Anthelmintic resistance in human helminths: Learning from the problems with worm control in livestock - reply

Abstract

re!;i~ance in intestinal nematode infections of ruminants is a major threat to morbidity control in anima!s. Geerts et c! correctly state that '... the key issue in development of resistance is the percentage contribution that helminths surviving therapy make to the next generation'. Fa~or~ that select for resistance include universal treatment (eg, drenching of entire herd), a genetically closed worm population (eg. single herd on a feed lot or other enclosed environment), and extremely frequent treatment (eg. a cycle of weeks or months). Tn, e issue was recently discussed in a ~,^,/HO Informal Consultation ~.2. which emphasized that chemotherapy-based control prc~rammes against human soil4ransmitted nematodes differ significantly from programmes applied in vetednar,/medicine. Treatment is normally targeted at a subpopulation at high risk for morbidity such as school-age children ] , women of childbeanng age' ~ and pre-school children. As only a small fraction of the worm population is exposed to the anthelmintlc, there ~s greatly enhanced probability of genetic dilution by the 'refugC population. Furthermore, treatment is also g~ven at inter~.'als greater than the nematodes' generation time, whid~ will also act against the creation of drug resistance. Experience with long-term schistosome chemotherapy programmes is bi~adly reassuring with re~ard to these cunclusions. A!though drug-resistant phenotypes have been encouqtered both in the field and laboratory since the 1970s, there is little evidence for ~hese becoming fixed within populations. C:~ssresistance has not been found between eraz~quante! arid the drugs of the oxamniquine family, Drug re~stan~.e has not endange.red Lhe e~ectiveness of any of the chemotherapy- based control programmes under way in several endemic countries. The theoretical and empirical evidence su~ests that the lessons of vetedna~l programmes are of lesser relevance to predicting the outcome of control strategies for human populations. This does not imply that we should be complacent. Hence, in spite of ~he fa.ct that r~:s,st~nce ;ms not emerged as a problem in human anthelmintic treatment, measures to monitor drug resistance are clearly necessary and some steps are being taken in that direction, An attempt is under way to fonm a network of scientists and policy makers from endemic countries and Europe to investigate issues related praziquantel usage and to monitor p~,ss~ble resistance. The aim of this initiative is to provide a framework for the rational answer to numerous questions ccqnected with antl-schistosom-J chemotherapy with praziquantel. Fdrthermore, WHO, through its technically supervised country programmes and its Collaborabng Centres, is carefully mor,ito,ing cul~. ~aLes dunng the course of large-scale control programmes for intestinal nematodes. References I Savioli, L e: