Only female mosquitoes take blood meals. Not all anopheline mosquitoes transmit malaria, but as a general rule irrigation development results in fauna simplification which favours vector species. Schistosomiasis is caused by parasitic trematode worms which in their adult form live in the blood stream of human hosts and which, to complete their lifecycle, need to pass a larval stage in certain species of aquatic or amphibious snails. The ecological requirements of these so-called intermediate host snails are a key determinant in the distribution of the disease. Aquatic weeds provide an important substrate for the snails.
Human behaviour is, therefore, the other key determining factor.
Pigs are the main amplifying host of the virus and migratory birds are suspected to play a role in the distribution of the virus over large distances. The association with the irrigated environment only exists where anophelines are the vectors, i.e. in Central Africa and where Mansonia mosquito larvae can develop attached to the roots of aquatic weeds, in South and South East Asia. There is only one documented case of an irrigation scheme in West Africa where a steep canal gradient created a favourable condition for blackfly breeding. Spillways of dams are well known to create this risk, but, on the other hand, impoundments will eliminate any breeding in the inundated parts. This section looks at the human health risks as a discussion of the environmental impacts.