Manual for Diagnosis of Screw-worm Fly - xcs consulting

1. Introduction
Agriculture, Fisheries and Forestry - Australia
Myiasis is the word used to describe an infestation of living vertebrate tissue by the larvae of
flies (Diptera) and is derived from the greek, myia – a fly. It is a widespread condition among
livestock and man (James 1947, Zumpt 1965).
Myiasis has been classified according to the habits of the fly species: obligate if the larvae
can only exist on living tissues, facultative, where larvae that normally feed on dead tissue
develop on living tissue if the female opportunistically lays on a living host. Myiasis is also
classified according to the site of infestation: traumatic (wound), aural, ocular, nasal and
oral and associated sinuses, anal and vaginal, enteric etc. Some species of flies produce
subdermal myiasis when access is via unbroken skin such as the Bot flies (Gasterophilus
and Cuterebra spp.) and the Warble flies (Hypoderma spp.).
There are at least 20 species of obligate myiasis-producing Diptera, more than 50 species
which are responsible for facultative myiasis and a further 35 species recorded as accidental
agents of myiasis (James 1947). Two of the most important of the obligate myiasis flies are
the Old World screw-worm fly (Chrysomya bezziana) and the New World screw-worm fly
(Cochliomyia hominivorax).
Identification of the fly species implicated in myiasis in our region has confirmed that in most
cases of cutaneous myiasis recorded in Africa, Arabia, India and SE Asia, the species
involved is C. bezziana. For example, in Malaysia C. bezziana was involved in 84% and 95%
of cattle myiases (Basset and Kadir 1982, Rajamanickam et al.1986). In Papua New Guinea
95% of recorded myiases involved C. bezziana (Norris and Murray 1964). In India, 99% of
myiases in cattle were due to C. bezziana (Narayan and Pillay 1936). A myiasis survey in the
Sultanate of Oman showed that C. bezziana was implicated in 93% of myiases in goats and
sheep.
The New World screw-worm fly, Co. hominivorax, was formerly distributed throughout the
southern U.S.A. and is present in Central America, the Caribbean and tropical and subtropical
South America. During summer months, Co. hominivorax used to expand its range
northwards, sometimes infesting up to one third of temperate continental U.S.A. (3 million
km 2). Recent eradication programs have eliminated Co. hominivorax from the U.S.A, Mexico
and the Caribbean Islands of Curacao, Netherlands Antilles and Puerto Rico (Graham 1985,
Krafsur et al. 1987). Co. hominivorax was accidently introduced to Libya in 1988 (Al-Azazy
1989, Gabaj et al.1989), and was eradicated using sterile flies from Mexico (FAO 1992).
Co. hominivorax and C. bezziana appear to be very similar in many biological and ecological
respects. Co. hominivorax is slightly larger than C. bezziana with smaller eggs and a higher
fecundity (e.g. native Co. hominivorax lay 200-400 eggs per clutch compared with 190-250
eggs in C. bezziana). Female screw-worm flies (SWF) of both species lay their eggs on the
edge of wounds or in and around moist body orifices, the resulting larvae invade the tissues
of the host, producing lesions which may lead to loss of condition through inappetence,
maiming, infertility if the genitals are infested, or death (Humphrey et al. 1980).
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