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SUMMARY
The New World screwworm (NWS), Cochliomyia hominivorax (Coquerel), and the Old World screwworm (OWS), Chrysomya bezziana Villeneuve, are both obligate parasites of mammals during their larval stages. (In this chapter, the term 'New World' refers to the Americas and the term 'Old World' refers to Europe, Africa and Asia.) Both species are in the subfamily Chrysomyinae of the family Calliphoridae of the order Diptera (true flies). Larvae feeding on the skin and underlying tissues of the host cause a condition known as wound or traumatic myiasis, which can be fatal. Infestations are generally acquired at sites of previous wounding, due to natural causes or to animal husbandry practices, but they may also occur in the mucous membranes of body orifices.
Female flies are attracted to wounds at the edges of which each female lays an average of 175 (OWS) to 340 (NWS) eggs. The larvae emerge within 12-24 hours and immediately begin to feed, burrowing head-downwards into the wound. After growth involving two moults, the larvae leave the wound and drop to the ground into which they burrow to pupate. The duration of the life-cycle off the host is temperature dependent, being shorter at higher temperatures, and the whole cycle may be completed in less than 3 weeks in the tropics.
Treatment is generally effected by application of organophosphorus insecticides into infested wounds, both to kill larvae and to provide a residual protection against reinfestation. Preventive measures include the spraying or dipping of susceptible livestock with organophosphorus compounds and, more recently, use of avermectins as subcutaneous injections to animals 'at risk'. Strict control of the movement of animals out of affected areas also acts as a preventive measure.
Identification of the agent: The larvae of NWS and OWS can be easily confused with each other and with the larvae of other agents of myiasis. Accurate diagnosis involves the identification of larvae extracted from the deepest part of an infested wound. The mature, third instar larvae are most reliable for this purpose and those of NWS can be identified by their darkly pigmented dorsal tracheal trunks extending from the twelfth segment forward to the tenth or ninth. This pigmentation is unique to the larvae of NWS among the species encountered in wound myiasis. Confirmation of OWS relies on the recognition of a characteristic combination of spinulation, the number of lobes on the anterior spiracles (4-6), and pigmentation of secondary tracheal trunks.
In the adult stage, species in the genus Cochliomyia can be separated from other genera involved in wound myiasis by confirmation of a body colour that is usually a metallic blue/green with three dark longitudinal stripes always present on the thorax. The separation of NWS from the very similar C. macellaria and the identification of adult OWS are discussed in this chapter.
Serological tests: At present there are no applicable serological tests, nor are they indicated in the identification of this disease. However, serology may have a future role in studies of the prevalence of myiasis.
Requirements for vaccines and diagnostic biologicals: There are no vaccines or biological products available except for the use of sterilised male flies in the sterile insect technique (SIT). In this technique, vast numbers of sterilised male flies are sequentially released into the environment, where their matings with wild females produce infertile eggs, leading to an initial population reduction and, progressively, eradication.
A. INTRODUCTION
The New World screwworm fly (NWS), Cochliomyia hominivorax (Coquerel), and the Old World screwworm fly, Chrysomya bezziana Villeneuve, are members of two genera of the subfamily Chrysomyinae of the Dipteran family Calliphoridae (blowflies). Both species are obligate parasites of mammals and, rarely, birds. Despite being in different genera and geographically separated, the two species have evolved in remarkable parallel. They have almost identical life histories because they fill identical parasitic respective niches in their geographical zones. The following discussion will relate to both species, except where indicated.
Unlike most other species of blowflies, adult female screwworms do not lay their eggs on carrion. Instead, they lay them at the edges of wounds on living, injured mammals or at their body orifices. Virtually any wound is attractive, whether natural (from fighting, predators, thorns, disease, and/or tick and insect bites) or man made (from shearing, branding, castrating, de-horning, docking, and/or ear-tagging). Commonly infested natural wounds are the navels of newborn animals and the vulval and perineal regions of their mothers, especially if traumatised. If eggs are deposited on mucous membranes, the larvae can invade natural body openings such as the nostrils and associated sinuses, the eye orbits, mouth, ears, and genitalia.
Within 12-24 hours of the eggs being laid, larvae emerge and immediately begin to feed on the wound fluids and underlying tissues, burrowing gregariously head-downwards into the wound in a characteristic screwworm fashion. As they feed, tearing the tissue with their hook-like mouthparts, the wound is enlarged and deepened, resulting in extensive tissue destruction. Infested wounds often emit a characteristic odour, which can be the first indication that at least one animal in a group is infested. Although the odour is not always apparent to humans, it is obviously highly attractive to gravid females (18), which lay further batches of eggs so increasing the extent of the infestation. A severe infestation that is left untreated may result in the death of the host.
