Old world screwworm (Chrysomya bezziana)

Multiple species

Old World screwworm (NWS), Chrysomya bezziana Villeneuve, is an obligate parasite of mammals, including humans, during their larval stages. It belongs 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 eggs. The larvae emerge within 12–24 hours and immediately begin to feed, burrowing head-downwards into the wound. After developing through three larval stages (instars) involving two molts, 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 (especially doramectin) as subcutaneous injections to animals ‘at risk’. Strict control of the movement of animals out of affected areas also acts as a preventive measure. 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. The zoonotic implications are considerable because humans, especially the young, elderly or infirm, can be infested, with severe and sometimes fatal consequences.