The Contribution of Legionella anisa to Legionella Contamination of Water in the Built Environment

<i>Legionella</i> bacteria can proliferate in poorly maintained water systems, posing risks to users. All <i>Legionella</i> species are potentially pathogenic, but <i>Legionella pneumophila</i> (<i>L. pneumophila</i>) is usually the primary focus of testing. However, <i>Legionella anisa</i> (<i>L. anisa</i>) also colonizes water distribution systems, is frequently found with <i>L. pneumophila</i>, and could be a good indicator for increased risk of nosocomial infection. Anonymized data from three commercial <i>Legionella</i> testing laboratories afforded an analysis of 565,750 water samples. The data covered July 2019 to August 2021, including the COVID-19 pandemic. The results confirmed that <i>L. anisa</i> commonly colonizes water distribution systems, being the most frequently identified non-<i>L. pneumophila</i> species. The proportions of <i>L. anisa</i> and <i>L. pneumophila</i> generally remained similar, but increases in <i>L. pneumophila</i> during COVID-19 lockdown suggest static water supplies might favor its growth. Disinfection of hospital water systems was effective, but re-colonization did occur, appearing to favor <i>L. pneumophila</i>; however, <i>L. anisa</i> colony numbers also increased as a proportion of the total. While <i>L. pneumophila</i> remains the main species of concern as a risk to human health, <i>L. anisa</i>'s role should not be underestimated, either as a potential infection risk or as an indicator of the need to intervene to control <i>Legionella</i>'s colonization of water supplies.