Escherichia coli O157 : H7 outbreak associated with an improperly chlorinated swimming pool

A cluster of gastrointestinal illnesses, including one case of hemolytic-ur emic syndrome and one culture-confirmed Escherichia coli O157:H7 infection, followed a trailer park pool party. We interviewed a cohort of party atten dees and park residents. A primary case was defined as the first gastrointe stinal illness within a household between 5 July and 20 July in which the t iter of IgG antibody to E. coil O157 (if determined) was elevated. Of 51 pa rty attendees and trailer park residents, 18 developed a gastrointestinal i llness, including 10 who met the definition of a primary case. Swimming in the pool significantly increased the risk of primary illness (relative risk = 6.3; 959% confidence interval = 1.8-18.9). No other exposure was signifi cantly associated with primary illness, after pool exposure was controlled for. The implicated pool had little to no chlorine added during the period of 4-10 July. This outbreak provides new evidence of the importance of prop er pool maintenance in controlling the spread off. coil O157:H7.