References And Further Reading 347

Myiasis is the invasion of a living vertebrate animal by fly larvae. This invasion may or may not be associated with feeding on the tissues of the host. Myiasis-causing flies are represented by a diversity of species. Some are rarely involved in myiasis, whereas for others it is the only way of life. Many of these same fly species also feed on carrion. Among flies, dietary proteins are required for growth, egg production, and development. Proteins may be obtained by adult flies, by their larvae, or by both. In the case of larval diets, proteins are assimilated, stored, and carried through the pupal stage for subsequent use by the reproducing adult fly. A larval diet rich in proteins dictates that there is less need for adults to seek proteins. Thus myiasis is a means of exploiting a rich protein source by the larva for its own growth and, in some cases, for reproduction by the adult.

Myiasis can be classified based on the degree to which fly species are tied to a host. Three types of myiasis generally are recognized: accidental, facultative, and obligatory myiasis. In accidental myiasis, also called pseudomyiasis, the fly larvae involved normally are not parasitic but under certain conditions may become so.

Accidental myiasis generally results when fly eggs or larvae contaminate foods ingested by a host animal. An example of this can result from the ingestion of food contaminated with eggs or larvae of pomace flies and fruit flies (Drosophila spp.). The 50 or so fly species involved include those which typically are free-living in all stages and rarely are parasitic. In most cases these flies pass unharmed through the host's alimentary tract, but they can cause discomfort, nausea, diarrhea, and a plethora of related problems on their way through. In some cases, symptoms can be severe. Invasion of the alimentary tract can occur in two ways: either through ingestion of contaminated food or by retroinvasion through the host's anus. There is some doubt as to whether or not these cases are true myiasis because there is scant evidence that any fly development takes place after the ingested eggs hatch.

Facultative myiasis involves larvae which can be either free-living saprophages or parasites. These flies are opportunistic, having the ability to exploit living tissue. An example of facultative myiasis is the invasion of open sores on livestock by maggots of blow flies which normally frequent carrion.

Kettle (1995) recognizes three types of facultative myiasis: primary myiasis, involving those species which can initiate myiasis; secondary myiasis, involving species which continue myiasis, but only after it is started by primary species; and tertiary myiasis, involving species which join the primary and secondary species just prior to host death. Facultative myiasis species are the evolutionary bridge linking saprophagous feeders to those restricted to feeding on living tissues.

Many of these facultative myiasis flies are able to shift from dead to living tissue and back again with alacrity (e.g., Cochliomyia macellaria, Wohlfltartia nuba). In a sense, these are borderline parasites that are capable of invading a sick or injured host and continuing their larval development after the death of that host. The adult flies are attracted to open wounds or chronic surface sores with purulent exudates.

In obligatory myiasis the maggots of the fly species involved are always parasitic; they require a living host for their development. Examples are primary screwworms and bot flies. Included here are those species which cause temporary obligatory myiasis, such as nestling maggots and floor maggots. In this type of myiasis, the maggots do not keep continual contact with their host. Occasional parasitism of atypical hosts by obligate myiasis-producing flies is called incidental myiasis.

Myiasis also can be categorized in relation to the site of maggot invasion or subsequent development in the host. Thus, the descriptives gastrointestinal, urogenital, ocular, nasopharyngeal, auricular, and cutaneous are antecedent to the word myiasis, indicating the general site of maggot infestation.

Gastrointestinal myiasis refers to fly larvae in the alimentary tract of a host. This can be accidental myiasis, such as the ingestion of false stable fly eggs or larvae in uncooked fruits, or obligatory myiasis, such as the development of stomach bot flies in a horse. Enteric myisasis refers specifically to the intestinal tract. Urogenital myiasis is the invasion of the urethra and/or genitalia by fly larvae. This can occur when a host is debilitated and the urogenital openings exposed or when the host has a urogenital infection producing exudates that attract flies. Cases of urogenital myiasis usually involve blow flies and flesh flies. Ocular myiasis is the invasion of eye tissues by fly larvae; most cases are caused by the sheep nose bot fly (Oestrus ovis) and infrequently by rodent bot flies (Cuterebra spp.). Nasopharyngeal myiasis is the invasion of nasal and deep oral cavities and recesses by fly larvae. As with urogenital myiasis, this often is associated with a microbial infection but also can be caused by nose bot flies in healthy hosts. Auricular myiasis is the invasion of ears by fly larvae, usually caused by blow flies or flesh flies. Cutaneous myiasis involves invasion of the skin, usually by blow flies, flesh flies, screwworms, or certain bot flies. When cutaneous myiasis is associated with a break, laceration, or open sore in the host's skin, it is called traumatic myiasis. Cases of temporary myiasis involve intermittent contact between a fly larva and its host.

Myiasis apparently has evolved along different lines in different groups of flies. Gastrointestinal myiasis and urogenital myiasis, for example, appear to represent a transition from species contaminating host foods to those associated with host excretions. They usually involve free-living species. Obligatory cutaneous myiasis appears to have evolved from carrion-breeding flies and from preda-ceous flies which prey on them. The origin of obligatory nasopharyngeal myiasis is less clear but probably is linked with host secretions associated with upper respiratory infections. Ocular and auricular myiasis are characteristically either accidental or incidental in nature, resulting in damage to tissues at those respective sites. Temporary myiasis appears to have evolved from nest associates or lair-frequenting scavenger species which fed on organic morsels.

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