Cochliomyia hominivorax - the New World Screwworm fly (NWSF)
Possibly the earliest written record of New World Screwworm fly (NWSF) 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 230,000 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).
Third instar larvae of C. hominivorax are creamy white and have a typical "smooth" maggot shape, with a robust cylindrical body from 6 to 17 mm long and 1.6 to 3.5 mm wide (Laake et al, 1936). 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, backwardly-directed and bearing single or double points, help maintain the larva's position in the wound; they are arranged in irregular rows and are large compared with those of non-screwworm blowflies, the longest averaging 130µm. The anterior spiracles have six to twelve (usually seven to nine) relatively long, well-separated branches. 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.
Cochliomyia hominivorax larvae can be identified by the presence of dark pigmentation of the dorsal tracheal trunks extending forward from the twelfth segment to the tenth or ninth. Other species have less marked pigmentation of the dorsal tracheal trunks. Such pigmentation is unique to this species among those encountered in wound myiasis and is a major character for identification keys. This pigmentation is most easily observed in living larvae of New World screwworm; those in preservative may need dissection to remove opaque tissues covering the trunks. The secondary tracheal trunks are unpigmented, in contrast to those of the Old World Screwworm fly, Chrysomya bezziana, in which the primary tracheal trunks are unpigmented past segment twelve, but the secondary trunks are dark.
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.
The New World Screwworm fly, Cochliomyia hominivorax, is a true obligate parasite of mammals. 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 perianal regions of their mothers, especially if traumatised. If eggs are deposited on mucous membranes, the larvae can invade undamaged 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 (Hall, 1995), 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.
Screwworm larvae pass through three stages (or instars), separated by cuticular moults that facilitate rapid growth, and they reach maturity about 5–7 days after egg hatch. They then 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 need to mature their ovaries over 6-7 days, and they only become responsive towards males and mate 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, like a tiled roof, 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 340 eggs for NWSF (Spradbery, 1994). Following the first egg batch, further batches are laid at intervals of 3–4 days (Thomas and Mangan, 1989). 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 (Laake et al. 1936). Thus, the complete life cycle of NWS may take 2–3 months in cold weather (Parman, 1945), whereas in temperate conditions with an average air temperature of 22 °C, it is completed in about 24 days (James, 1947), and in tropical conditions averaging 29 °C it is completed in about 18 days (Thomas and Mangan, 1989).
Historically, the range of NWSF 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 (James, 1947). 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 NWSF from the USA, Mexico, Curacao, Puerto Rico, and the Virgin Islands and, in Central America, from Guatemala, Belize, El Salvador, Honduras, Nicaragua and, in 2000, Costa Rica (Graham, 1985; Baumhover, 2001; Wyss, 2001). 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 NWSF in the Americas. A NWSF eradication programme was also officially launched in Jamaica in July 1998, as part of a plan to eradicate the species from the entire Caribbean. This programme has encountered severe setbacks due to a complex combination of management and technical difficulties but is ongoing (Dyck et al., 2005).
Although NWSF 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 (Lindquist et al., 1992). 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 NWSF have been reported recently in Mexico, the USA, and even in the United Kingdom in tourists returning from endemic areas (Mallon et al., 1999).
- Hall, M.J.R. (1991) Screwworm flies as agents of wound myiasis. Pp. 8 17 in, World Animal Review, Special Issue "New World Screwworm: Response to an Emergency", October 1991, R.D.S. Branckaert (Ed.) 52 pp.
- Hall, M.J.R. (2004). New World screwworm (Cochliomyia hominivorax) and Old World screwworm (Chrysomya bezziana). Chapter 2.2.8., pp.370-379, in, Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (mammals, birds and bees), Fifth Edition, Volume 1. World Organisation for Animal Health, Paris, France, 1178 pp.
- Baumhover, A.H. (2001). A personal account of programs to eradicate the screwworm, Cochliomyia homnivorax, in the United States and Mexico with special emphasis on the Florida program. Florida Entomologist, 84, 162.
- Dyck, V.A., Reyes Flores, J., Vreysen, M.J.B., Regidor Fernández, E.E., Teruya, T., Barnes, B., Gómez Riera, P., Lindquist, D. & Loosjes, M. (2005). Management of area-wide integrated pest management programmes that integrate the sterile insect technique. Chapter 5.3, pp 525-545, in Sterile Insect Technique : principles and practice in area-wide integrated pest management, Dyck V.A., Hendrichs J. & Robinson A.S. (Eds), Springer, Dordrecht, xiv + 787 pp.
- Graham, O.H., Ed. (1985). Symposium on eradication of the screwworm from the United States and Mexico. Miscell. Pub. Entomol. Soc. Am. 62: 1–68.
- Hall, M.J.R. (1995). Trapping the flies that cause myiasis: their responses to host-stimuli. Ann. Trop. Med. Parasitol. 89: 333–357.
- James, M.T. (1947). The Flies that Cause Myiasis in Man. United States Department of Agriculture Miscellaneous Publication No. 631, USDA, 175 pp.
- Laake, E.W., Cushing, E.C. & Parish, H.E. (1936). Biology of the Primary Screwworm Fly, Cochliomyia americana, and a Comparison of its Stages with those of C. macellaria. United States Department of Agriculture, Technical Bulletin No. 500, USA, 24 pp.
- Lindquist, D.A., Abusowa, M. & Hall, M.J.R. (1992). The New World screwworm fly in Libya: a review of its introduction and eradication. Med. Vet. Entomol. 6: 2–8.
- Mallon, P.W.G., Evans, M., Hall, M. & Bailey, R. (1999). Something moving in my head. Lancet 354: 1260.
- Parman, D.C. (1945). Effect of weather on Cochliomyia americana and a review of methods and economic applications of the study. J. Econ. Entomol. 38: 66–76.
- Spradbery, J.P. (1994). Screw-worm fly: a tale of two species. Agric. Zoo. Rev. 6: 1–62.
- Thomas, D.B. & Mangan, R.L. (1989). Oviposition and wound-visiting behaviour of the screwworm fly, Cochliomyia hominivorax (Diptera: Calliphoridae). Ann. Entomol. Soc. Am. 82: 526–534.
- Wyss, J.H. (2001). Screwworm eradication in the Americas. Proceedings of the 19th Conference of the OIE Regional Commission for Europe, Jerusalem (Israel), 19–22 September 2000, Office International des Epizooties, Paris, France, 239–244.