A large outbreak of Legionnaires’ Disease in an industrial town in Portugal

Background: We describe the investigation and control of an outbreak of Legionnaires’ disease in Portugal in October, November and December 2014. Methods: Confirmed cases were individuals with pneumonia, laboratory evidence of Legionella pneumophila serogroup 1 and exposure, by residence, occupational or leisure tothe affected municipalities. 49 possible sources were reduced to four potential sources, all industries with wet cooling system, following risk assessment. We geo-referenced cases’ residences and the location of cooling towers defining four study areas 10 km buffer centeredon each cooling tower system. We compared the number of cases with expected numbers,calculated from the outbreak’s attack rates applied to 2011 census population. Using Stones’Test, we tested observed to expected ratios for decline in risk, with distance up to 10 km fourdirections. Isolates of Legionella pneumophila were compared using molecular methods.