Following two moults, the larvae reach maturity about 5-7 days after egg hatch. They stop feeding and leave the wound, falling to the ground into which they burrow and pupariate. The pupa develops within the puparium, a barrel-shaped protective structure formed by hardening and darkening of the cuticle of the mature larva. On completion of development, adult flies usually emerge from the puparium in the morning and work their way up to the soil surface, where they extend their wings for hardening prior to flight. Males become sexually mature and able to mate within 24 hours, but females mature their ovaries, become responsive towards males and mate only when about 3 days old. About 4 days after mating, female flies are ready to oviposit. They seek a suitable host and lay their eggs, all oriented in the same direction, firmly attached to each other and to the oviposition substrate. The numbers of eggs laid per batch vary depending on many factors (e.g. fly strain, disturbance during oviposition), but the average first batch has in the order of 175 eggs for OWS and 340 for NWS (38). Following the first egg batch, further batches are laid at intervals of 3-4 days (46). Adult flies live on average for 2-3 weeks in the field during which time they feed at flowers, and the females also take in protein, e.g. from serous fluids at animal wounds.
The rate of development of the immature stages is influenced by environmental and wound temperatures, being slower at low temperatures, although true diapause does not occur. This effect is most pronounced in the off-host pupal stage, which can vary from 1 week to 2 months' duration depending on the season (23). Thus, the complete life cycle of NWS may take 2-3 months in cold weather (33), whereas in temperate conditions with an average air temperature of 22°C, it is completed in about 24 days (23), and in tropical conditions averaging 29°C it is completed in about 18 days (46).
The degree to which NWS and OWS can tolerate cold has had a major influence on their distributions, best documented for NWS. Historically, the range of NWS extended from the southern states of the United States of America (USA), through Mexico, Central America, the Caribbean islands and northern countries of South America to Uruguay, northern Chile and northern Argentina (21). This distribution contracted during the winter months but expanded during the summer months, producing a seasonality at its edges and year round populations in the central areas - the New World tropics. Use of the sterile insect technique (SIT) in major programmes has resulted in eradication of NWS from the USA (6), Mexico (16), Curacao, Puerto Rico, and the Virgin Islands and, in Central America, from Guatemala, Belize, El Salvador, Honduras, Nicaragua and, in 2000, Costa Rica (49). The Central American eradication programme is continuing in Panama, where sterile flies were first released in July 1998. The ultimate objective is to establish a barrier zone in Panama that will become the future northern limit of NWS in the Americas. A NWS eradication programme was also officially launched in Jamaica in July 1998, as part of a plan to eradicate the species from the entire Caribbean. Although NWS is a New World species, in 1988, it was detected in Libya in North Africa where it threatened to become firmly established. However, it was eradicated in 1991 by an intensive SIT campaign (13, 24). The threat of spread of screwworms aided by modern rapid transport systems is ever present, necessitating constant vigilance from quarantine and other front-line animal health and medical officers in unaffected areas. Cases of NWS have been reported recently in Mexico, USA, and even in the United Kingdom (28).
The distribution of OWS is confined to the Old World, as the name suggests, throughout much of Africa (from Ethiopia and sub-Saharan countries to northern South Africa), the Gulf countries, the Indian subcontinent, and South-East Asia (from southern China [People's Rep. of] through the Malay Peninsula and the Indonesian and Philippine islands to Papua New Guinea) (21, 38, 42, 50). OWS myiasis has also been reported from Algeria (1), in a local shepherd, but in the absence of other reported cases, particularly animal cases, a continuing presence there seems unlikely. The situation in the Gulf area and surrounding regions is dynamic with recent reports confirmed from Iran (32) and Iraq (2). The climatic requirements of the two screwworm species are very similar and their potential distributions, if unrestrained, would overlap considerably (42).
Organophosphorus insecticides such as dichlofenthion, fenchlorphos, and in particular, coumaphos are recommended for the treatment of wounds infested with OWS and NWS (15, 34, 40). They have the effect of expelling the larvae, which die on the ground. To provide residual protection against reinfestation, they must be applied at 2-3-day intervals until the wound has healed. The contents of individual wound treatment sachets, e.g. 5 g of 5% coumaphos wettable powder, should be either sprinkled directly on to a wound or, more effectively, brushed into the wound as a paste after mixing with ordinary cooking oil (33 ml). Organophosphorus compounds may also be applied as aerosol sprays, in which marker dyes and bacteriostats are included, or as dusts that are puffed into the wound from plastic squeeze bottles. Dichlorfenthion is used in South America as a 1% aerosol to treat NWS cases and is also effective against OWS (34). Any larvae that die in the wound should be removed to prevent sepsis. Close attention should always be paid to the manufacturers' safety instructions.
