Legionnaires disease 2022

Legionnaires' disease in Denmark 2022

2022 recorded 287 cases of Legionnaires' disease (LD) among persons with permanent residence in Denmark. Hereof, 169 were men (20-95 years, median 73 years) and 118 were women (28-96 years, median 76 years). Among all LD cases, 56 (19.5%) died, including 32 men (18.9%) and 24 women (20.3%) within a month after debut or hospitalisation. Mortality has followed an increasing trend in recent years, averaging 11.5% in 2017-2019 and 14.5% in 2020-2021. Furthermore, an increasing share of those diagnosed with LD are elderly people (≥ 70 years). From 2017 to 2019, a mean 132 LD cases ≥ 70 years were recorded. In 2020/2021, an average of 151 patients were registered in that age group; in 2022, 180 cases ≥ 70 years were found (63%). In 2022, the mortality was also relatively high (27.2%) in the same age group (≥ 70 years).

The male-female ratio in 2022 was 1.4:1, which is in line with the level recorded in the preceding two years, but slightly lower than in previous years. The share of travel-associated cases (abroad) remained lower than before COVID-19, but has increased from approx. 5.5% during COVID-19 to 13.6% in 2022. The overall high percentage share of death is, in part, associated with an overall lower number off travel-associated cases than before COVID-19 as this group is characterised by a very low mortality (Table 1).

The highest number of LD cases was recorded in the months from September to November. The highest number of infected people in Denmark was recorded in October, when 30 cases were registered. Subsequently, a decline was observed throughout November and December, to 25 and 19 cases, respectively. As seen from Table 1, the age difference is rather large between travel-associated cases and cases linked to a hospital/an institution. Age and mortality alike are high for the group Unknown. Some of these cases are likely linked to a hospital/institution, but the circumstances of the infection remain unknown due to, e.g., underlying diseases and unclear information about LD disease onset date, and/or relief stays and/or multiple brief admission periods within the full possible incubation period (2-14 days).


Infection in Denmark

Overall, it is presumed that 248 of the disease cases recorded in 2022 had become infected in Denmark (86.4%), Table 1 and Table 2. This constitutes a slight decline in the number of cases infected in Denmark compared with the past two years, but remains at a higher level than before COVID-19 (2017-2019) when an average 210 annual Danish cases were recorded.


The 2022 distribution on regions and provinces (Table 2) was roughly in line with the distribution recorded in 2021, but a decline was seen in Copenhagen Subs. and an increase in North Zealand.

Once more, Copenhagen City recorded a higher detected incidence than in previous years. The island of Bornholm recorded a high incidence in 2022 but - due to its low population count - this incidence is sensitive to slight changes in the number of detected cases. A relatively large interregional incidence difference remains between the Capital Region of Denmark, the Zealand Region and the Region of South Denmark, on one hand, and the Central Denmark Region and the North Denmark Region, on the other. No outbreaks or clusters have been detected that may explain increases or differences.

Community-acquired infection in Denmark

Among 188 cases of community-acquired infection in Denmark, Table 1, 79 were culture verified (42.0%), including 40 (50.6%) with L. pneumophila serogroup 1 (SG1) (16 Pontiac and 24 non-Pontiac) and 19 with SG3 (24.1%). The remaining 19 cases had become infected with other serogroups, and in one L. bozemanae was cultured.

In 33 cases, Statens Serum Institut (SSI) has knowledge that a suspected water supply in the person’s home or another suspected infection source in the environment was tested. For 15 cases, a clinical isolate was available for comparison with environmental isolates. For a total of 10 of these cases, correspondence was found between the typing results of isolates from the patient and from water samples/environmental samples from the hot-water supply system in the patients’ own home, a wellness centre and privately-owned CPAP equipment. Among the 33 places tested, in 6 places L. pneumophila was not detected by culture. Thus, as in previous years, the share with detected infection from the water systems of the patients’ residences that could be verified by typing was around 50%. However, the cause of infection for a large share of the remaining LD cases with presumed infection in Denmark remains undetermined.

Infections during travels in Denmark

A total of five persons probably became infected while travelling in Denmark; three at different Danish camping areas and two during stays at two different Danish hotels. This includes one case at a hotel where another two cases were detected in 2021 (Annual report 2021). For two cases who had become infected while travelling in Denmark (hotels), the typing results from patient and environmental isolates were in agreement.

Institutional and hospital-acquired transmission in Denmark

A total of 38 cases with presumed or certain infection from healthcare institutions were notified; 19 from hospitals and 19 from other institutions such as nursing homes. This is the same relatively high level seen in 2021. A total of 17 of these cases were verified by culture, 7 L. pneumophila SG1 (3 Pontiac and 4 non-Pontiac) and 9 with other serogroups (3, 5, 10 and non-typeable) and 1 case of L. anisa. SSI has knowledge that water samples were tested in connection with 18 of the 38 cases. However, a match was found in only three of these cases, all from nursing homes or another institution.

