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New Study on Legionella in Public Water Systems

A recently published study, Monitoring distribution systems for Legionella pneumophila using Legiolert, examines how a new testing method could be used in monitoring Legionella in public water systems and at the same time may give some indication of the likelihood of finding Legionella in routine distribution system testing.  The study was conducted by Dr. Mark LeChevaillier, a long-time researcher in the drinking water field and is published in AWWA’s Water Science.  Legiolert is a culture based assay for L. pneumophila based on the most probable number.  The new method was developed by IDEXX Laboratories, developer of Colilert, a similar method commonly used in the water industry for coliform bacteria and E. coli.  IDEXX provided funding for the study.  The main objectives of the study were to assess utilities’ experiences with Legiolert and develop a protocol for responding to positive samples.

A total of 12 utilities participated in the study which was conducted between November 2017 and April 2018.  Participating systems covered both surface water and groundwater, chloramine and free chlorine, and also captured UV and Ozone.   As for evaluating the test method, a follow-up survey of utilities found the Legiolert test was easy to use and interpret.  As for test results, there were 679 water samples analyzed.  Legionella pneumophila was detected in 5.7% of 53 source water samples, 0.17%  of 576 distribution samples, with no positive samples from plant effluent (high disinfectant residuals). Another objective of the study was a protocol for responding to positive Legionella tests in the water system.  Many of the participating systems, and some that were initially interested but declined to participate cited uncertainty over how to respond to positive samples as a concern with their participation.  The study team collaborated with the state of Washington and developed a protocol for response that is consistent with OSHA and EU guidance.  The protocol begins with repeat sampling, triggers an RTCR Level 1 type assessment for multiple positive samples, outlines subsequent corrective actions, and includes appropriate coordination with regulators.

The study provides useful information for utilities and states about testing for Legionella, the possible scale of positive results, and reasonable response to those positives.  A second study is underway to compare results during a warmer season.