Schistosoma: background document for the WHO guidelines for drinking-water quality and the WHO guidelines on sanitation and health 

Overview

• There is strong evidence linking transmission of Schistosoma to inadequate sanitation, poor hygiene and lack of access to safe drinking-water. However, there is no evidence of transmission through ingestion of drinking-water. The provision of an adequate water supply contributes to disease prevention by reducing the use of infested water for domestic purposes such as bathing and washing clothes.
• Schistosoma are blood flukes, with most human infections caused by S. mansoni, S. haematobium and S. japonicum. Humans are the definitive vertebrate host, but a wide range of animals have been shown to be infected with human schistosomes. Hybridization between human and animal schistosomes represent a potential concern for zoonotic transmission.
• Schistosoma are transmitted by skin contact through domestic (e.g bathing, laundry), occupational (e.g. fishing) and recreational use of contaminated water. Transmission involves a complex two-host life cycle starting with contamination of freshwater with eggs from infected humans (or animals) via faeces or urine, reproduction in an intermediate snail host, and release of cercariae that infect the vertebrate host by penetrating the skin. Inside hosts, they develop into adult flukes, mate and produce eggs. Person-to-person transmission does not occur.
• Schistosomiasis involves acute and chronic symptoms that present in two main forms: intestinal (S. mansoni, S. japonicum, S. mekongi, S. intercalatum) and urogenital schistosomiasis (S. haematobium). Symptoms are typically based on immune responses to eggs and are influenced by amount and location of eggs in the body.
• Schistosoma are endemic in tropical and subtropical regions. Geographical distribution of individual species varies as each has a specific range of snail hosts. Most infections occur in low-income populations with inadequate access to safe drinking-water and effective sanitation. People of all ages are susceptible to infection, but school-age children are particularly vulnerable due to increased swimming and fishing in contaminated water and inadequate hygiene.
• Schistosome eggs in urine and faeces are detected via microscopic examination and molecular methods. Qualitative methods for the detection of cercariae in standing water are available.
• In endemic areas, host snails are susceptible to invasion by schistosome eggs in faeces and urine. Once released from the host snail, cercariae can remain infective for 1 to 3 days in freshwater.
• Systematic approaches that identify and control faecal and urine contamination of surface waters, snail control programmes and provision of safe drinking-water to reduce exposure to contaminated water through domestic use is needed to manage risks from Schistosoma.
• Schistosome eggs should be removed by conventional wastewater and drinking-water treatment processes through size exclusion (e.g. sand filtration, membranes) or settling processes (e.g. sedimentation basins, stabilization ponds). Disinfection processes, including chlorination and UV light, are effective for inactivating cercariae.
• Due to the complex life cycle and environmental survival of Schistosoma, E. coli (or, alternatively, thermotolerant coliforms) is not a suitable indicator for the possible presence or absence of Schistosoma in drinking-water.