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

Overview

• There is moderate evidence linking transmission of Fasciola with unsafe drinking-water and inadequate sanitation. Fascioliasis transmission through ingestion of contaminated surface water is supported by indirect evidence, including statistically significant associations of fascioliasis and other waterborne parasitic infections.
• Fasciola are vector-borne zoonotic pathogens. Sheep, goats, cattle, buffalo, pigs and equines are the primary hosts of Fasciola. Humans can also be hosts.
• Fasciola have a two-host lifecycle involving release of eggs from infected hosts, passage through lymnaeid snails and shedding of cercariae which transform into infectious metacercariae upon attachment to freshwater plants. Consumption of uncooked freshwater plants (e.g. watercress) is the main route of transmission, but contaminated water may also be important. Person-to-person transmission is not possible.
• Asymptomatic infection occurs in 12% to 15% of cases. The acute phase of symptomatic fascioliasis lasts for 3 to 4 months and includes fever, abdominal pain, gastrointestinal disturbance, urticarial and respiratory symptoms. The chronic phase can last from 9 to 14 years, with symptoms including fatty food intolerance, nausea, jaundice, pruritus, right upper-quadrant abdominal tenderness and biliary colic.
• In endemic areas, prevalence peaks in 9- to 11-year-olds; in developed countries, prevalence peaks in 30- to 40-year olds. Fascioliasis follows seasonal patterns influenced by temperature and rainfall, which vary depending on latitude and altitude and can include year-long transmission, mono-seasonal transmission and bi-seasonal transmission.
• Evidence indicates that metacercariae are highly infective, but the infective dose has not been determined.
• Microscopic methods are available to detect and identify metacercariae in faeces, wastewater, on plants and in water. Molecular methods are also available.
• Fasciola are found in freshwaters inhabited by lymnaeid snails. Metacercariae can survive for long periods of time in water.
• Systematic approaches that identify and control faecal contamination along the whole sanitation service and drinking-water supply chains are needed to manage microbial risks, including from Fasciola. Since animals are the primary source of Fasciola, a comprehensive approach that also deals with the management of animal faeces is necessary to control transmission.
• Reduction of Fasciola by wastewater and drinking-water treatment varies depending on the process. Removal is based on physical removal (e.g. size exclusion). Fasciola metacercariae are likely to be resistant to disinfection processes such as chlorination.
• Due to the complex life cycle and environmental survival of Fasciola, E. coli (or, alternatively, thermotolerant coliforms) is not a suitable indicator for the possible presence or absence of Fasciola in drinking-water.