Health impact evaluation of wastewater use in Mexico.

Wastewater from Mexico city is used to irrigate over 85 000 hectares, mainly of fodder and cereal crops in the Mezquital Valley. A cross-sectional study method is being used to test the impact of exposure to raw wastewater and wastewater from storage reservoirs on diarrhoeal disease and parasitic infections in farmworkers and their families. The study population in the rainy season survey included 1 900 households: 680 households where the farmworker is exposed to untreated wastewater (exposed group), 520 households exposed to reservoir water (semi exposed group), and 700 households where the farmworker practices rain-fed agriculture (control group). Preliminary analysis of the data from the rainy season study (dry season study in progress) has been carried out. Current information indicates that the risk of Ascaris lumbricoides infection is much higher in the exposed group than in the semi-exposed and the control groups. According to the procedures employed Entamoeba histolytica infection was more frequent in individuals aged 5 to 14 years exposed to wastewater, than in those in the control group. When diarrhoeal disease rates were analyzed, children under age 15 years from exposed households had a significantly higher prevalence than controls. The final results of this study are expected to aid decisions on low cost treatment options and other health protection measures within the National Wastewater Reuse Programme in Mexico. The implications for assessment of the WHO guidelines for the use of wastewater in agriculture are discussed.