[Waterborne outbreak of Shigella sonnei caused by consumption of public supply water].

Objective: To investigate a waterborne outbreak in the population of València d'Àneu (Lleida, Spain).

Methods: A cohort study of consumption of mains water, bottled water and spring water was carried out. The sample was obtained by telephone contact with all private numbers in the municipality. We surveyed 58.3% of the population census (105/180) for water consumption, number of glasses drunk daily and symptoms. The water supply in the municipal system was sampled, and the presence of Enterobacteriaceae in the stool samples of 10 patients was determined. The association of each type of water was studied with estimation of relative risks (RR) and 95% confidence intervals (95% CI).

Results: The overall attack rate was 64.8% (68/105). The epidemic curve was consistent with an exposure of 6 days. Consumption of public supply water was associated with gastroenteritis (RR=4.2, 95% CI: 1.5-11.9). In addition, a dose-response relationship was found (χ(2)=34.1; p <0.001). There was a higher risk of illness in consumers of 2-3 glasses per day (RR=4.3; CI 95%: 1.9-9.9) and in those drinking more than three glasses per day (RR=4.9; 95% CI: 2.2 -10.9). The chlorinator of mains water froze and stopped working. Six stool cultures were positive for Shigella sonnei.

Conclusions: A waterborne outbreak of S. sonnei was caused by contamination of the public water supply. Investigation of illegal dumping of wastewater was recommended. Municipalities should ensure proper disinfection systems and prevent spillage near deposits.