Dermal and subdermal myiasis
Wound or traumatic myiasis
Myiasis attributed to new and old world screwworms
Geographical distribution of new and old world screwworm flies
Biology and life cycle of the new world screwworm
Identification of the new world screwworm
Bibliography
M.J.R. Hall
The infestation of animal and human wounds by the larvae of Diptera results in wound or traumatic myiasis. It may involve species of fly whose larvae feed only on diseased and dead tissue or, more seriously, it may involve species that are obligate parasites and feed on the living tissues of their hosts. Most significant among the latter group is the New World screwworm, Cochliomyia hominivorax (Coquerel). The most important insect pest of livestock in the Americas, it was established in 1988 for the first time in the Old World, in the Libyan Arab Jamahirya. The New World screwworm as an agent of wound myiasis is considered in comparison to other fly species, in particular the Old World screwworm, Chrysomya bezziana (Villeneuve).Dr Martin J. H. Hall is Deputy Head, Medical & Veterinary Division, Department of Entomology, Natural History Museum, London. The author would like to express his gratitude to his colleagues Dr Paul Ready and Nigel Wyatt for their comments on early drafts of this article, and to Peter York and Phil Crabb for their photography of maggot and fly specimens.
The term myiasis (then spelt myiasis) was first proposed by Hope (1840) to refer to diseases of humans originating specifically with dipterous larvae, as opposed to those caused by insect larvae in general, scholechiasis (Kirby and Spence, 1815). 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" (Zumpt, 1965). There are two main systems for categorizing myiasis: anatomically, in relation to the location of the infestation on the host (see Table 1), and entomologically, according to the level of dependence on the host (see Table 2).
The anatomical system of classification was first proposed by Bishopp (Patton, 1922) and later modified by James (1947), (see Table 1). The system is useful for practical diagnosis (Zumpt, 1965) and so is used below. However, Patton (1922) found 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 fly (see Table 2).
1. Classification of myiases according to their anatomical position in or on the host animal
Classement des myiases selon leur position anatomique dans ou sur l'animal hôte
Clasificación de las miasis en función de su posición anatómica dentro del animal huésped o sobre él
|
Zumpt |
Bishopp |
James |
|
Sanguinivorous |
Bloodsucking |
Bloodsucking |
|
Dermal/subdermal |
Tissue-destroying |
Furuncular |
|
Subdermal migratory |
Creeping |
|
|
Traumatic/wound |
||
|
Anal/vaginal |
||
|
Nasopharyngeal |
Infestations of the head passages |
Nose, mouth and sinuses |
|
Aural |
||
|
Ocular |
||
|
Intestinal |
Intestinal/urogenital |
Enteric |
|
Anal/vaginal |
||
|
Urogenital |
Intestinal/urogenital |
Bladder and urinary passages |
|
Anal/vaginal |
Note: The division of myiases into five rows is based on the grouping of Zumpt (1965) in the first column. The second and third columns show the comparable groupings of Bishopp (Patton, 1922) and the modification of these by James (1947).
In Patton's categorization, 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. Zumpt (1965) 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) (Kettle, 1984), (see Table 2).
2. Classification of myiases according to the parasitic relationship of the Diptera with the host
Classement des myiases selon la relation parasitique du diptère avec l'hôte
Clasificación de las miasis en función de la relación parasitaria de los dípteros con el huésped
|
Group |
Subgroup |
Remarks |
|
Specific/obligatory |
|
Parasite dependent on host for part of its life cycle |
|
Semi-specific/facultative |
Primary |
Normally free-living but may initiate myiasis |
|
Secondary |
Normally free-living and unable to initiate myiasis but may be involved once animal is infested by other species |
|
|
Tertiary |
Normally free-living, but may be involved in myiasis when host is near death |
|
|
Accidental/pseudomyiasis |
|
Normally free-living larvae that may be accidentally ingested and cause pathological reactions |
Sources: Patton, 1922; Zumpt, 1965; Kettle, 1984
In addition, Patton (1922) defined a third group of myiasis-causing species, those that cause accidental myiases when their eggs or larvae are ingested by the host. Zumpt (1965) termed these pseudomyiases.
