Acknowledgements This work received financial support from the UNDP/World Bank/WH0 Special Program for Research and Training in Tropical Diseases (TDR) contract 980268 and from the 0ffice for Schistosomiasis Control of Hunan Health Bureau of the People's Republic of China. We thank Shunke Zhang, Ganwei Yan, Xinshong Luo, Zhongjie Li and Qinren Zeng for their excellent technical assistance and Xiaoye Wu for her precious help. We thank our colleagues at Department of Parasitology for their support.