Predictors of legionella occurrence in dental unit waterlines of a highly colonized dental hospital

Introduction. Legionella is frequently detected in Dental Unit Waterlines (DUWLs). Although such a high occurrence is not necessarily associated with high risk for Legionnaire’s disease among patients and staff, it is prudent to monitor DUWLs for Legionella periodically. Since this procedure is long and expensive, surrogate markers are frequently used. Aim. To investigate whether surrogate markers are predictive of Legionella detection in DUWLs in a highly colonized dental hospital. Material and methods. DUWLs from a dental hospital where legionellae were detected intermittently throughout a period of ten years was considered. The investigated predictors were total viable flora (TVF) at 37°C and at 22°C, Pseudomonas (legionellae competitor) occurrence and season. Multivariate analysis was made and, using the best fitting logistic regression model, the probability to detect legionellae in water from DUWLs was estimated. Results. Legionellae were detected in 52% water samples collected in summertime and never detected in wintertime at levels ranging between 0 and 200 colony forming units(CFU)/L. The odds ratio of legionellae occurrence were 25.0 for Pseudomonas undetected vs. detected, 108.3 for summertime vs. wintertime, 2.2-2.3 for TVF levels at 37°C and 22°C >200 CFU/mL vs. ≤200 CFU/mL. A 29% probability to detect legionellae from DUWLs, where Pseudomonas was undetected, TVF levels were >200 CFU/mL and in summertime, was estimated. Conclusion. Despite legionellae were ubiquitous in the dental hospital during the study period, in the most favourable conditions for Legionella growth (lack of competitor, high biofilm and hot weather), legio nellae were detected in almost one third of DUWLs.