Dermal and subdermal myiasis, or cutaneous myiasis, is the invasion of skin tissues by larvae of Diptera that cause burrows or boils in the dermal layers, invade and enlarge existing wounds or form wounds themselves. Some species that are mainly considered as agents of intestinal, nasopharyngeal or sanguinivorous myiases are included in the following discussion, as they may be encountered in or on cutaneous tissues.
Examples of 18 different genera of flies that may be encountered in cases of cutaneous myiasis are shown in Figures 1 (adults) and 2 (mature larvae). They belong to seven families, Calliphoridae (see Figure 1, Examples 1 to 8), Sarcophagidae (Examples 9 and 10), Muscidae (Example 11), Phoridae (Example 12), Cuterebridae (Example 13), Gasterophilidae (Example 14) and Oestridae (Examples 15 to 18). Only the first three families are important in wound or traumatic myiasis, so only brief mention is made of the latter four families.
The Phoridae, scuttle flies, include facultative parasites such as the cosmopolitan Megaselia scalaris (Figure 1, Example 12). Their larvae are occasionally found as secondary invaders in cases of wound myiasis, the females being attracted to oviposit on foul-smelling sores.
The Cuterebridae, Gasterophilidae and Oestridae are all obligate parasites. The most important cuterebrid is Dermatobia hominis (Figure 1, Example 13). Sometimes called the tórsalo, or human bot fly, it is a very serious pest of cattle in Central and South America. Its larvae create boil-like swellings where they enter the skin. Cuterebra species cause myiasis of rodents, lagomorphs and occasionally humans in North America.
Species in the genus Gasterophilus (Figure 1, Example 14) of the Gasterophilidae are all termed bot flies. Their larvae develop in the digestive tract of equids (intestinal myiasis). Originally restricted to the Palaearctic and Afrotropical regions, Gasterophilus species now have a worldwide distribution. The Oestridae include a number of important veterinary parasites. Hypoderma species (Figure 1, Example 15) are heel flies, warble flies or cattle grubs, whose larvae migrate from sites of oviposition by a subcutaneous route and in nerve tissues to the back, where they develop in "warbles" which spoil the host's hide. They are a great problem to cattle production in the Holarctic region. Some species attack deer in a similar manner. Larvae of the Old World genus Rhinoestrus (Figure 1, Example 16), nasal bot flies, infest the nasal sinuses of their hosts (nasopharyngeal myiasis) and are, generally, very host specific. Rhinoestrus purpureus attacks horses and donkeys. Cephalopina titillator (Figure 1, Example 17), the camel nasal bot fly, is the only species in its genus. Its larvae develop in the nasal cavities of camels wherever camels naturally occur. The genus Oestrus includes Oestrus ovis (Figure 1, Example 18), the very important sheep nasal bot fly, whose larvae develop in the head sinuses and nasal passages of sheep and goats in all sheep-farming areas of the world.
Two of the calliphorid genera illustrated in Figures 1 and 2 are also not involved in wound myiasis. Cordylobia species include Cordylobia anthropophaga (Figure l, Example 8), the Tumbu fly of Africa, which causes a boil-like (furuncular) type of myiasis like that caused by D. hominis in the Americas. It is commonly found infesting humans. Dogs are the domestic animals most frequently affected. Bloodsucking larvae of the African species Auchmeromyia luteola (Figure l, Example 7), the Congo floor maggot, are atypical myiasis-causing species, as they do not live on or in the host, but suck the blood of sleeping humans and burrow-dwelling animals (sanguinivorous myiasis).
4. Identification key for agents of wound myiasis - Clé pour l'identification des agents de la myiase des plaies - Clave de identificación de los agentes de las miasis cutáneas
Wound or traumatic myiasis is the infestation of animal and human wounds by dipterous larvae. It may be benign, as when secondary species confine their activities to diseased and dead tissue, or it may be malign, as when the obligate and primary species attack living tissue.
