Expert comment: Legionella cases are increasing in Switzerland – but the overall risk remains low

"The number of reported cases of Legionnaires’ disease, a severe form of pneumonia that is caused by Legionella spp. bacteria, has increased in Switzerland in recent years. Even so, with around 500 to 600 cases reported annually, the disease remains relatively rare, and the overall risk for healthy people is low. The current cluster in Basel-Stadt nevertheless shows why it is important to understand how Legionella infections occur and how research can contribute to better prevention. 

Legionella bacteria occur naturally in freshwater, building plumbing and soil – examples are compost, garden hoses, showers, or cooling towers, decorative fountains or carwash facilities – and we regularly encounter them. Most strains found in the environment pose little risk to human health. Certain strains can, however, multiply in man-made water systems and spread through fine water droplets. Infection occurs when these aerosols are inhaled or contaminated water is aspirated. Legionnaires’ disease is generally not transmitted from person to person and swallowing contaminated water does not normally cause infection. It is therefore important to closely monitor trends in reported cases and respond rapidly to any clusters when a sudden increase in cases is observed. This surveillance is already carried out by the Federal Office of Public Health and the cantonal authorities.

At Swiss TPH, we are working closely with the Swiss Federal Office of Public Health, microbiologists, bioinformaticians, and clinicians from 20 university and cantonal hospitals to further improve the detection of Legionnaires’ disease in the clinic, strengthen surveillance, and investigate potential sources of infection.

Our research points to two priorities for effective surveillance of Legionnaires’ disease. First, cases should be identified early and documented as completely as possible, including information about where people may have been exposed. This makes it easier to identify unusual clusters and initiate targeted investigations.

Second, whenever possible, Legionella bacteria should be isolated from patients’ respiratory samples and genetically characterised. Information about possible exposure can then help investigators decide where to collect environmental samples, such as from water systems. By comparing the genetic profiles of bacteria found in patient and environmental samples, researchers can identify possible links and trace potential sources of infection.

Ideally, these genetic data would be brought together in a national database, similar to the approach used during the COVID-19 pandemic. This could help identify patterns across locations and over time, trace potential sources more rapidly and guide targeted measures. The cantons already play an essential role in surveillance and outbreak investigation. A national data platform could complement and support their work by facilitating the exchange of information across cantonal boundaries.

Together, these measures could strengthen the prevention and surveillance of Legionnaires’ disease, both during occasional large outbreaks such as the recent one in Basel-Stadt and for the many individual cases reported across Switzerland each year."

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