Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance/Blow flies

Blow-flies and similar flies (Diptera, Calliphoridae)

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Characters of Blow-flies, Screw-worm flies, and Flesh-flies

These calliphorid flies are large, stout, and their body bears many long thick setae in the form of bristles. The body surface is like polished metal colored blue, green or black. Wings are usually clear and always have a full network of veins over their surface. Mouthparts of adults are always of the sponging type, without piercing parts. The adults feed on liquids from their hosts or environment, and they do not pierce host skin to suck up blood .

Photograph shows a Chrysomya Screw-worm adult fly and mature larva. The sponging type of mouthparts hang down below the head. The colors of a live fly would show red/brown eyes and shiny blue/green abdomen.

The larvae are usually the stage related to disease, causing either facultative myiasis (an optional infestation) or obligate myiasis (an essential infestation). Most infestations with these larvae are one of three types: a mass of larvae abrade and feed at surface of skin; a mass of larvae burrow vertically into skin; single larvae develop as furuncles, like boils, in the skin. Usually these larvae look similar to the free living maggots typical of the muscid flies, with segmental bands of spines and rasping mouthparts. The shape and patterns of the posterior spiracles are crucial for identification. The posterior spiracles of calliphorid larvae usually have a trio of slits surrounded by a peritreme. This contrasts with posterior spiracles of oestrid larvae which form a pair of large flat plates with numerous small openings (see Botflies). The life-cycle is a normal complete metamorphosis, with three larval stages and a free living pupa.

Diagram of feeding at skin represents a superficial infestation by immature larvae of a Screw-worm fly. The proportions larvae to skin are not accurate.

There are many genera and species of parasitic calliphorid flies. Some of them are highly host specific, for example Elephantoloemus indicus, whose larvae burrow in the skin of Asian elephants. Only those mostly commonly causing disease to domestic animals and people are included here.

Glossary

• Facultative myiasis = This type of myiasis caused by flesh-eating larvae is not essential to the reproduction and survival of the fly population; the eggs may be laid on carrion and other rotting material and develop there.
• Furuncle = An abscess or boil-like development in the skin of a host animal where a single myiasis larva is developing.
• Hypopleuron = An area like a plate on the lower posterior side of the thorax, that may or may not have a row of setae (9 on Calliphora).
• Labellum = This organ of the mouthparts of dipteran flies is in the form of a sponge in Blow-flies and similar (6 on Calliphora).
• Maggot = General vernacular term for larvae of muscid and calliphorid flies; usually free living larvae but also refers to larvae causing myiasis.
• Peritreme = A sclerotized area around the posterior spiracles of dipteran larvae (3 on Calliphora).
• Spiracle = Opening of the respiratory system of insects and acarines; anterior and posterior spiracles occur on dipteran larvae (2 and 3 on Calliphora).
• Stem vein = A thick vein at the base of wings of dipteran flies (10 on Calliphora).
• Strike or Blowfly strike = Vernacular name for the type of superficial myiasis caused by Lucilia and similar Blow-flies.

Calliphora (Calliphoridae)

Characters: larva, adult, lateral. 1- Rasping mouthparts of larva. 2- Anterior spiracle has eight to ten openings. 3- Posterior spiracles have a thick peritreme with two internal projections, and 3 openings as long ovals. 4- Antenna has large arista with setae on both sides. 5- Palps are short relative to the labium of the mouthparts. 6- Mouthparts have a sponging labellum, without piercing parts. 7- Eyes are dark brown. 8- Thorax and abdomen are usually shiny dark blue but some species are mid brown; also bristle-like setae on thorax are conspicuously long. 9- Thorax has on its hypopleuron a row of stout setae (compare with Musca in Houseflies ). 10- Wings have stem veins without a row of stout seta. 11- Thoracic squama of the wing bears long thin setae. 12- Vein 4 is angled sharply up toward the outermost edge.

Hosts: Sheep may be infested with larvae.

Signs and disease: Larvae of Calliphora species sometimes cause superficial myiasis in the skin of sheep, contributing to the disease often known as Blowfly-strike. The larvae burrow into the skin head first, exposing just their spiracles to the surface .

Lucilia (Calliphoridae)

Characters: larva, adult, lateral. 1- Rasping mouthparts of larva. 2- Anterior spiracle has 7 to 10 openings. 3- Posterior spiracles have a thick peritreme with one internal projection. 4- Wings have stem veins without a row of stout setae. 5- Vein 4 is angled sharply up toward the outermost edge. 6- Thoracic squama bears no setae on its upper surface. 7- Thorax has on its hypopleuron a row of stout setae. 8- Thorax and abdomen are metallic bright green. 9- Eyes are mid brown. 10- Antenna has a large arista with setae on both sides.

Hosts: Sheep, and sometimes other domestic animals. Female flies, commonly Lucilia sericata and L. cuprina, often lay eggs at sites on skin that are already wounded, or contaminated with feces, or sites infected with Dermatophilus congolensis bacteria, or similar infections. Lucilia species are facultative parasites, and the adults cause no direct harm .

Signs: Infestations of larvae are superficial on the skin, with the larvae not burrowing down into the dermis. An infestation resulting from a single egg batch will produce a wide and bloody lesion infested by several hundred mature maggots.

Disease: Blowfly-strike by Lucilia species causes pain, stress, hide-damage, and anemia. In severe cases an acute and potentially fatal toxemia develops due to ammonia excreted by the larvae. These blowflies can cause serious loss of production for sheep farmers.

Phormia (Calliphoridae)

Characters: larva, adult, lateral. 1- Anterior spiracle has up to 10 openings. 2- Posterior spiracles have a thick peritreme, but it does not form a complete circle. 3- Wings have stem veins with a row of stout seta. 4- Vein 4 is angled sharply up toward the outermost edge. 5- Thoracic squama bears faint white setae on its upper surface. 6- Thorax has on its hypopleuron a row of stout setae. 7- Thorax and abdomen are metallic dark blue or dark green. 8- Eyes are mid brown.

Hosts: Phormia regina is another species that may contribute to Blowfly-strike on sheep. It is a facultative parasite.

Signs and disease: As for Lucilia, however Phormia flies are less widespread than Lucilia in their role as a cause of myiasis.

Chrysomya (Calliphoridae)

Characters: larva, adult, lateral. 1- Anterior spiracles have from 4 to 13 openings. 2- Some species have larvae with many fleshy tubercles projecting from most segments, other species are smooth between the bands of spines, as shown here. 3- Posterior spiracles have a peritreme that is incomplete. 4- Stem vein of wing has a row of setae. 5- Vein 4 of wing is angled sharply up toward the outermost edge. 6- Thoracic squama is large, angular and white, and it has setae on its upper surface. 7- Dorsal surface of thorax has stripes slightly darker than their background. 8- Hypopleuron of thorax has a row of stout setae. 9- Thorax and abdomen are metallic green or blue/green. 10- Setae on top of thorax are relatively short. 11- Eyes are red/brown or orange. 12- Antenna has arista with setae on both sides.

Hosts: Chrysomya bezziana, C. megacephala (Screw-worm flies) and other species are obligate parasites in their larval stage on cattle and other livestock species, and wild bovids.

Signs: The larvae are described as screw-worms because they burrow head first into the skin of their host, individually resembling screws for wood-work. A batch of eggs laid by one female at a slight wound in the skin leads to a wide lesion packed with several hundred mature maggots. Intact moist skin at nostrils and elsewhere may also be invaded.

Disease: Cutaneous myiasis of the Screw-worm type causes much pain and distress and can be fatal if the infestation starts at a wound on the ear or navel and penetrates deeply.

Distribution: These are known as the Old-World Screw-worm flies, distributed in tropical and sub-tropical Africa and Asia .

Cochliomyia (Calliphoridae)

Characters: larva, adult, lateral. 1- Anterior spiracles have 7 to 10 openings. 2- Posterior spiracles have a peritreme that is incomplete. 3- Stem vein of wing has a row of setae. 4- Vein 4 of wing is angled sharply up toward the outermost edge. 5- Thoracic squama has setae on its upper surface. 6- Dorsal surface of thorax has distinct dark stripes. 7- Hypopleuron of thorax has a row of stout setae. 8- Thorax and abdomen are metallic blue/green. 9- Setae on top of thorax are relatively dense and long. 10- Eyes are red/brown or orange. 11- Antenna has arista with setae on both sides.

Hosts: Cochliomyia hominivorax and C. macellaria (Screw-worm flies) are obligate parasites in their larval stage on cattle and other livestock species, horses, also rarely humans (after whom C. hominivorax is named).

Signs and symptoms: As for Chrysomya; these Screw-worms create a wide lesion when many of them from one batch of eggs burrow directly into the skin.

Disease: Cutaneous myiasis of the Screw-worm type causes much pain, distress, loss of production, and can be fatal if the infestation starts at a wound on the ear or navel and then penetrates deeply.

Distribution: These ectoparasites are known as the New-World Screw-worm flies, distributed widely in tropical and sub-tropical areas of the Americas. (Note that Chrysomya and Cochliomyia are closely similar, but Cochliomyia are restricted to the Americas. There was an accidental introduction on transported livestock of C. hominivorax into Libya, but it was eradicated there before it could spread further. Also C. hominivorax has been eradicated from the USA, Mexico and other Central American countries using sterile insect technique .

