The rise in Legionnaires’ disease cases challenges prevention efforts

Ahead of World Water Day (22 March), we are focusing on Legionella, a bacterium found in aquatic environments that can cause Legionnaires' disease, a severe respiratory illness. Despite advances in detection and prevention, case numbers continue to rise, highlighting the need to strengthen control measures in high-risk facilities.

Legionella was first identified in 1976 in Philadelphia, USA, when over 200 pneumonia cases were reported among attendees of an American Legion convention (hence the name Legionella and the term Legionnaires' disease). At that time, it was discovered that the source of infection was the water system of the hotel where the convention had taken place.

Following this discovery, the scientific community conducted numerous studies on this newly identified species. Today, it is known that infection only occurs through exposure to a contaminated water source, by inhaling aerosols containing Legionella. The disease typically presents as pneumonia and primarily affects vulnerable individuals (those with underlying health conditions, weakened immune systems or older adults). It is important to note that person-to-person transmission has not been documented.

Legionella occurs naturally in damp environments such as rivers, lakes and moist soil, but usually poses no risk in these settings due to its low concentration. The risk to humans arises in artificial water systems such as domestic water networks, cooling towers, humidifiers, ornamental fountains or irrigation systems. The bacterium enters these systems via the mains water supply and can reach high concentrations because the conditions in these facilities often favour its growth (temperatures around 35°C). Human infection occurs through aerosol-generating elements such as showers, taps or sprinklers.

In the early years after the discovery of Legionella, significant resources were invested in developing techniques to characterise the bacterium in order to trace environmental sources and confirm links between patient and environmental isolates. Over the past two decades, efforts have shifted towards control and prevention in high-risk installations (potential sources of infection), resulting in several Royal Decrees being issued in Spain. The most recent was published in July 2024 and outlines all the actions required of various water installations to prevent the spread of the bacterium to humans.

Despite the efforts being made in Spain, the incidence of Legionnaires' disease continues to increase. Over the period 2013-2023, the number of cases per 100,000 population has risen from 1.8 to 4.6. This trend has been observed not only nationally, but also across Europe and in countries such as the United States. Mortality has also increased in recent years, although only within the over-65 age group.

Several factors may explain this rise in incidence. We now have better diagnostic tools and greater access to them, allowing for more efficient detection. In addition, more Legionella-positive samples have been detected in water from high-risk facilities, meaning more environmental sources are colonised and a greater proportion of the population is potentially exposed. Lastly, the proportion of vulnerable individuals has increased in recent years, with a growing number of immunocompromised people and more individuals in the at-risk age group.

The only way to reverse this upward trend is to improve the condition of water systems to reduce public health risks. This goal can be achieved by identifying an effective biocide to eliminate the bacterium from water and by encouraging facility managers to carry out regular testing, disinfection and all necessary preventive actions.

This article was written by Noemí Párraga, leader of the Legionnaires' disease Study Group (GELeg) of the Clinical and Environmental Infectious Diseases Study Group (CEID) at IGTP.