Direct prevention of screwworm infestation can be achieved by spraying or dipping of livestock with coumaphos (0.25% aqueous suspension of 50% wettable powder) or other organophosphorus insecticides at the maximum concentration prescribed for external parasite control. The effects of such treatment are twofold: firstly, the treatment kills larvae directly and provides residual protection; secondly, the treatment kills ticks and other external parasites, which means that there are fewer wounds available as sites for oviposition. Synthetic pyrethroids have potential for control of screwworm larvae in wounds, but there have been few reported trials of their effect on screwworms (e.g. Permethrin versus NWS, 35). Dipping or spraying of a group of animals would be indicated if any member of the group was found to be infested, or if animals were traversing or leaving an infested area, or following wound-inducing animal husbandry practices, e.g. shearing.
A single subcutaneous injection of ivermectin (200 µg/kg) was effective against OWS in preventing navel strike of newborn calves (34) and scrotal strike of castrated calves (39). Ivermectin also prevented re-strike of treated wounds of adult cattle. Cattle treated with a sustained-release bolus of ivermectin developed no OWS myiasis from 14 to 102 days after treatment (48). However, because of the negative effects on dung-breeding fauna, it was recommended that boluses be reserved for use in containing outbreaks of OWS. Early results suggested that ivermectin may be ineffective against NWS (Mackley & Brown, in ref. 16), but more recent studies demonstrated that it can produce a significant reduction in the incidence of navel and scrotal myiasis due to NWS (7, 26). Although results of ivermectin trials show variation, results of doramectin trials are overwhelmingly positive (17). There has been an increasing number of publications reporting that a subcutaneous injection of doramectin (200 µg/kg) was up to 100% effective as a NWS prophylactic, preventing infestation of artificial wounds, umbilical or castration wounds of calves, and infestation of post-parturient cows, for up to 12-14 days post-treatment (4, 30, 31). Effectiveness depended on factors such as cattle breed and degree of challenge. In one comparative trial, doramectin and ivermectin, both at 200 µg/kg subcutaneous injection, gave 100% and 50% protection, respectively, against NWS myiasis, experimentally induced 2 hours after treatment (29). Doramectin also provided complete protection for 21 days and partial protection (56%) at 28 days post-treatment (29). In another, larger, comparative trial, doramectin had a mean efficacy of 94.6% (range 53.3-100%) compared with 43.7% (range 0-100%) for ivermectin (10). Abamectin (subcutaneous injection, 200 µg/kg) gave good, but not 100%, prevention of post-castration myiasis by NWS (3). Pour-on formulations of moxidectin, eprinomectin and doramectin gave poor protection against OWS myiasis (48) when compared with injectable formulations of doramectin against NWS. There are early indications that fipronil (a phenyl-pyrazole) might be effective as a preventive of post-castration myiasis. Similarly, topical application of an insect growth regulator (IGR), dicyclanil, to castration wounds in cattle gave good protection (>90%) against NWS myiasis (5). IGRs are very specific to insects and, therefore, are less hazardous in the environment than many other groups of insecticides.
Indirect prevention of screwworm flies infestation includes the avoidance of wounding procedures at the times of year when screwworm are numerous, the careful handling of livestock to minimise wounding, the removal of sharp objects (e.g. wire strands) from livestock pens, and the use of measures to reduce other wound-causing parasites, in particular ticks, e.g. by dipping and by insecticide impregnated ear-tags.
To prevent the spread of the screwworms beyond present limits, strict observation of the requirements for international trade, as set out in the OIE Terrestrial Animal Health Code, is necessary.
B. DIAGNOSTIC TECHNIQUES
| 1. | Identification of the agent
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| | Identification of the eggs and first instar larvae of the agents of myiasis based on morphology is difficult, and, because these stages are seldom encountered during the collection of specimens from infested wounds, they will not be considered further here.
