Our data confirm that polyparasitism is very common in rural Côte d'Ivoire and that people have clear perceptions about the morbidity caused by some of these parasitic infections. Our findings can be used for the design and implementation of sound intervention strategies to mitigate morbidity and co-morbidity.

keywords Schistosoma mansoni, praziquantel, resistance correspondence Y.S. Liang, Jiangsu Institute of Parasitic Diseases, Meiyuan, Wuxi, Jiangsu 214064, P. R. China. The tolerance of Schistosoma mansoni to praziquantel has been reported in some endemic regions (Fallon et al. 1995; Stelma et al. 1995; Ismail et al. 1996; Guisse et al. 1997). To establish the reasons for clinical failures of praziquantel, a simple, quick and economic assay is required to detect resistance. Ideally this will involve the use of eggs or miracidia since these are the stages of the parasite life cycle which can easily be obtained from the faecal material of infected humans. As praziquantel causes changes in the shape of miracidia (Coles 1979), this was used as the basis for designing a test for resistance. In 24-well flat bottom microplates we observed the effect of praziquantel on miracidia hatched from eggs obtained from the faeces of mice infected with six isolates of S. mansoni. Two isolates were praziquantel-susceptible (one from Puerto Rico and one a mixture of isolates from Puerto Rico, Brazil, Egypt and Kenya), and four isolates were praziquantel-insusceptible, including a laboratory-selected praziquantel-resistant population (Fallon & Doenhoff 1994) and three Senegalese isolates. The cessation of swimming of miracidia was observed in different concentrations of praziquantel at various times and then the morphological changes were checked by adding a drop of Lugol's iodine. When the miracidia of both the susceptible and insuscep-tible isolates were exposed to 10-3 and 10-4 M praziquantel, they immediately contracted in the middle part of their bodies, giving the shapes of an unequal dumbbell or calabash, with the greater mass at the anterior end. In 5 10-6 M prazi-quantel 100% of miracidia from the susceptible isolates immediately changed shape, whereas only 11-15% of those from the insusceptible isolates did. Thus by addition of Lugol's iodine immediately after administering praziquantel, an objective measure of susceptibility could be obtained. After 1 minute in 10 6 M praziquantel 52% to 100% of susceptible miracidia had changed shape, and after 5 min 100% had done so compared with 3% to 15% and 9% to 18% of the insus-ceptible miracidia. Susceptibility could also be detected by determining whether miracidia had stopped swimming but this was less easy to read as a test than change in shape. By exposing freshly hatched miracidia to 10 6 M prazi-quantel and observing change in shape over one minute it should be possible to determine whether failed therapy is due to the presence of praziquantel-tolerant worms. It is planned to investigate this in field trials in China. The work was supported by the UNDP/World Bank/WHO Special Programmme for Research and Training in Tropical Diseases. References Coles GC (1979) The effect of praziquantel on Schistoma mansoni. Journal of Helminthology 53, 31-33. Fallon PG, Sturrock RF, Niang AC, Doenhoff MJ (1995) Short report: diminished susceptibility to praziquantel in a Senegal isolate of Sch...

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