Parasitologie Biologie der Humanparasiten

Abstract

6.2 Flies as parasites of warm-blooded animals

Muscids or cyclorrhaphic diptera develop from a maggot (larva without a separate head and no legs) to a coarctate pupa (larval skin hardens to give the tegument) and a blowfly-like calliphorid fly (fig. 6.9, page 252). Parasitic maggots cause myiasis in man, paired and unpaired ungulates and other mammals and birds. Larvae detected in situ breathe via their abdominal pair of spiracles, which are characteristically structured (fig. 6.12, page 255).

The characterstic life cycle of a myiasis-causing fly is described for Oestromyia leporina (fig. 6.10, page 253). Eggs are deposited into the fur of Microtus arvalis, the common vole. A first stage larva hatches after 36-40 hours, penetrates into the subcutaneous connective tissue, migrates to the skin, forms a small hole to the open air, turns so that its abdomen contacts the air and moults to a second stage larva, which grows and moults to the third and finally fourth stages. The latter stage leaves the host and pupates outside. Hatched flies copulate and lay eggs without feeding, except for drinking water. - The mouthparts of the imagoes are reduced to a great extent (fig. 6.11, page 254). By cooling the pupal stages to –4 ⁰C for two days, the annual cycle can be reduced to 4-5 months. The exposure of mice carrying the third stage larvae to a 14-hour day for 2-5 days shortens the cycle to 3 months.

A first evolutionary line to myiasis begins with the calliphoridae, which feed on faeces and carcasses. Lucilia sericata feeds on purulent wounds cleaning them of necrotic tissue, even when covered with dressing. Other species also attack healthy tissue, e.g. parasitizing in the nasal openings of frogs, toads and salamanders and causing rhinomyiasis.

Cordylobia anthropophaga the tumbufly of Africa, south of the Sahara, oviposits on sand or clothes with minute traces of urine. In addition to man, dogs and pigs are often attacked, and rarely horses. The main hosts, however, are rats in the holes of which abundant exuviae are found.
Cochlyomyia macellaria (syn. Callitroga americana, Chrysomyia macellaria) feeds mostly only on necrotic tissue. It is observed from Argentina to Texas in cattle, sheep and man.
Chrysomyia (syn. Callitroga) hominivorax, the screw-worm living in the same regions also feeds on healthy tissue. The holes left in the skin of cattle, goats and sheep make the leather useless. The fly was eradicated from the Caribbean island Curaçao by the sterile male technique (see below).

A second evolutionary line is plausible from an ectoparasitic life to mining inside skin: Dermatomyiasis.

Aucheromyia luteola (Calliphoridae) deposits eggs on the resting places of mammals and sleeping mats of man. The nightly active larvae (Congo floor maggot) scratch the skin, lick seeping blood and hide themselves during the day. They pupate without mining in the skin.
Dermatobia hominis (Cuterebridae), the human botfly, also attacks cattle and sheep. It adheres its eggs to other muscidae, mosquitos and ticks for transport to the hosts (phoresia). The subcutaneously mining larvae develop to 20 mm in length in two and a half months and cause warble tumours. After the larva have gone, the wounds rapidly heal but the leather is subsequently useless.
Hypoderma bovis (cattle botfly) deposits its eggs singly in flight, attacking the host’s skin. Cattle flee in panic. The mining maggots migrate to the spinal cord channel, moult to third stages and return to the skin.
Oestrus ovis (sheep nostril fly) in flight oviposits in the nose and the eyes of sheep, goat and cattle. The larvae are expelled by sneezing.

A third evolutionary line leads to intestinal myiasis or enteromyiasis. The family of gasterophilidae belongs to the more primitive acalyptrata of the cyclorrhapha, indicating its own separate line.

Gasterophilus intestinalis (horse bot) of horse and donkey. The eggs are deposited onto the forelegs. The hatched larvae are licked from the skin and anchor at first in the tongue or cheek. They then moult and migrate to the epiglottis where they moult again before they are swallowed to the stomach. Damage may be considerable if the larvae accumulate in large masses. The larvae ready for pupation are evacuated via the faeces. The anterior stigma of the second thoracic segment and the posterior spiracles of the abdomen are highly differentiated according to the portion of life spent without connection to the open air (fig. 6.13, page 257).
Gyrostigma spec. is found in the rhinoceros.
Cobboldia loxodontis oviposits to the tusks of elephants; the larvae develop similarly in the mouth cavity and finally in the stomach but return to the mouth, fall out and pupate outside.

The control of Cochliomyia hominivorax was carried out in Curaçao 1950 by the introduction of males sterilized by X-rays in Orlando/Florida. To control the effect, goats were wounded artificially and staked outside. The number of eggs deposited into the wounds were registered; no eggs could be detected after five months. A similar campaign was successful after 16 months in Libya; the fly had been introduced into Libya from America in 1988 and had caused 13000 cases in cattle and wild animals. In both campaigns, success was based on the fact that they were carried out in regions limited by natural borders.

Fannia (Anthomyiidae): the larvae of the genus are sometimes found in the bladder or rectum of man. F. canicularis (the small house fly) deposits its eggs on moist food and the larvae develop in the stomach but not further. They are observed by chance in vomit or medical samples. All body cavities accessible directly from outside may temporarily harbour parasitic larvae of flies and cause dermato-, entero-, gastro-, kolpo-, zysto, rhino- and ophthalmo-myiasis.

Flies as stationary blood-feeders

Permanent parasitic cyclorrhaphic flies live in fur and plumage. They are grouped as pupipara because they propagate by the delivering of singly prepupae (maggot larvae ready for pupation).

Family Hippoboscidae: Hippobosca equina, the louse fly of horses, is dorso-ventrally flattened. Both sexes retain their wings lifelong, whereas in Lipoptena cervi, the louse fly of red deer, the female loses its wings after settling on the host. Craterina pallida, the louse fly of the swift, carries only small tapering wings. Unable to fly, the flies crawl from nest to nest. On a single bird, only 1-3 flies are present, whereas on the mammalian hosts, louse flies are regularly abundant. Melophagus ovinus, the louse fly of sheep (fig. 6.14, page 258), is permanently parasitic and the vector of Babesia ovis. The pupae stick to hairs.

Family Nycteribiidae: the louse flies of bats are wingless; the oblong head is tipped backward into a dorsal groove of the thorax giving the imago the shape of a six-legged spider ("spider fly"). Copulation takes place at the end of the bats’ hibernation. Nycteribia pedicularia lives on the ubiquituous Myotis species. Wild bats caught and kept in captivity are deserted by the flies in the first days.

Family Streblidae: the tropical and subtropical louse flies of bats comprise about 80 species. The head is normally positioned and the wings range from well developed to missing completely. In Ascodipteron spec. (Australia, India, Africa) both sexes are winged at copulation but thereafter the female loses its wings.

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