Case studies of emergency management of screwworm.

Screwworm myiasis, caused by infestation of even minor wounds by the obligative parasitic larval stages of the New World screwworm (NWS) (Cochliomyia hominivorax) or Old World screwworm (OWS) (Chrysomya bezziana) flies, is a major cause of livestock morbidity and mortality in tropical and sub-tropical regions of the world. The two parasites occur in different hemispheres but are remarkably homologous. Animal health emergencies result from the invasion of new territories by the parasites or, in the case of NWS, reinfestation of areas from which the parasite had been eradicated after great effort and expense. The author reviews the biology of the parasites and the effects of screwworm, in addition to prevention of infestation upon the introduction of animals. Examples of three programmes or events are described. The first is the eradication of previously exotic NWS from an epizootic in Libya before the parasite spread to become enzootic in the Mediterranean Basin and eventually other areas of the Eastern Hemisphere. The second example reviews the serious consequences of the extension of the range of OWS into Iraq where conditions at the time were favourable for propagation and unfavourable for control. The third example describes the NWS programme strategy in North and Central America which, for forty years, has been to progressively achieve eradication and then protection of areas from north to south on that continent, employing the sterile insect technique (SIT). Outbreaks in areas where screwworm has already been eradicated divert costly programme resources and slow progress southwards, and are considered emergencies. Some problems encountered and the solutions found during the height of the eradication programme in Mexico are described. Although to date eradication of screwworms has only been accomplished with the application of SIT, this technique alone will not eradicate the pest. The author describes other elements which are required to control or eradicate screwworms. Programmes for this highly mobile parasite encompass large geographic areas and consequently require active and continuous international participation.