IMPACT OF WASTEWATER IRRIGATION ON INTESTINAL INFECTIONS IN A FARMING POPULATION IN MEXICO, RU1 }4EZQUfTAL VALLSY

An opportunistic study was carried out in central Mexico, where one of the world's largest wastewater reuse schemes for agricultural production is located. This scheme provided a unique opportunity to assess the health impact of exposure to wastewater of different qualities on intestinal parasitic infections and diarrhoeal diseases. The central objective of the study was to evaluate the effect of hydraulic retention on reducing the health risks associated with wastewater use. Exposure groups were defined according to eligibility procedures and to the quality of irrigation water. Microbiological quality was measured using nematode eggs and faecal coliforms as indicators. The exposure groups involved households: a) exposed to untreated wastewater; b) exposed to wastewater retained in a single reservoir; c) exposed to wastewater which had passed through two reservoirs in series, and been retained for some time in both; and d) nonwastewater-exposed households (controls). The study outcomes included risk of Ascaris lumbricoides, Entamoeba histolytica and Giardia lamblia infections, as well as the risk of diarrhoeal diseases. The study design was based on two cross-sectional surveys (rainy and dry seasons), and the analyses focused on both comparison of risks between the different exposure groups as well as the identification of at-risk groups. The two surveys involved different intermediate groups (b) and (c) above and the main purpose was to assess the effects of single versus double hydraulic retention. They are distinguished mainly for this purpose, rather than the evaluation of possible seasonal fluctuations of the study outcomes. Other variables (Le. socioeconomic, hygiene and sanitation) were analysed as confounders using a multivariate model. In young children the prevalence rates of A. lumbricoides infection were considerably higher in the raw wastewater group (13.7%), and lower prevalences were observed with decreasing exposure (11.8% in the single reservoir group, 3.3% in the double reservoirs group, and 0.6 2.5% in the control group). A high prevalence rate of G.lamblia was observed in children (17 20.5%), but no association with untreated wastewater was found. The prevalence rates of E. histolytica infection in children from the various exposure groups ranged between 4.8 7.0%, but were considerably higher in older individuals: 15.7 and 16.5% in the two surveys among the raw wastewater group, compared with 13.2% and 14.7% respectively in the controls. In addition, a high prevalence of diarrhoeal diseases (two-weeks recall period) was found in the rainy season, particularly in young children from the raw wastewater exposure group, and lower prevalences were observed with