Infection acquired during foreign travel

A total of 39 patients (13.6%) probably became infected during foreign travel, Table 1. This was more cases than in 2020/2021, but fewer than before COVID-19. With 15 cases, Italy was the most frequent travel destination among the cases infected with LD. Hereof, 10 cases formed part of three clusters counting 6 cases at a hotel, 2 cases at different hotel and another 2 cases at a third hotel. Eight other cases had travelled to the United Arab Emirates, primarily to Dubai. Among the remaining cases, two were associated with each of the following countries; Greece, Thailand, Turkey, Germany and Vietnam. Seventeen of the travel-associated cases were culture verified, 14 (82.4%) with SG1 Pontiac subgroups (see Table 3), which is 45.9% of all SG1 Pontiac cases in 2022 and contributing to the overall proportion of SG1 Pontiac in 2022 being higher than during COVID-19. A total of 19 of the cases were positive by Legionella pneumophila urinary antigen test (UAT), which generally has a high sensitivity in travel-associated cases.

Laboratory-confirmed cases

L. pneumophila was detected by culture in 121 patients. L. bozemanae was detected in two patients, whereas L. anisa and L. londiniensis were detected in one patient each. Thus, in 2021, 125 of a total of 287 cases (43.6%) were detected by culture, which is slightly higher than the level recorded in 2020 and 2021. The distribution for L. pneumophila by serogroup and subgroup is presented in Table 3.


In addition to serogrouping, the sequence type (ST) of the clinical L. pneumophila isolates was also assigned by whole-genome sequencing. In all, 53 different STs were detected. As in the previous years, the most frequently detected ST was ST1 (belonging to SG1, both Pontiac and non-Pontiac) with 29 isolates (24%), which is at the same level as seen previously. The second most frequently recorded ST was ST87 with 7 isolates (5.8%) belonging to SG3. Thus, the serogroup distribution was roughly as before COVID-19. A total of 264 patients had positive Legionella PCR (92.0%). L. pneumophila urinary antigen test (UAT), which mainly detects SG1, was positive in 57 cases (19.9%), but could - if it had been performed in all cases - have been positive for nearly 40% of cases (sensitivity approx. 40%, but depending on the serogroup distribution). Not all SG1 cases test positive by UAT, particularly cases with a non-Pontiac infection may test negative. Lower respiratory tract samples from 9 UAT-positive patients tested negative by PCR. A total of 33 of the 264 PCR-positive patients (12.5%) were reported as positive for L. species non-pneumophila (n=23) or L. species (n=10) without result for L. pneumophila. However, five of these cases were established to be infection with L. pneumophila by PCR at SSI and/or positive UAT/culture. A total of 22 cases, all from the Capital Region of Denmark, were confirmed to have become infected by another species than L. pneumophila, including four culture-verified cases.


As in previous years, the share of detected infection from patients’ housing facilities was approx. 50% among those investigated. Therefore, it must be assumed that some cases are not infected via the hot water systems of the houses, but from other external sources of infection. The decrease in the number of cases in winter months does not indicate that any energy savings resulting in lower water temperatures due to the energy crisis at the end of 2022 resulted in more cases than expected. Prevention of high levels of Legionella in hot-water systems is based on keeping the hot-water temperature above 50 °C in all parts of the hot water system, and that all taps are flushed regularly with hot water at a minimum temperature of 50 °C.

The increasing share of elderly people among the diagnosed LD cases is probably linked to the fact that more elderly people are tested by Legionella PCR. Since 2017, there has been almost a doubling in the number of people ≥70 years old who are being examined for Legionella. This is especially seen in the Capital Region.

The fact that the 22 cases definitely infected by another species than L. pneumophila had all been diagnosed in the Capital Region of Denmark may indicate that under-diagnosing occurs in the rest of Denmark. We encourage conducting concurrent PCR for Legionella species and L. pneumophila. A positive PCR for Legionella species only has a very low predictive value for diagnosis of Legionnaires' disease (50-60%) as some samples must be considered false positives or of unknown clinical value. It is also important to remain aware that some mild cases or cases seen in the early phase of the course (e.g., without productive cough) of Legionnaires' disease are not PCR positive (probably around 20-30%) as the Legionella content may be very low and it may be difficult to obtain sampling material of a sufficient quality. Some of these patients have an SG1-infection and may, in many cases, be detected by UAT, whereas some particularly mild cases will probably test negative with both of these tests.

This annual report is also described in EPI-NEWS no. 22/2023.