Record spike in Legionnaires’ disease in 2024

Malta recorded its highest-ever annual number of Legionnaires’ disease cases in 2024, official statistics show. 

The water-borne disease is a potentially fatal bacterial infection often linked to poorly maintained air-conditioning and plumbing systems. 

The annual report of the Infectious Disease Prevention and Control Unit (IDCU) shows that 30 cases were reported last year, up from 13 in 2023. Of these, 22 involved Maltese nationals. 

A spokesperson for the Health Ministry confirmed that legionella, as it is also known, “may have contributed to five deaths”, all of whom were individuals aged over 64 with underlying health conditions or weakened immune systems. 

While health authorities noted that the increase could partly reflect “better detection and reporting”, they also cited other contributing factors, including “rising temperatures favouring bacterial growth, and increased population and tourism”. 

All cases reported last year were sporadic and unlinked, meaning no clusters emerged in the same establishment. 

Investigations revealed that half of the cases in 2024 were connected to elevated legionella levels in household water systems, such as showers, taps, and stagnant warm water—conditions that create an ideal environment for bacterial growth. 

Seven accommodation establishments were investigated, but only one was found to have high legionella counts. Precautionary cleaning and disinfection were carried out in all seven, with follow-up measures taken to ensure safety. 

Last year, MaltaToday had flagged a resurgence of the disease, particularly among locals, who accounted for nine of the 13 cases in 2023. That figure had been the third-highest on record, surpassed only in 2004 and 2020 (during the pandemic) when 16 cases were reported. A similar figure (13 cases) was registered in 2011. However, these figures pale in comparison to the 30 cases recorded in 2024. 

The 22 cases among Maltese residents in 2024 highlight a growing trend—more infections are now being contracted in domestic settings. Between 1989 and 2012, authorities recorded 58 local cases and 71 linked to travel. But between 2013 and 2024, there were 93 cases among Maltese nationals and 38 among non-Maltese nationals. With the additional 30 cases in 2024, the total reported since 2013 now stands at 131. 

What is Legionnaires’ disease? 

Legionnaires’ disease is a type of pneumonia caused by Legionella bacteria. It has an overall fatality rate of around 15%, with increased risk for smokers and people with underlying conditions. 

The disease is contracted by inhaling contaminated water droplets from hot tubs, showers, or air-conditioning units, but is not spread from person to person. 

The Legionella microbe thrives in stagnant, warm water and biofilm—the coating inside pipes that forms when water circulation is poor. Symptoms include fever, chills, cough, and sometimes muscle aches and headaches. The disease is treatable with antibiotics, particularly when caught early, though long-term side effects such as fatigue may persist. 

The disease was first identified following an outbreak in 1976 at the Bellevue-Stratford Hotel in Philadelphia, where American Legion veterans had gathered. Within two days, attendees began falling ill. A total of 221 people required medical treatment, and 34 died. 

How to prevent Legionellosis 

While household plumbing systems are generally low risk, the risk increases when parts of the system are unused for extended periods. Authorities recommend shutting off water systems if a property is left vacant and flushing out stagnant water by running taps for at least five minutes upon return. Opening windows during flushing helps disperse any aerosols. 

Households are also advised to regularly check and insulate water tanks to prevent water from warming up. Water from bowsers not certified by the Health Department should be avoided. 

Hotel establishments carry a higher risk, and hotel owners are legally obliged to register and regularly disinfect cooling towers and evaporative condensers. The health authority can shut down establishments where confirmed cases of legionellosis are recorded. These regulations were introduced in 2006 following a spike in cases in 2004. 

Climate change and Legionnaires’ disease 

Across both Europe and the United States, cases of Legionnaires’ disease have risen sharply since 2000, with climate change increasingly recognised as a key contributing factor. Warmer, wetter conditions favour the growth of Legionella in water systems, and extreme weather events can facilitate its spread. 

Data from the US Centers for Disease Control and Prevention (CDC) show a 6.5-fold increase in Legionnaires’ disease cases, rising from around 0.4 to 2.7 per 100,000 people between 2000 and 2019. 

Similarly, data from the European Centre for Disease Prevention and Control (ECDC) show the infection rate in Europe doubled from 1.2 per 100,000 in the early 2010s to 2.4 per 100,000 by 2021. 

According to the ECDC, changes in climate and weather patterns “can impact both the ecology of Legionella in the environment and the exposure to water aerosols containing the bacteria.” The bacteria thrive in water temperatures between 25–45°C, meaning that longer hot seasons and heatwaves extend the danger period. Milder winters and earlier springs also help the bacteria flourish in regions previously considered too cool. 

In Malta, 14 of the 30 cases in 2024 occurred in the fourth quarter, coinciding with increasingly warmer autumn months. Another eight cases were reported in spring. 

Public health agencies emphasise the importance of maintaining clean water systems, improving surveillance, and keeping vulnerable systems disinfected to prevent further outbreaks.