Investigation of Typhoid Fever and Cholera Transmission by Raw Wastewater Irrigation in Santiago, Chile

Abstract

Santiago, the capital city of Chile, has suffered for years from high rates of typhoid fever, reaching peaks as high as 210 cases/100,000 in 1977 and 1982. Many officials suspected that the use of raw wastewater to irrigate 13,500 ha of vegetables and salad crops may have been one of the modes of transmission. However, control measures have in general been ineffective. In April, 1991 an outbreak of 41 cases of cholera occurred in Chile probably initiated by the penetration of cholera cases from adjacent Peru which was undergoing an explosive cholera epidemic. Investigations showed that there was strong circumstantial evidence supporting the hypothesis of typhoid and cholera transmission by wastewater irrigated crops. From data gathered it is shown that while there was little seasonal typhoid fever variation in the rest of the country, there was a decided summer peak in Santiago that coincided with the peak irrigation season and harvesting of sewage irrigated vegetables. In the 1991 cholera outbreak, which occurred during the irrigation season, 68% of the cases had consumed wastewater irrigated salad crops eaten uncooked. Salmonella typhi and Vibrio cholerae were isolated by “Moore” pads from raw wastewater in canals leading to irrigated vegetable plots. Other direct and indirect evidence supports the case of wastewater irrigated vegetables as the main mode of transmission. Emergency cholera control measures, including heavy chlorination of raw wastewater, partially settled by flowing in slow moving irrigation canals, are reviewed.