The three major species of obligate parasites encountered in wound myiasis are the New World screwworm, Cochliomyia hominivorax, the Old World screwworm, Chrysomya bezziana, and Wohlfahrt's wound myiasis fly, Wohlfahrtia magnifica. Nine genera belonging to the families Calliphoridae, Sarcophagidae and Muscidae are considered in some detail. This list includes those species most likely to be encountered in traumatic wound myiasis, but it is by no means exhaustive. A large number of other species are capable of invading wounded tissues, depending on the host, its habitat, the nature of the wound and so forth.
Calliphoridae
Cochliomyia species. The two species of the New World genus Cochliomyia encountered in wound myiasis are Cochliomyia hominivorax (Figure 1, Example 1) and Cochliomyia macellaria. The former is considered in detail below. Cochliomyia macellaria has often been implicated in myiasis because of misidentifications of C. hominivorax. The larvae of C. macellaria may be very abundant on carrion. When involved in myiasis they are only secondary invaders, feeding on the edge or surface of the wound and not producing the pocket-like lesions characteristic of the primary screwworms (James, 1947).
Phormia and Protophormia species. These closely related genera are approximately confined to areas north of the Tropic of Cancer and are not found in Africa. The important species are Phormia regina (Figure 1, Example 5) and the more northern Protophormia terraenovae (Figure 1, Example 6). 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 (James, 1947; Smith, 1986).
Lucilia species. 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 Lucilia cuprina, and in many temperate areas including Europe and North America, where the important species is Lucilia sericata (Figure 1, Example 3).
The adult flies are metallic green or coppery green and are therefore known collectively as greenbottles. The life history of the two species involved in myiasis is very similar. Female Lucilia species lay their eggs on carcasses, in neglected, suppurating wounds and, in particular, on the wool of sheep soiled with urine, faeces or blood. In addition to sheep, other animals attacked include horses, cattle and humans (Zumpt, 1965; Smith, 1986). Lucilia sericata has been used to assist the healing of deep wounds in humans, a treatment termed "maggot therapy", whereby the larvae ingest necrotic tissues and stimulate the healing process. However, Lucilia species may also be a medical problem in bed-ridden patients unable to clean or otherwise care for themselves.
Calliphora species. This genus includes the well-known bluebottle flies, of which there are numerous species throughout the world. The two most important species are Calliphora vicina (Figure 1, Example 4) and Calliphora vomitoria, which share similar biologies. Females are attracted for oviposition to any decaying matter, of which carrion is most suitable. Calliphora croceipalpis is found in sub-Saharan Africa. Calliphora species are usually only involved in myiasis as secondary species, but C. vicina, in particular, may be a primary invader (Zumpt, 1965; Smith, 1986).
Chrysomya species. This Old World genus is analogous to the New World genus Cochliomyia. It includes the Old World screwworm fly, Chrysomya bezziana (Figure l, Example 2), an obligate parasite in wounds. The life cycle (see Figure 3) and habits of C. bezziana and the appearance of wounds infested by it are very similar to those of Cochliomyia hominivorax, and therefore it is the species with most potential to be mistaken for the New World screwworm. There is a remarkable parallel between the two screwworm species, which appear to occupy an exactly equivalent parasitic niche in their natural ranges. Adult C. bezziana females only oviposit on live mammals, depositing 150 to 500 eggs at sites of wounding or in body orifices such as the ear, nose and urinogenital passages. The larvae hatch after 18 to 24 hours, moult once after 12 to 18 hours and moult a second time about 30 hours later. They feed for three to four days and then drop to the ground and pupate. The pupal stage lasts for seven to nine days in tropical conditions, but up to eight weeks in the subtropical winter months (Zumpt, 1965).
A number of other Chrysomya species have been associated with wound myiasis. Those considered most important in North Africa are Chrysomya albiceps and Chrysomya megacephala. Of lesser importance are Chrysomya chloropyga, Chrysomya putoria and Chrysomya marginalis.
