Human helminth infections--future research foci.

Abstract

It was encouraging to read that plans are being made to improve our understanding of the impact of helminths on human health and to promote their control in Africa [1xHuman helminth infections – future research foci. Olsen, A. et al. Trends Parasitol. 2001; 17: 303–305Abstract | Full Text | Full Text PDF | PubMed | Scopus (5)See all References[1]. There is a major international effort on malaria control, which predominantly features the problem of drug resistance. However, there was no mention under Box 4 (recommendations for helminth control) in the article by Olsen et al. on the monitoring for anthelmintic resistance or the use of strategies that would reduce the probability of resistance developing [1xHuman helminth infections – future research foci. Olsen, A. et al. Trends Parasitol. 2001; 17: 303–305Abstract | Full Text | Full Text PDF | PubMed | Scopus (5)See all References[1].We defined the presence of drug resistance in Schistosoma if, in any trial, the efficacy is significantly less than in a fully susceptible population (assuming that other factors which affect drug efficacy have been taken into account) [2xDefining resistance in Schistosoma. Coles, G.C. and Kinoti, G.K. Parasitol. Today. 1997; 13: 157–158Abstract | Full Text PDF | PubMedSee all References[2]. On this basis, there already appears to be drug resistance in hookworms [3xFailure of pyrantel in treatment of human hookworm infections (Ancylostoma duodenale) in the Kimberley region of North West Australia. Reynoldson, J.A. et al. Acta Trop. 1997; 68: 301–312Crossref | PubMed | Scopus (90)See all References, 4xFailure of mebendazole in treatment of human hookworm infections in the southern region of Mali. Clercq, D. et al. Am. J. Trop. Med. Hyg. 1997; 57: 25–30PubMedSee all References]. Resistance to praziquantel has been detected in schistosomes from Egypt [5xCharacterisation of isolates of Schistosoma mansoni from Egyptian villagers that tolerate high doses of praziquantel. Ismail, M.S. et al. Am. J. Trop. Med. Hyg. 1996; 55: 214–218PubMedSee all References[5] and in three isolates of Schistosoma mansoni from Senegal [6xIn vitro responses of praziquantel-resistant and -susceptible Schistosoma mansoni to praziquantel. Liang, Y.S. et al. Int. J. Parasitol. 2001; 31: 1227–1235Crossref | PubMed | Scopus (60)See all References[6]. This information, together with the clear warnings already issued [7xChemotherapy of human nematodes: learning from the problems in sheep. Coles, G.C. J. R. Soc. Med. 1995; 88: 649–651See all References, 8xAnthelmintic resistance in human helminths: learning from the problems with worm control in livestock. Geerts, S. et al. Parasitol. Today. 1997; 13: 149–151Abstract | Full Text PDF | PubMed | Scopus (49)See all References], must be considered when planning human helminth control.Van Wyk [9xRefugia – overlooked as perhaps the most potent factor concerning the development of anthelmintic resistance. Van Wyk, J.A. Onderstpoort J. Vet. Res. 2001; 68: 55–67PubMedSee all References[9] has argued strongly and, I believe correctly, that the most important issue in the development of resistance to anthelmintics in animals is the number of parasites that are in refugia (not exposed to anthelmintics) rather than the frequency of treatment. Where drought reduces the number of free-living larval stages, few anthelmintic treatments are required for resistance to develop [10xDrought and flock isolation may enhance the development of anthelmintic resistance in nematodes. Papadopoulos, E. et al. Vet. Parasitol. 2001; 97: 253–259Crossref | PubMed | Scopus (53)See all References[10] because the next generation of worms comes largely or exclusively from those surviving treatment. Thus, anthelmintic treatment during the dry season will encourage the development of resistance in human nematodes. The mass use of anthelmintics or schistosomicides is likely to run into serious problems of resistance, unless the parasite is highly sensitive to the anthelmintic used (e.g. praziquantel and S. japonicum in China) [11xSusceptibility of Schistosoma japonicum to praziquantel in China. Liang, Y.S. et al. Trop. Med. Int. Health. 2001; 6: 707–714Crossref | PubMed | Scopus (22)See all References[11]. If the development of resistance in worms is almost inevitable with total reliance on chemical control, then effective vaccines must be developed or control must rely primarily on improved hygiene. Health education and improved sanitation are more important than the ‘easy’ option – using anthelmintics.