Defining myiasis




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myiasis

Defining myiasis

The term myiasis was first ) to refer to diseases of humans originating specifically with dipterous larvae, as opposed to those caused by insect larvae in general, scholechiasis ( Hope presented a table of myiasis cases which included several from Jamaica resulting from unknown larvae, one of which led to death. Some of the larvae were described as being of an unidentified blue fly; these are almost certainly very early references to myiasis caused by the New World screwworm, Cochliomyia hominivorax (Coquerel).

Myiasis has since been defined as 'the infestation of live vertebrate animals with dipterous larvae, which, at least for a certain period, feed on the host's dead or living tissue, liquid body substances, or ingested food' There are two main systems for categorizing myiasis: anatomically, in relation to the location of the infestation on the host (see Table 1); or according to the parasite's level of dependence on the host

The anatomical system of classification was first proposed and later modified by The system is useful for practical diagnosis and so is used below. However, Patton ( it to be unsatisfactory when considering evolutionary and biological relationships, because individual species could be assigned to more than one group and different groups contained species with different levels of dependence on the host. He put forward instead a system based on the degree of parasitism shown by the In Patton's categorisation, there are two main groups of myiasis-causing species: the specific parasites, which must develop on live hosts; and the semi-specific parasites, which usually develop on decaying organic matter, such as carrion, faeces and rotting vegetation, but may also deposit their eggs or larvae on live hosts. termed the specific parasites obligatory and the semi-specific parasites facultative. The facultative species may be further differentiated depending on whether they are able to initiate myiasis (primary species) or only invade after other species have initiated it (secondary and tertiary species) In addition, a third group of myiasis-causing species, those that cause accidental myiases when their eggs or larvae are ingested by the host. these pseudomyiases.Flies that may be encountered in cases of cutaneous myiasis primarily belong to four families, Calliphoridae, Sarcophagidae, Muscidae, and Oestridae. The first three families are involved primarily in wound or traumatic myiasis.

Wound or traumatic myiasis

The primary purpose for developing this identification key is to aid in identifying cases of traumatic myiasis caused by the three major species of obligate parasites encountered in wound myiasis. The New World screwworm fly, Cochliomyia hominivorax, the Old World screwworm fly, Chrysomya bezziana, and Wohlfahrt's wound myiasis fly, Wohlfahrtia magnifica.Wound or traumatic myiasis is the infestation by dipterous larvae of primarily the cutaneous tissues in animals and humans, usually at the sites of natural (orifices) and unnatural (wounds) openings into the body. It may be deleterious, as when the obligate and primary species attack the host's healthy tissues or it may be benign, as when secondary species confine their activities to diseased and dead tissue. In the later situation carefully controlled myiasis can even be of benefit to the host in 'maggot therapy' (Sherman et al., 2000).

Calliphoridae

Auchmeromyia

Bloodsucking larvae of the African species Auchmeromyia senegalensis the Congo floor maggot, are atypical myiasis species as they do not live on or in the host, but suck the blood of sleeping humans and burrow-dwelling animals such as warthogs (sanguinivorous myiasis).

Cordylobia

Cordylobia includes three species: C. anthropophaga is the Tumbu fly of Africa which causes a boil-like (furuncular) type of myiasis, particularly of man and dogs. Eggs are deposited on dry, shaded ground, especially if contaminated by urine/faeces, or on drying laundry. Larvae hatch in 1-3 days and remain just under the soil surface until activated by host body heat. They then emerge, burrow into the host and grow for 8-15 days in a furuncle. Antelopes and the African Giant Rat are important hosts of C. rodhaini.

Cochliomyia

The two species of the New World genus, Cochliomyia, most frequently encountered in cases of wound myiasis are C. hominivorax and C. macellaria. The New World screwworm fly, C. hominivorax, is a true obligate parasite of mammals: females lay eggs at the edges of wounds on living mammals (or on mucous membranes); within 24 hours, larvae emerge and immediately begin to feed on the underlying tissues, burrowing gregariously head-downwards into the wound; larvae reach maturity about 5-7 days after hatching and leave the wound, falling to the ground into which they burrow and pupate. On completion of development, adult flies emerge from the puparium and mate within 1-3 days. About four days after mating, females seek a suitable host and lay an average of 200 eggs (range 10-490) in a flat, shingle-like batch. Further batches are laid at intervals of three days with an average of four batches per female. Adult flies live for 2-3 weeks on average and may disperse great distances. The literature on the New World screwworm is extensive and scattered, but earlier publications may be accessed rapidly by reference to the bibliography of Larvae of C. macellaria involved in myiasis are only secondary invaders, feeding on the edge or surface of the wound. A rare case of C. minima myiasis of a dog in Puerto Rico has been described

Chrysomya screwworms

The life cycle of Chrysomya bezziana (Old World screwworm), its habits and the appearance of wounds infested by it are very similar to those of Cochliomyia hominivorax. The two species appear to occupy an exactly equivalent parasitic niche in their natural ranges. Adult female Ch. bezziana only oviposit on live mammals, depositing 150-500 eggs at sites of wounding or in body orifices such as the ear, nose and urinogenital passages. The larvae hatch after 18-24 hours, moult once after 12-18 hours and a second time about 30 hours later. They feed for 3-4 days and then drop to the ground and pupate. This species has been recorded on 21 host species at a zoo in Malaysia ). Similar reports exist for Cochliomyia hominivorax and Wohlfahrtia magnifica.

Chrysomya species other than screwworms

Chrysomya albiceps is a facultative parasite and normally lays its eggs on carcasses. The first instar larvae feed on exudations of the decomposing flesh, but second and third instars are, in addition, predacious, feeding on other blowfly larvae. They may even be cannibalistic. Whilst the eggs are normally laid on carcasses, they may also be laid on neglected wounds where the larvae can cause tissue destruction. Chrysomya albiceps and the similar Ch. rufifacies are frequently involved in secondary myiasis in sheep.

Lucilia

Members of this genus are responsible for the condition known as 'blowfly strike' of sheep in a number of countries including South Africa and Australia, where the species responsible is L. cuprina, and in many temperate areas including Europe and North America, where the important species is L. sericata. Female Lucilia lay their eggs on carcasses, in neglected, suppurating wounds and, in particular, on the wool of sheep soiled with urine, faeces or blood. Lucilia sericata has been used to assist the healing of long-term wounds in man, a treatment termed 'maggot therapy' (larva therapy or biosurgery), whereby the larvae ingest necrotic tissues and stimulate the healing process (Sherman & Pechter, 1988; Thomas et al., 1996; Sherman et al., 2000). Lucilia bufonivora is an obligate parasite of toads and frogs.



Calliphora

The two most important species are C. vicina and C. vomitoria which share similar biologies. Females are attracted for oviposition to any decaying matter, of which carrion is most suitable. Calliphora are usually only involved in myiasis as secondary species, but C. vicina> , in particular, may be a primary invader

Phormia, Protophormia

These closely related genera are, approximately, confined to areas north of the Tropic of Cancer. The important species are Phormia regina and the more northern Protophormia terraenovae. They are very similar in appearance and habits, both usually breeding in carrion, but also recorded in wound myiasis. Protophormia terraenovae may, in particular, be a serious parasite of cattle, sheep and reindeer (

Protocalliphora

Flies in the genus Protocalliphora are obligate, blood-feeding parasites of nestling birds in the Old and New Worlds but, in general, their effects are not serious.



Other Calliphorids causing obligate myiasis

Flies in the genus Booponus develop as larvae in boils under the skin of bovids and deer of the Old World. In that respect they are somewhat like Cordylobia, but they lay their eggs directly on the host. Elephantoloemus indicus is the sole representative of its genus and develops in the skin of Indian elephants - many thousands of larvae can impart a honey-comb appearance to the skin due to numerous scars.


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