In the Netherlands, all laboratory-confirmed LD cases are notifiable and reported by clinicians or medical microbiologists to the public health service (PHS). The PHS uses a structured questionnaire to conduct a source finding investigation and collects all relevant data, including demographic, diagnostic, underlying conditions, smoking, travel history, and all possible locations, activities, or other potential sources of exposure in the 14 days before illness onset. Using those data, the PHS in collaboration with the National Reference Laboratory for Legionella (Legionella Source Identification Unit) decides which water or soil samples should be collected from potential sources of infection. The reference laboratory collects and analyzes all samples, including genotyping of Legionella isolates.

Newly installed brand A water heaters were identified as the most probable source of infection in this outbreak because all the available data from epidemiologic and environmental investigations showed a clear link between the water heaters and cases of L. pneumophila ST37 infection. Our case–control study confirmed that cases were associated with newly installed brand A water heaters with a large and highly significant OR. However, because of the partially retrospective design, data on the brand of water heater were missing for many (25/56) controls. Therefore, we performed a sensitivity analysis assuming strong bias toward the null, which still showed a statistically significant association (OR 13.02 [95% CI 2.78–60.92]). Of note, the L. pneumophila ST37 patient isolates and environmental strains found in this cluster were also closely related by cgMLST to isolates from 3 LD cases reported in 2012, 2015, and 2019 in patients not related to this cluster. That finding aligns with previous studies reporting on the genomic population structure of L. pneumophila isolates that made similar observations for some STs, indicating that isolates can be genetically closely related but not epidemiologically linked (15–17).

At the start of the outbreak, the drinking water supply was also considered as a possible source of infection but was swiftly ruled out after assessing the geographic distribution of the cases, the ORs, and detection of L. pneumophila ST37 only in warm, not cold, water samples. The geographic distribution of the LD cases and their homes across the country showed that they were connected to various drinking water companies and nothing indicated that L. pneumophila ST37 was in drinking water supplied by those companies. That observation is supported by environmental samples taken during 2012–2021 as part of source investigation by the reference laboratory of the Netherlands. The reference laboratory sampled 984 sources, including the homes of 316 LD patients, and found L. pneumophila ST37 in only 0.4% of all sources (18,19). Furthermore, only 3.4% of L. pneumophila clinical isolates during that period were ST37 (18,19). Of note, the sampling results in cluster 1 indicated this strain possibly has a high virulence because a low concentration (<1.000 CFU/L) of ST37 caused multiple LD cases. Therefore, ST37 would have been found more often in potential sources or in the clinical isolates if the strain were commonly in drinking water in the Netherlands, even in low concentrations. Finally, we also looked at possible introduction of ST37 in the drinking water system by other sources. In the building of cluster 1, a device to control the water pressure had been replaced during 2022, but that device was not replaced in the building in cluster 2 or for any of the other cases. Source finding history did not show any other sources of L. pneumophila ST37 introduction into the water system of the homes of the patients or any other common sources of exposure.

On the basis of the questionnaire and temperatures measured at sampling, we did not find specific risk factors related to the temperature settings or other common issues or errors with the water heaters. At most homes, the hot water temperatures at the tap were >55°C. Some water heaters were set on Eco mode (energy efficient), resulting in low temperatures for the first few minutes. How often and for how long the patients used the hot water is not known.

The brand A water heater is also sold in several other countries in Europe. To identify possible cases in those countries, we issued a warning through EpiPulse, an online infectious disease surveillance portal for Europe, but no other countries reported LD cases. However, several countries communicated that they might be unable to detect such cases because detailed case finding information with environmental investigations and typing of isolates are not routinely performed in those countries. Furthermore, unlike the Netherlands, many countries use biocides in their drinking water system, which could explain why no cases have been observed in other countries. In the Netherlands, chemical or thermal disinfection is not mandatory after water heater installation, but such measures might reduce risk for LD.

The brand A water heater had a flow-through system without a storage tank, so the legionellae growth cannot be explained by stagnant water with temperatures of 25°C–50°C degrees or thermal stratification in the hot water storage tank, which is known to pose a higher risk for legionellae growth (20). However, a plausible explanation for introduction of legionellae into the water system is that a small amount of test water containing L. pneumophila ST37 remained in the heater after the production process and that microorganisms were able to create a biofilm, enabling L. pneumophila to survive. All heaters are tested with water for possible leakage before distribution. During the investigation into the clusters, the reference laboratory confirmed that a small (≈30 mL) amount of water was in new water heaters still in storage. We hypothesize that after water heater installation, L. pneumophila was released into the hot water system in the home and was able to grow in the pipes, shower hoses, and shower heads, subsequently causing infection. However, before the safety warning, no L. pneumophila ST37 was detected by the reference laboratory in samples from the small amount of water in the water heaters still in storage.

Further research is necessary to assess the risk for L. pneumophila contamination in water heaters causing infections and provide evidence-based policy recommendations for infection prevention and control. Previous research has documented that L. pneumophila is able to enter a viable but nonculturable state and later resuscitate in the right environment (21,22), which might explain how the bacteria in the LD clusters survived in storage for months. However, more data are needed to elucidate how L. pneumophila ST37 entered the water heaters, how the bacteria were able to grow in the plumbing system despite high water temperatures, and whether energy-saving designs like Eco Mode result in lower temperatures and an increased risk for bacterial growth. Further research also is needed to determine whether L. pneumophila infection is specific to brand A water heaters or if similar infections could occur in water heaters from other brands as well.

In conclusion, L. pneumophila in plumbing systems is widely reported, often in relation to water temperatures of 25°C–50°C, stagnant water, or low biocide levels. The risk for legionellae growth in thermostatic valves and biofilms in shower hoses also is well known (23–26). This study demonstrated the value of legionellae control measures during production or installation of water devices and that a new device could pose a risk for introduction of L. pneumophila into a water system. Manufacturers of water heaters or fittings for plumbing systems that are tested with water should ensure devices are clean and dry before packaging and should control for L. pneumophila growth in the test water. Furthermore, this outbreak emphasizes how vital an adaptable surveillance system, environmental investigations, typing of isolates, and alert public health workers are for detecting previously unrecognized sources of L. pneumophila.