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| | Larvae collected for diagnosis should be removed from the deepest part of the wound to reduce the possibility of collecting non-screwworm species, which may infest the shallower parts of the wound. Living specimens should first be examined for pigmentation of the dorsal tracheal trunks (Figure 1) and then be preserved in 80% ethanol and returned to the laboratory for examination under a dissecting microscope at up to x50 magnification (for further techniques see refs 12, 20, 37, 50). Optimal preservation of larvae, in their natural extended state, is made by killing them by brief immersion (15 seconds) in boiling water before storage in 80% ethanol.
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| | Second instars larvae: Second instars larvae have only two spiracular slits in each of the posterior spiracular plates compared with the three slits of third instar larvae (Figures 2 and 3). Second instars of NWS can be diagnosed by the presence of dark pigmentation of the dorsal tracheal trunks, for over half their length in the terminal segment. Other species have less extensive pigmentation of the dorsal tracheal trunks, for example, these trunks are pigmented for no more than one-third of their length in the twelfth segment of OWS. The anterior spiracles of second instar NWS have from seven to nine branches compared with about four branches in OWS (22). More positive identification may be gained by rearing living, immature larvae to third instars. This can be done on the standard meat medium used for large-scale rearing of NWS before the introduction of gel diets, i.e. in the proportion of 1 litre water, 1.3 kg ground horse meat, 50 g dried bovine blood, and 1.5 ml formalin (44), mixed and maintained at 35-38°C and 70% relative humidity. For simply rearing up larvae for identification, the exact meat and blood types are not essential, and more readily available fresh blood could be used instead of dried blood.
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| | Third instars larvae: Third instars larvae of both NWS and OWS have a robust, typical maggot shape, with a cylindrical body from 6 to17 mm long and from 1.1 to 3.6 mm in diameter, pointed at the anterior end (23, 37). Fully mature larvae of both NWS and OWS develop a reddish-pink tinge over the creamy white colour of younger larvae. Both screwworm species have prominent rings of spines around the body and these spines appear large and conspicuous under a microscope when compared with most non-screwworm species, the longest averaging 130 µm. In NWS the spines can be either single or double pointed, but in OWS they are always single pointed. The anterior spiracles of NWS each have from six to eleven well separated branches, but usually from seven to nine (Figure 2). In OWS, the anterior spiracles each have from three to seven branches, but usually from four to six (Figure 2). The latter character should not be used on its own to identify OWS, because third instars larvae of the obligate myiasis-causing species Wohlfahrtia magnifica (Diptera: Sarcophagidae), whose distribution overlaps that of OWS in the Middle East, have similarly branched anterior spiracles. Hence, in using any identification key, such as that in Figure 1, it is essential that each specimen be taken through the whole key to avoid misidentifications. On the posterior face of the terminal segment of both NWS and OWS, the posterior spiracular plates all have a darkly pigmented, incomplete peritreme enclosing three straight, slightly oval-shaped slits, which point towards the break in the peritreme. These diagnostic features are illustrated in Figure 3. Of greatest diagnostic value are the dorsal tracheal trunks, which extend forwards from the posterior spiracular plates and are darkly pigmented up to the tenth or ninth segment in NWS (Figure 1; see also refs 12, 14, 17, 20, 21, 37, 50 for identification keys). This feature is seen most easily in living larvae. Those in preservative may need dissection to remove opaque tissues covering the trunks. The dorsal tracheal trunks of OWS are darkly pigmented only in the twelfth segment. However, in OWS the secondary trachea branching off the dorsal tracheal trunks are pigmented from the twelfth segment forwards to at least the tenth segment (confirmed in specimens throughout the range, from Malaysia, Bahrain and Zimbabwe; M.J.R. Hall, unpublished). Conversely, in NWS these secondary trachea are not pigmented, only the dorsal trachea are. Hence, the tracheal pigmentation appears reversed between the two screwworm species.
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| | Adult: Adult flies needed for identification purposes are often collected using wind-oriented traps (8) and sticky traps (37) baited with a synthetic odour, swormlure-4 (27). Alternative sampling systems, using electrocuting grids or sticky surfaces at odour-baited visual targets, have been used for research purposes (18). Identification of adult flies is seldom required for the diagnosis of myiasis, because the larval stages are those most apparent to livestock owners and veterinary personnel. However, a brief description follows.
