Monitoring Legionella in Drinking Water: Should We Focus on L. pneumophila or All Species to Effectively Protect Public Health?

Legionella pneumophila is responsible for the majority of reported Legionnaires' disease cases worldwide. However, environmental monitoring of building plumbing systems often targets a broad range of Legionella species, raising the question of whether monitoring should focus exclusively on L. pneumophila or include all Legionella species. This review examines the policy and public health implications of both strategies by assessing case attribution data for Legionnaires' disease, the environmental prevalence of Legionella species, and the validity of using non-pneumophila counts as indicators for L. pneumophila. Although over 30 species can cause illness, L. pneumophila dominates culture-confirmed cases despite the frequent detection of L. non-pneumophila species in building plumbing and other known sources. Ecological differences between species, including growth temperatures and disinfection resistance, arguably limit the suitability of L. non-pneumophila species as reliable indicators for L. pneumophila. As a result, using all Legionella species counts to inform risk management may lead to excessive interventions without proportional public health benefits. We conclude that routine monitoring should prioritize L. pneumophila to ensure targeted, cost-effective, and health-relevant risk management. Broader monitoring may be warranted in high-risk settings or where local epidemiological data justify a more inclusive approach. These findings support risk-based regulatory frameworks that align monitoring targets with public health outcomes.