a smaller first radial cell. 4- Wings are semi-transparent white, without patterns, and contrasting with black body of midge.

Hosts: Cattle, sheep, horses, birds, and humans are the hosts - as for Culicoides.

Signs and disease: Irritation, biting-stress and dermal hypersensitivity are caused - as for Culicoides.

Distribution: Tropical and sub-tropical regions mainly are inhabited.

Characters of Sand-flies (Psychodidae)[edit]

These flies are all typical nematocerans, with long antennae consisting of many similar segments. Sand-flies are nearly as small as ceratopogonid midges, but structurally are more like mosquitoes, with complex veins in their wings and long thin legs.

The females feed on blood; larva and nymphs inhabit surface of soil. Sand-flies are most important in medical entomology because of their transmission of Leishmania species of protozoa, causing visceral and cutaneous Leishmaniasis in many tropical regions. Additionally, Leishmaniasis is a serious problem for domestic dogs where these flies are abundant. Species in the genera Phlebotomus and Lutzomyia are most important. These genera are closely similar and only Phlebotomus is shown here.


Phlebotomus (Psychodidae)[edit]

Phlebotomus female lateral.png

Characters: female, lateral. 1- Antennae are of typical nematoceran type: long, multi-segmented and covered in fine setae. 2- Wings are long, with a complex pattern of veins and covered in long thin setae. 3- Thorax and abdomen are thickly covered with long thin setae. 4- Males are characterized by their abdominal claspers. 5- Legs are slender and long. 6- Mouthparts consist of a short set of piercing parts and the associated sensory palps.

Hosts: Humans and many species of livestock animals, wild mammals, birds, and reptiles are used as hosts. These flies feed during night-time.

Signs and disease: These flies do not usually feed in large numbers or cause obvious irritation or biting-stress. Phlebotomus and Lutzomyia species are principally important as vectors of many Leishmania species of protozoa that cause cutaneous and visceral leishmaniasis [14].

Distribution: Sand-flies inhabit tropical and sub-tropical regions: Phlebotomus in Africa and Asia, Lutzomyia in the Americas.


Characters of Blackflies and Buffalo-gnats (Simuliidae)[edit]

These flies are nematocerans, with relatively short antennae that consist of many similar and compact segments. These flies are small, compact, and dark colored; they are larger than Culicoides midges but smaller than Haematobia horn-flies.

Female Blackflies feed on blood. Larva and nymphs inhabit fast running clear rivers. Blackflies are notorious in association with River-blindness of humans. The importance of Blackflies to domestic animals is mainly by causing severe to intolerable biting-stress to cattle or horses when they appear seasonally close to running water [15].


Simulium (Simulidae)[edit]

Simulium female lateral.png

Characters: female, lateral. 1- Palps are long, formed of 5 segments. 2- Mouthparts are short, with the piercing elements hidden within the labium when not in use. 3- Antennae are short, without setae and consist of 11 similar segments. 4- Eyes are large relative to rest of body. 5- Thorax is distinctly humped in shape. 6- Thorax and abdomen are mostly without setae, except for the basal scale on the first abdominal segment. 7- Wings have a simple pattern of veins, with several thick veins near the leading edge and only thin veins in the rest of the wing; scales and colored patterns are absent.

Hosts. Livestock species, horses, poultry, humans, and many wild animals are used as hosts.

Signs and disease: These include: irritation, avoidance behavior, biting-stress, dermatitis, and acute allergic reactions to saliva of the feeding flies. These flies swarm around their hosts in daytime. Massive swarms of these flies can directly cause death of livestock from acute stress. Species of simuliids of this and other genera that are important for this stress include: Cnephia pecuarum, Austrosimulium pestilens, Simulium arcticum and S. ornatum. In Africa Simulium species are vectors of Onchocerca volvulus causing human onchocerciasis or River-blindness [16]. Simuliids are biological vectors of Leucocytozoon protozoa to poultry birds, and of Onchocerca gutterosa and O. gibsoni to cattle causing bovine onchocerciasis.


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References[edit]

  1. Smart, J. (1948) A Handbook for the Identification of Insects of Medical Importance. London, British Museum (Natural History).
  2. Snow, K.R. (1990) Mosquitoes. Slough, Richmond Publishing Co. Ltd. ISBN 0-85546-276-0.
  3. Edwards, F.W. (1941) Mosquitoes of the Ethiopian Region. vol 3, Culicine adults and pupae. London, British Museum (Natural History).
  4. Foote R.H. & Cook D.R. (1953) Mosquitoes of Medical Importance. Washington, Agricultural Research Service U.S. Department of Agriculture.
  5. Gillett, J.D. (1972) Common African Mosquitos and their Medical Importance. London, William Heinemann. ISBN 0-433-11750-8.
  6. Subramanian, S., et al. (1997) The relationship between microfilarial load in the human host and uptake and development of Wuchereria bancrofti microfilaeiae by Culex quinquefasciatus: a study under natural conditions. Parasitology, 116: 243-255.
  7. Gubler, D.J. (2004) The changing epidemiology of yellow fever and dengue, 1900 to 2003: full circle? Comparative Immunology, Microbiology and Infectious Diseases, 27: 319-330. doi:10.1016/j.cimid.2004.03.013.
  8. 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.
  9. Klein, T.A., et al. (1991) Comparative susceptibility of anopheline mosquitoes to Plasmodium falciparim in Rondonia, Brazil. The American Journal of Tropical Medicine and Hygiene. 44: 598-603.
  10. Gillies, M.T. & De Meillon, B. (1968) The Anophelinae of Africa South of the Sahara. Johannesburg, The South African Institute for Medical Research.
  11. Kettle, D.S. (1995) Medical and Veterinary Entomology. Wallingford: CAB International. ISBN 0-85198-968-3.
  12. 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.
  13. 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.
  14. Svobodova, M., et al. (2009) Cutaneous leishmaniasis caused by Leishmania infantum transmitted by Phlebotomus tobbi. International Journal for Parasitology, 39: 251-256. doi:10.1016/j.ijpara.2008.06.016.
  15. Freeman, P. & De Meillon, B. (1953) Simuliidae of the Ethiopian Region. London, British Museum (Natural History).
  16. Fischer, P., et al. (1993) Parasitological and clinical characterization of Simulium neavei transmitted onchocerciasis in western Uganda. Tropical Medicine and Parasitology. 44: 311-321.