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| | i) | NWS: The body length is usually 8-10 mm long and has a deep blue to blue-green metallic colour, with three dark longitudinal stripes on the dorsal surface of the thorax. This combination of colour and pattern is not shared by any other species commonly involved in wound myiasis except the secondary screwworm of the New World, Cochliomyia macellaria (Fabricius). These two Cochliomyia species can be separated by the presence of black setulae on the fronto-orbital plates of the head of NWS compared with only light yellow hairs on the fronto-orbital plates of C. macellaria. The fifth (=fourth visible) abdominal tergite of NWS has only a very slight lateral pollinose dusting, whereas that of C. macellaria has a dense dusting, pro-ducing a pair of distinct, lateral, silvery-white spots. In addition, females of NWS have a dark brown-black basicosta, whereas those of C. macellaria have a yellow basicosta (Figure 4; see also refs 11, 14, 23, 37).
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| | ii) | OWS: The body is up to 10 mm long and has a metallic blue, bluish-purple or blue-green colour, i.e. it is very similar to NWS, but without the thoracic stripes. The lower squama (s in Figure 4) also differs from NWS, being distinctly covered with fine hairs over its entire upper surface in OWS and other Chrysomya species, whereas in NWS it is hairless above, except near the base. Adults of OWS can be distinguished from other Chrysomya found in cases of myiasis by the combination of black-brown to dark-orange-coloured anterior thoracic spiracles (rather than pale yellow, creamy, or white), with waxy-white, lower squamae (rather than blackish-brown to dirty-grey) (37, 50).
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Fig. 1. Identification key for the diagnosis of third instars larvae of Cochliomyia hominivorax and Chrysomya bezziana from cases of wound myiasis. To avoid misidentifications, it is essential that the key is worked through from the first step for each specimen.
Fig. 2. Head and first two thoracic segments of third instars larva of Cochliomyia hominovorax
(as = anterior spiracle; inset is the anterior spiracle of Chrysomya bezziana).
Fig. 3. Characteristics of third instars larva of Cochliomyia hominivorax: (A) whole larva, lateral aspect;
(B) posterior face of terminal segment; (C) posterior spiracular plate; a = anterior spiracle; b = button adjacent to opening in peritreme; p = peritreme; sl = spiracular slit; sp = spines. (After Laake et al. [23].)
Fig. 4. Characteristics of adult Cochliomyia hominivorax; note longitudinal thoracic stripes;
b = basicosta; p = fronto-orbital plate, indicated from above on whole Cochliomyia hominivorax and laterally on
head of typical calliphorid fly; s = lower squama, surface hairless except at base; v = stem vein with
hairs on dorsal posterior surface.
| | In addition to the standard morphological techniques discussed previously, more recent techniques for identification of screwworms and their geographical origins include cuticular hydrocarbon analysis (9), analysis of mitochondrial DNA (19, 25, 45), and use of random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR) (36). Problems with identification of larvae or adults from cases of myiasis can be referred to the Food and Agriculture Organisation of the United Nations Collaborating Centre on Myiasis-Causing Insects and Their Identification (Department of Entomology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom. Fax: [44.171] 938.89.37. E-mail: mjrh@nhm.ac.uk).
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| 2. | Serological tests
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| | No standardised serological tests are presently available, nor are they indicated for diagnosis of this disease. However, experimental studies have shown that serological techniques have potential value in future investigations of the prevalence of screwworm infestations in animal populations to detect antibodies to screwworm post-infestation (47).
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C. REQUIREMENTS FOR VACCINES AND DIAGNOSTIC BIOLOGICALS
There are no biological products such as vaccines, available currently. However, research towards development of potential vaccines is being conducted (43). The only proven method of eradication of NWS relies on a biological technique, the sterile insect technique, SIT (16, 24), which has also been applied experimentally to OWS (41). In this technique, male flies sterilised in their late pupal stage by gamma irradiation are sequentially released into the wild in vast numbers. Any of their matings with wild females result in infertile eggs only, leading to a progressive population reduction and, eventually, eradication. In operational situations, SIT is supported by the insecticide treatment of screwworm-infested wounds in livestock, by strict control of livestock movement, by the quarantining of infested animals and by an active publicity campaign. SIT is very expensive because of the cost of continuous production and aerial dispersion of sterile flies. Historically, it has been considered cost effective only when used as an eradication strategy in situations where the geography would favour such a programme (e.g. references 13, 24). Presently, only one New World sterile screwworm production facility exists, at Tuxtla Gutiérrez in the south of Mexico. Another will be built in Panama (for further information contact: USDA/APHIS, 4700 River Road, Riverdale, Maryland 20737, USA). An experimental facility to produce sterile OWS opened in Malaysia in 1998 (for further information contact: Institut Haiwan, Box 520, 86009 Kluang, Johor, Malaysia).
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