Schistosomiasis or bilharzia is a tropical disease caused by worms of the genus Schistosoma. The transmission cycle requires contamination of surface water by excreta, specific freshwater snails as intermediate hosts, and human water contact. The main disease-causing species are S haematobium, S mansoni, and S japonicum. According to WHO, 200 million people are infected worldwide, leading to the loss of 1.53 million disability-adjusted life years, although these figures need revision. Schistosomiasis is characterised by focal epidemiology and overdispersed population distribution, with higher infection rates in children than in adults. Complex immune mechanisms lead to the slow acquisition of immune resistance, though innate factors also play a part. Acute schistosomiasis, a feverish syndrome, is mostly seen in travellers after primary infection. Chronic schistosomal disease affects mainly individuals with long-standing infections in poor rural areas. Immunopathological reactions against schistosome eggs trapped in the tissues lead to inflammatory and obstructive disease in the urinary system (S haematobium) or intestinal disease, hepatosplenic inflammation, and liver fibrosis (S mansoni, S japonicum). The diagnostic standard is microscopic demonstration of eggs in the excreta. Praziquantel is the drug treatment of choice. Vaccines are not yet available. Great advances have been made in the control of the disease through population-based chemotherapy but these required political commitment and strong health systems.

Schistosomiasis is a tropical parasitic disease, caused by blood-dwelling worms of the genus Schistosoma. The main human species are S mansoni (occurring in Africa and South America) and S japonicum (South and East Asia) causing intestinal and hepatosplenic schistosomiasis, and S haematobium (Africa) causing urinary schistosomiasis. Severe symptoms develop in predilected people with heavy and long-standing infections. Acute schistosomiasis, a flulike syndrome, is a regular finding in travel clinics. Although prevalences can be high, most infected people show limited, intermittent, or aspecific symptoms. The diagnosis of schistosomiasis relies on microscopic examination of stools or urine, serologic tests, and imaging. Praziquantel is the drug of choice, active against all species in a single or a few oral doses. Current control strategies consist mainly of preventive therapy in communities or groups at risk.

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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 praziquantelinsusceptible, including a laboratory-selected praziquantelresistant 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 insusceptible 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 ϫ 10 -6 M praziquantel 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 Ϫ6 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 insusceptible 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 Ϫ6 M praziquantel 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. ReferencesColes 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 Schistosoma ...

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