Chrysomya albiceps is a facultative parasite and normally lays its eggs on carcasses, preferably among clusters of other blowfly eggs. The first instar larvae feed on exudations of the decomposing flesh, but second and third instars are also predacious, feeding on other blowfly larvae. They may even be cannibalistic. Although the eggs are normally laid on carcasses, they may also be laid on neglected wounds, where the larvae can cause tissue destruction. Chrysomya albiceps is frequently involved in secondary myiasis in sheep, following initial damage by Lucilia species.
The larvae of C. albiceps and the related Chrysomya rufifacies (Australasia and the Orient) are commonly known as hairy maggots because of the fleshy projections on their bodies (see Figure 4). The larvae of Chrysomya varipes also have fleshy projections, but they have fewer than the hairy maggots, are smaller when they mature in similar conditions (11 mm compared to 18 mm) and are known only from Australia (Zumpt, 1965).
Chrysomya megacephala is commonly called the oriental latrine fly because of its habit of breeding in faeces as well as on carrion and other decomposing organic matter. It may occur in large numbers around latrines and may also become a nuisance in slaughterhouses and open-air meat and fish markets. The larvae may become involved in wound myiasis of humans and animals. It appears to be a recent introduction to North Africa, having been reported previously in Africa only from South Africa and some islands in the Madagascar region (Zumpt, 1965).
Of the less important species, C. chloropyga and C. putoria are very closely related and occur in sub-Saharan Africa, the former species being a primary and secondary cause of sheep myiasis in South Africa. Chrysomya marginalis is a very common carrion breeder in Africa south of the Sahara, but is only rarely encountered in wound myiasis. Chrysomya inclinata is a less common carrion breeder in Africa, also rarely found in cases of myiasis (Zumpt, 1965). Chrysomya mallochi is a minor agent of myiasis in Australia and New Guinea (Zumpt, 1965; Smith, 1986).
Sarcophagidae
In this family, commonly called flesh flies, the two genera of importance in myiasis are Sarcophaga sensu lato and Wohlfahrtia. Females are larviparous, depositing first instar larvae rather than eggs.
Flies in the genus Sarcophaga sensu lato are very alike in all stages and extremely difficult to identify to species. Many species breed in excrement, carrion and other decomposing organic matter and may occasionally be involved in myiasis, but little is known of their larval stages. Sarcophaga cruentata (= haemorrhoidalis) (Figure 1, Example 10) is one of the most common species and breeds mainly in faeces (Zumpt, 1965; Smith, 1986).
The most important agent of myiasis in the genus Wohlfahrtia is Wohlfahrtia magnifica (Figure 1, Example 9), an obligate parasite of warm-blooded vertebrates in southeastern Europe, southern and Asiatic Russia, the Near East and North Africa. Some 120 to 170 larvae are deposited near wounds or body openings of humans and other animals such as sheep, goats, cattle, horses, donkeys, pigs, dogs, camels and geese. Camels and sheep appear to suffer most. The Larvae feed and mature in five to seven days and then leave the wound for pupation (Zumpt, 1965; Kettle, 1984).
Wohlfahrtia nuba also infests wounds of livestock in North Africa and the Near East, but it probably feeds only on dead or diseased tissues rather than on living tissues (James, 1947). Wohlfahrtia vigil and Wohlfahrtia opaca are North American species whose larvae tend to penetrate the host's skin, individually producing furuncles like those caused by Cordylobia species. They are therefore not usually associated with wound myiasis.
Muscidae
Members of the family Muscidae are not generally considered to be of significance in myiasis, but they may be involved as secondary invaders. Of the genus Musca, the ubiquitous Musca domestica (Figure 1, Example 11), the common housefly, is most often reported in cases of myiasis, although its role as a mechanical vector of various pathogens is far more important. Its larvae can develop in a large range of decomposing organic matter, animal or vegetable.
Possibly the earliest written record of New World screwworm myiasis was that of a United States cavalry officer who reported the death of horses, "occasioned by worms" in 1825 (Scruggs, 1975). Cochliomyia hominivorax was first described from adult flies bred from a human infestation; the name hominivorax means "man-eater" (Coquerel, 1858). Despite the danger posed to humans by this screwworm, it is primarily a veterinary pest. Thus, when 230000 animal cases were reported from the southern United States in 1935, only some 100 human cases were indicated. Cattle, horses, sheep, goats, pigs and dogs are frequently recorded as hosts (James, 1947), and many species of wildlife are affected (Lindquist, 1937). The Old World screwworm is similarly catholic in its selection of host. It was first given the name of screwworm in 1909 in association with a human infestation in India (Patterson, 1909), although it was not named Chrysomya bezziana until 1914. Cases of human myiasis due to C. bezziana appear to be more common in India than in Africa (Patton, 1922; Zumpt, 1965). Other hosts include cattle, water-buffalo, sheep, goats, horses, donkeys, dogs, camels, elephants (Zumpt, 1965), impala, bushbuck, waterbuck, giraffe, lion, white rhinoceros (L.E.O. Braack, personal communication), eland, black rhinoceros (M.D. Kock, personal communication) and 21 zoo species. It is probable that infestations of wildlife are more common than a review of the literature would suggest. They are not often reported because predation removes injured animals before they can be examined and because wounded, infested animals tend to seek dense cover where they are not observed (Lindquist, 1937). The marked pathological reactions to screwworm infestations are described in detail by Humphrey, Spradbery and Tozer (1980).
In Zimbabwe, C. bezziana was reported to be the second most serious insect pest of cattle after the tsetse fly, Glossina species. However, C. bezziana has not had an economic impact in Africa comparable to that of C. hominivorax in the Americas. There are a number of possible reasons for this. The distribution of tsetse fly in Africa overlaps that of C. bezziana to a considerable extent (see Figure 5), and historically cattle are not maintained in areas infested by tsetse. Thus, in general terms, cattle have not been available to C. bezziana throughout its range. Where cattle and screwworm have coincided, the resistance of native breeds may have reduced the potential problem of infestations (Zumpt, 1965). Furthermore, since African cattle tend to be managed in small numbers and under close supervision, they can be treated more promptly for myiasis than would be possible under open ranching conditions in either Africa or the Americas. The regular dipping of cattle for tick control in Zimbabwe and South Africa has also had a very beneficial effect on screwworm control there. The impact of ox-pecker birds, Buphagus species, on the control of myiasis in Africa is worthy of further investigation (Attwell, 1966). Specialized feeders equivalent to the ox-peckers are not found in the natural range of the New World screwworm.
The degree to which C. hominivorax can tolerate cold has had a major influence on its distribution. Historically, its range extended from the central and southern United States through Mexico, Central America, the Caribbean Islands and the northern countries of South America to Uruguay, northern Chile and northern Argentina (James, 1947), (see Figure 5). This distribution contracted during the winter months but expanded during the summer months, so that populations were seasonal at the edges of the distribution and year-round elsewhere. Following a major control programme in the United States and Mexico utilizing the sterile insect technique (SIT), (Knipling, 1960), the northern limits of C. hominivorax are now the border regions between Mexico, Guatemala and Belize (Graham, 1985). In 1988, C. hominivorax was introduced into the Libyan Arab Jamahiriya in North Africa, and it appears to have become firmly established there (El-Azazy, 1989; Gabaj et al., 1989).
The Old World screwworm is at present confined to the Old World; it is found throughout much of Africa (from south of the Sahara to northern South Africa), on the Indian subcontinent and in Southeast Asia (from southern China through the Malay peninsula and the Indonesian and Philippine islands to New Guinea), (James, 1947; Zumpt, 1965). It has also been introduced into several countries on the west coast of the Persian Gulf (see Figure 5). The climatic requirements of the two screwworm species are similar, and their potential distributions if unrestrained would overlap considerably (Sutherst, Spradbery and Maywald, 1989).
