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The South-to-North Water Diversion Project (SNWDP) is currently the key, national, water-conservation project in China, designed to optimise the use of water resources and relieve the water shortages in the north of the country. As one of the main water intakes for the project, that of the Eastern Route Scheme (ERS), is a breeding site for Oncomelania hupensis (the intermediate host of Schistosoma japonicum), there is concern that the snail may be carried far to the north, in the water passing through the project. To see if they could survive and breed to the north of their current range in China, O. hupensis were collected in marshland near Nanjing City and transferred to cages, on the banks of fish ponds, in the cities of Zhenjiang (in Jiangsu province, at 32 degrees 10'N), Xuzhou (in the same province but at a latitude of 34 degrees 23'N) and Jining (in Shandong province, at 35 degrees 23'N). Except over the first 6 months in Xuzhou, the snails moved north of their natural distribution did not survive and reproduce as well as those in Zhenjiang, and all those transferred to Jining died out within 1 year. Although the snail populations in Xuzhou survived for 7-8 years and retained their infectivity to S. japonicum, histological and histochemical studies revealed abnormalities in the reproductive organs of these snails. It is concluded that, unless global warming significantly increases the minimum winter temperatures in northern China, the SNWDP is unlikely to result in the northward spread of schistosomiasis japonica.

If there is a change in the biological characteristics of schistosomes associated with the development of resistance to praziquantel, this could affect the transmission and pathology of the diseases they cause. To investigate this possibility, the host-parasite relationships of five praziquantel-resistant and five praziquantel-susceptible isolates of Schistosoma mansoni were compared in Biomphalaria glabrata snails and outbred CD(1) albino mice. Whether praziquantel-resistant or -susceptible, all the laboratory-selected isolates gave similar results in B. glabrata. However, the snails infected with any of three field-collected isolates from Senegal, each of which has been shown to be praziquantel-resistant, shed fewer cercariae and survived longer than the snails infected with the drug-susceptible or laboratory-selected, drug-resistant isolates. There were no differences between isolates in terms of their infectivity to mice. However, mice infected with any of four of the five drug-resistant isolates shed more eggs in their faeces than mice carrying the drug-susceptible parasites, and mice infected with any of the resistant isolates also had larger numbers of eggs in their tissues. Although granuloma sizes were generally similar for all isolates, the praziquantel-resistant isolates may be more pathogenic in mice than the susceptible ones because of their relatively high egg productions.