In July 2017, 7 cases of Legionnaires' Disease (LD) were declared to the Public Health Department. This was unusually high considering the region has an annual average of 5 declarations of LD. Also, address of residence of cases indicated they lived in a 7 km radius from each other. This spacio-temporal aggregate prompted an outbreak investigation. The infectious disease team questioned patients and family. Questionnaire revealed that all of them spent time outside, in Trois-Rivieres (TR), Quebec, Canada. As well, a few patients had everyday activities that were almost exclusively in the downtown area. At the beginning of the investigation, the environmental health team rapidly validated cooling tower's (CT) microbiological water quality. They were under the intervention threshold prescribed by the provincial regulations. Concomitantly, other potential sources of a Legionella were investigated in regards to their outbreak potential. The epidemiological survey highlighted the information on the temporal distribution of this outbreak (critical period between July 4th and 17th) and the concentration of patient's displacements in the city center of TR, confirming spatial distribution, which made it possible to refine environmental investigation. Therefore, culture results of a CT in this area were scrutinized in detail and irregularities were found. This CT had the results of recurrent interfering flora. Thus, the CT was resampled and results showed contamination at 630 000 CFU/I (under the current health standard of 1 000 000 CFU/I). Subsequently, the CT was closed. Ultimately, the Legionella strain found in respiratory specimen of the patients was a genotypic match to the one cultured from the CT and had never been documented in Quebec before. Major surveys require the participation of multiple skills from different horizons and use collective intelligence. This outbreak illustrates the complementary of the epidemiological and environmental investigations.
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