5. Distribution of New World screwworm (red) and Old World screwworm (green). The northern limits of the New World screwworm are its historical summer limits. in the aftermath of the sterile insect programme, the present northern limit in Central America is along the line joining the two red arrows - Distribution de la lucilie bouchère (rouge) et de Chrysomya bezziana (vert). Les limites septentrionales de la lucilie bouchère représentent ses limites estivales historiques. Après l'exécution du programme de lutte par la méthode de l'insecte stérile, la limite septentrionale actuelle en Amérique centrale se situe le long de la ligne joignant les deux flèches rouges - Distribución del gusano barrenador del ganado (rojo) y del gusano barrenador del Viejo Mundo (verde). Los limites septentrionales del gusano barrenador del ganado son sus limites históricos del verano. Tras la aplicación de la técnica de los insectos estériles, el limite septentrional actual en América Central sigue la línea situada entre las dos flechas rojas
The New World screwworm fly, C. hominivorax, is a true obligate parasite of mammals. Female screwworms do not lay their eggs on carrion; instead they lay them at the edges of wounds on living mammals or on mucous membranes associated with natural body openings such as the nostrils and sinuses, the eye orbits, mouth, ears and vagina. Virtually any wound is attractive, whether natural, for example from fighting, predators, thorns or disease or from tick, insect or vampire bat bites, or human-inflicted, such as from shearing, branding, castrating, dehorning, docking or ear-tagging. The navels of newly born animals are a common site of infestation, as are the vulval and perineal regions of their mothers, especially if traumatized during labour (see Figure 6).
Within 24 hours of oviposition, larvae emerge and immediately begin to feed on the underlying tissues, burrowing gregariously head-downward into the wound. As they feed, the wound is enlarged and deepened, resulting in extensive tissue destruction. Infested wounds have a characteristic odour attractive to gravid females, which lay further batches of eggs, resulting in up to 3000 larvae in a single wound (Lindquist, 1937). In a severe, untreated infestation death may occur.
The larvae reach maturity about five to seven days after hatching and leave the wound, falling to the ground, into which they burrow to pupate (Travis, Knipling and Brody, 1940). On completion of development, adult flies, 8 to 10 mm long, usually emerge from the puparium in the morning and mate within one to three days. About four days after mating, female flies, which are autogenous for at least the first gonotrophic cycle, are ready to oviposit. They seek a suitable host and lay 10 to 400 eggs (average 200) in a flat, shingle-like batch, all eggs oriented in the same direction. Batches are laid at intervals of three days (Thomas and Mangan, 1989), with an average of four batches per female. Adult flies live for two to three weeks on average (Knipling, 1960) and may disperse great distances, exceptionally up to 290 km in less than two weeks (Hightower, Adams and Alley, 1965), but more usually at a maximum rate of 40 to 55 km per week (USDA, 1952).
The rate of development of the immature stages is strongly influenced by temperature, being slower at low temperatures although true diapause is not entered. This effect is most pronounced in the pupal stage, which can vary from one week to two months in duration depending on the season (Laake, Cushing and Parish, 1936). Thus, the complete life cycle (see Figure 7) may take two to three months in cold weather (Parman, 1945), while in temperate conditions with an average air temperature of 22°C it is completed in approximately 24 days (Laake, Cushing and Parish, 1936) and in tropical conditions of about 29°C it is completed in about 18 days (Thomas and Mangan, 1989).
The literature on the New World screwworm is extensive and scattered, but it may be accessed rapidly by reference to the bibliography of Snow, Siebenaler and Newell (1981) and to an annual update prepared by Dr D.B. Taylor (USDA, ARS, BRL, PO Box 5674, Fargo, ND 58105, USA).
The most important stage in the life cycle requiring identification, and that most commonly encountered by veterinarians and livestock owners, is the larval stage. Larvae of the different species encountered in wound myiasis are superficially very similar (see first row of Figure 2), and C. hominivorax larvae may be easily confused with those of other species. Accurate diagnosis involves the extraction of larvae from the deepest part of an infested wound to reduce the possibility of collecting non-screwworm species, which may infest the shallower parts of the wound. Living specimens should be examined for pigmentation of the tracheal trunks (see Figure 8) and should then be placed directly into a tube containing 70 percent alcohol and returned to the laboratory for examination under a dissecting microscope at up to 50x magnification. For further techniques see Zumpt (1965) and Smith (1986).