Cordylobia (Calliphoridae)

Characters: larva, adult, lateral. 1- Anterior spiracles are in form of an irregular clump of openings. 2- Larvae are stout and covered in small sharp spines. 3- Posterior spiracles are on a circular plate and the openings are variously wavy in outline. 4- Stem vein of wing is without setae. 5- Thoracic squama of wing is without setae. 6- Abdomen and thorax are dull yellow/brown; adults are large. 7- Hypopleural setae of the thorax are distinct. 8- Thorax dorsal surface bears a small number of large bristle-like setae and numerous small setae. 9- Antenna has an arista with setae on both sides.

Hosts: Dogs, cats, humans, rodents and wild bovids are used as hosts for the larvae.

Signs, symptoms and disease: Furuncular myiasis of the skin causes pain, distress and malaise. Cordylobia species are exceptional amongst myiasis causing flies: the females lay eggs on soil or in nest of their hosts, this is nidicolous behavior. They also will lay eggs on damp clothes hung out to dry. Thus adult flies are rarely associated with the diseased hosts. Larvae emerging from eggs then independently seek their host to invade them through their skin.

Distribution: Cordylobia inhabits Africa south of the Sahara. Note that humans who suffered infestation in Africa by larvae of the Tumbu-fly, or of Lund's-fly, may present their myiasis problem in other parts of the world after travelling .

Auchmeromyia (Calliphoridae)

Characters: larva, adult, lateral. 1- Anterior spiracles of larva have a bunch of openings. 2- Larvae are dark brown, without spines, but with tubercles. 3- Posterior spiracles have 3 straight and horizontal openings; the peritremes are indistinct. 4- Vein 4 of wing bends up toward leading edge. 5- Stem vein of wing has no bristles. 6- Thoracic squamae have no setae. 7- Second abdominal segment of female is longer than first or third segments. 8- Hypopleural bristles are present. 9- Color of adults is dull yellow/brown or red/brown.

Hosts: Wild pigs, warthogs, other large burrowing animals are the usual hosts, but also rarely humans may be parasitized.

Disease and distribution: Auchmeromyia senegalensis, the Congo Floor-maggot is the important representative of this genus. The genus is characterized by its larvae being adapted for blood feeding on their hosts whilst remaining free-living within the host's burrow. This can be described as an unusual form of myiasis .

Sarcophaga (Sarcophagidae)

Characters: larva, adult dorsal. 1- Anterior spiracles have numerous openings. 2- Larval segments have bands of small spines. 3- Posterior spiracles each have 3 straight vertical openings and an incomplete peritreme. 4- Arista of antenna has setae at base but is plain anteriorly. 5- Thorax has 3 dark longitudinal stripes. 6- Abdomen has pattern of dark rectangles. 7- Vein 4 of wing bends towards the leading edge. 8- Legs end in large pulvilli and paired claws. 9- Hypopleural bristles are present on side of thorax.

Hosts: Usually species of Sarcophaga are entirely free living, similar to typical blow-flies. Sarcophaga haemorrhoidalis is usually a typical carrion fly, but sometimes it feeds as a facultative parasite of cattle, sheep and humans.

Signs and disease: The larvae may cause superficial myiasis, and also contaminative transmission of bacteria from carrion to domestic animals and humans.

Wohlfahrtia (Sarcophagidae)

Characters: larva, adult dorsal. 1- Anterior spiracles have up to 14 openings. 2- Posterior spiracles have in incomplete peritreme and the openings of the spiracles are approximately parallel to each other. 3- Legs are long; they end in pairs of large pulvilli. 4- Thorax bears a row of setae on its hypopleuron (see on Calliphora). 5- Thorax and abdomen are dull light grey with distinctive patterns of black stripes and spots. 6- Wing has vein 4 curving sharply up towards the outer leading edge. 7- Antenna has arista without any setae.

Hosts: Larvae infest sheep, goats, cattle, camels, horses, dogs and humans. Larvae of Wohlfartia species are obligate parasites.

Signs, symptoms and disease: Cutaneous myiasis is caused as larvae burrow into skin. Larvae of some species cause lesions packed with larvae similar to those of Screw-worm flies, others cause furuncular lesions in the skin, like abscesses or boils, with separate individual larvae. Pain and stress is severe, production losses can be significant, and hosts may be killed by the infestation. Infestations of humans are rare and consist of only a few larvae, but these tend to be at the nose, with serious consequences .

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