Stephen T. McGarvey, Gemiliano D. Aligui, Jonathan D. Kurtis, Arve Lee Willingham III, Hélène Carabin, Remigio Olveda
Schistosomiasis is one of the six diseases of most concern to the World Health Organization and among human parasitic diseases ranks only behind malaria in terms of socioeconomic and public health importance in tropical and subtropical areas. There are an estimated 600 million individuals residing in schistosomiasis endemic regions with approximately 200 million infected at any time (WHO 1993). Schistosoma japonicum is endemic to the Philippines, the People's Republic of China, and Indonesia. It is unique among the major schistosomes infecting humans in that its intermediate host snail is amphibious and it is the only schistosome for which zoonotic transmission is considered important with domesticated animals serving as important reservoir hosts of the parasite (Jordan et al. 1993). About 69-75 million individuals are at risk of infection from S. japonicum, including approximately 1.5 million current infections (Chitsulo 1998).
The purpose of this chapter is to describe and interpret the results from several community-based studies on S. japonicum in the Philippines and China, and to present the rationales for two new interdisciplinary research projects in the Philippines. This is not an exhaustive review of all studies on S. japonicum done by the many research groups and individuals in the endemic nations. The emphasis is on the insights obtained from the studies conducted in the 1980s and 1990s by our research group and how they led to new research with an explicit human ecological and interdisciplinary set of perspectives.
The ecology and transmission of schistosomiasis in the Philippines and China results from the interaction of the parasite S. japonicum with its complex life cycle, human and other mammalian definitive hosts, regular exposure of hosts to the cercaria, the infective form of the parasite in fresh water, tropical or semi-tropical rainfall and temperature patterns, all within the context of rural poverty and the lack of sanitation systems (Jordan et al. 1993). Faeces of infected humans and other mammals contain S. japonicum eggs, which hatch in water into miracidia and then infect the intermediate host snail species. The cercerial form emerges from infected snails, penetrates the skin of potential hosts and after complex developmental changes and migration within hosts over several weeks, adult worms take up residence in the mesenteric veins. Male and female worms find one another