Second instar larvae of C. hominivorax may be identified by the presence of dark pigmentation of the dorsal tracheal trunks for over half their length in the terminal segment. Other species have less marked pigmentation of the dorsal tracheal trunks; for example, it extends for no more than a third of their length in the twelfth segment of second instar C. bezziana. In the mature, third instar larvae of the New World screwworm, the pigmentation of the dorsal tracheal trunks extends from the twelfth segment forward to the tenth or ninth (see Figure 8). Such pigmentation is unique to this species among those encountered in wound myiasis and is a major character for identification keys (see Figure 4). It is most easily observed in living larvae; those in preservative may need dissection to remove opaque tissues covering the trunks.
Third instar larvae of C. hominivorax are creamy white and have a typical maggot shape, with a cylindrical body from 6 to 17 mm long and 1.6 to 3.5 mm wide (Laake, Cushing and Parish, 1936), (see Figure 8). Fully mature larvae average 15 to 16 mm in length and develop a faint reddish tinge. The prominent rings of spines around the body give the screwworm its name because of their resemblance to the threads of a screw. The spines are large compared with those of non-screwworm blowflies, the longest averaging 130 m m, and they help maintain the larva's position in the wound (see Figure 9). On the posterior face of the terminal segment, the posterior spiracles have a darkly pigmented, incomplete peritreme enclosing three straight, slightly oval-shaped slits, which point toward the break in the peritreme (see Figure 4).
Adult flies are encountered much less frequently than larvae. Cochliomyia species may be separated from those of other genera by confirmation of a deep blue to blue-green metallic body colour with three dark longitudinal stripes on the thorax (see Figure 10). This pattern is unique to C. hominivorax among species causing wound myiasis in the Old World. In the New World C. macellaria is very similar, but the species may be separated by reference to Laake, Cushing and Parish (1936), James (1947) and FAO (1990).
For complete identification keys to larvae and adults see James (1947), Zumpt (1965), Smith (1986) and FAO (1990).
Attwell, R.I.G. 1966. Oxpeckers and their associations with mammals in Zambia. Puku, Occas. Pap. Dep. Game Fish. Zambia, 4: 17-48.
Coquerel, C. 1858. Note sur les larves appartenant a une espèce nouvelle de diptère (Lucilia hominivorax). Ann. Soc. Entomol. France, 27: 171-176.
El-Azazy, O.M.E. 1989. Wound myiasis caused by Cochliomyia hominivorax in Libya. Vet. Rec., 124: 103.
FAO. 1990. Manual for the control of the screwworm fly, Cochliomyia hominivorax, Coquerel. Rome, FAO. 93 pp.
Gabaj, M.M., Awan, M.A.Q., Wyatt, N.P., Pont, A.C., Gusbi, A.M. & Benhaj, K.M. 1989. The screwworm fly in Libya - a threat to the livestock industry of the Old World. Vet. Rec., 125: 347-349.
Graham, O.H., ed. 1985. Symposium on eradication of the screwworm from the United States and Mexico. Misc. Pub. Entomol. Soc. Am., 62: 1-68.
Hightower, B.G., Adams, A.L. & Alley, D.A. 1965. Dispersal of released irradiated laboratory-reared screw-worm flies. J. Econ. Entomol., 58: 373-374.
Hope, F.W. 1840. On insects and their larvae occasionally found in the human body. Trans. R. Entomol. Soc. London, 2: 256-271.
Humphrey, J.D., Spradbery, J.P. & Tozer, R.S. 1980. Chrysomya bezziana: pathology of Old World screwworm fly infestations in cattle. Exp. Parasitol., 49: 381-397.
James, M.T. 1947. The flies that cause myiasis in man. USDA Misc. Pub. No. 631. 175 pp.
Kettle, D.S. 1984. Medical and veterinary entomology. London and Sydney, Cross Helm. 658 pp.
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