Study Identifies Primary Source of Community-Acquired Legionnaires' Disease Risk

Despite being one of the most prevalent types of Legionnaires' disease, tracking the source of cases of community-acquired Legionnaires' disease (CALD) is notoriously difficult. An in-depth study in Berlin over three years found that the presence of a virulent type of Legionella pneumophila in household water was a significantly greater predictor for CALD than the overall concentration of all Legionella species.

Led by a group of German researchers from 2016 to 2019, the study was presented in the peer-reviewed scientific journal PLOS One. Although it was originally published in November 2020, the study recently garnered attention at the 10th Annual Conference on Legionella in Yokohama, Japan this past September.

As public health professionals and regulators around the world work to mitigate potentially deadly Legionella risks, the three-year study takes on new significance. In Europe, member states continue to adapt national legislation to the updated EU Drinking Water Directive before the Jan. 12, 2023, deadline. Knowing how to best utilize environmental testing to minimize risk could make an enourmous difference for labs and public health.

Focus on Residential Drinking Water

The study, called LeTriWa after the German terms for "Legionella in drinking water," was a massive collaborative effort involving the Robert Koch Institute, the German Environment Agency, the national Legionella lab, the State Department for Health and Social Affairs in Berlin, and a dozen health authorities and hospitals across the city.

CALD cases account for at least 70% of reported cases in Germany—more than the total cases associated with travel and hospitals combined, according to the study. CALD's sporadic nature and the difficulty of environmental sampling make it especially challenging to link known cases with the source of infections.

The researchers adopted a novel methodology to pinpoint sources, using what they referred to as an analytical comparative approach. Over three years, 111 patients with confirmed cases of CALD agreed to participate in the study. Several weeks after each participant's symptoms began, researchers took standard water samples from the household. They also requested additional urine and lower respiratory samples for further testing.

Besides analyzing patient samples and water samples, the research team conducted questionnaire-based interviews. For each case, the group recruited two control patients in the same age range who had been admitted to the same hospital for an illness other than pneumonia. Control patients also filled out the questionnaire, and the researchers took standard household water samples from these patients' homes.

Type vs. Amount

Using an appraisal matrix that grouped source and evidence types, the LeTriWa team attributed as many Legionnaires' disease cases as possible to a specific source of infection. Ultimately, they succeeded with about half.

Attributed cases mainly came from a direct residential drinking water (RDW) source, such as a showerhead. Through their analyses, the researchers also discovered that "wearing inadequately disinfected dentures" led to a small percentage of cases and recommended that dentures as an infectious source be investigated further. The median age in the study was 67.

When the team analyzed water samples, they checked the Legionella concentration as well as the serogroup and monoclonal antibody (MAb) subtype. "An important type is the MAb 3/1-positive strains that are believed to be particularly virulent," the researchers wrote. MAb 3/1 is a specific type of L. pneumophila. The study also distinguished four MAb 3/1-positive bacteria subtypes: Knoxville, Philadelphia, Benidorm, and France/Allentown.

"The most outstanding characteristic of RDW leading to infection was MAb 3/1-positivity, whereas the degree of Legionella concentration in the standard household water samples was not relevant," the researchers wrote. "Risk for LD seems to be conferred primarily by the type of Legionella rather than the amount."

Dovetailing with other researchers' findings in England and Wales published in the European Journal of Clinical Microbiology & Infectious Diseases, the Berlin study results suggest that the risk in residential water only exists if it contains the MAb 3/1-positive strain.

Significant for Environmental Testing

Martin Exner, medical director of the University of Bonn's Institute for Hygiene and Public Health, highlighted the Berlin study at the recent 10th Annual Conference on Legionella during his video presentation on regulation in Germany.

"The level of Legionella concentration with any Legionella seems to be a good indicator of the technical condition of the drinking water systems, but it did not allow a good prediction of the occurrence of cases of community-acquired Legionnaires' disease," he said.

Citing the LeTriWa findings, Exner pointed out that the previous focus in Germany on testing for all Legionella species should be replaced or supplemented by a stronger focus on L. pneumophila that also integrates more consideration for serogroups and further virulence markers.

Germany, along with several other European states, already requires Legionella and temperature monitoring, but the updated EU Drinking Water Directive leaves exact testing parameters up to each member state. Each member state has until Jan. 12, 2023, to adapt national legislation to the directive, which recommends monitoring priority premises such as hospitals, schools, and hotels for Legionella if they don't already.

France and certain provinces of Canada already focus environmental testing on L. pneumophila, and the Dutch government is pursuing a "hybrid" approach that monitors for more Legionella species in locations serving the immunosuppressed, but otherwise primarily targets L. pneumophila.

"There are more than 60 known Legionella species; however, according to leading health bodies, roughly 96% of Legionnaires' disease is caused by just one specific species, Legionella pneumophila, which is also responsible for the deadly form of pneumonia," EURACTIV reported this year.

Weighing the Human Cost

Much is at stake. Cases continue to emerge across Europe, including deadly outbreaks in Belgium and the Netherlands. Some energy conservation efforts conflict with the guidelines for maintaining hot and cold water temperatures to prevent Legionella growth. New legislation must minimize any reliance on limited resources while maximizing protection for the public.

A 2020 study by Italian researchers published in the scientific journal Pathogens pointed to several factors supporting exclusive L. pneumophila monitoring. "Laboratories may save time, human resources and money, and they can employ those saved resources to analyzing additional samples or locations, instead of identifying other Legionella species, which represent a much lower health risk," the researchers wrote.

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Alyssa Danigelis

Alyssa Danigelis is a professional freelance journalist who covers business, sustainability, energy, science, and technology. She received a bachelor’s degree from Mount Holyoke College and a master’s degree from Columbia University’s Graduate School of Journalism. Having grown up in Burlington, Vermont, she spent formative time in Boston and pounded the pavement for years in New York City before moving to sunny Colorado, where she currently resides.