Legionnaires disease in Denmark 2021

Legionnaires' disease in Denmark 2021

2021 saw a total of 276 recorded cases of legionnaires' disease (LD) among persons with a fixed residence in Denmark, including 168 men (range 29-96 years, median 72 years) and 108 women (range 29-97 years, median 75 years). With 39 deaths within a month after onset or hospitalisation, the overall mortality was 14.1% and roughly the same for both sexes. This share is higher than normally, but in line with the level recorded in 2020 (14%). The male/female ratio was 1.6:1. The proportion of men is lower than normally, but the share is in line with that recorded in 2020. The lower sex ratio and the higher share of deaths are probably explained by the lower number of travel-associated cases than is normally seen. These cases are typically men, they are generally younger than the other cases and their mortality is therefore lower. The highest number of cases was recorded in July, August and September with 32 to 34 monthly cases (a total of 98; 35.5% of all cases).


Infection in Denmark

Overall, it is presumed that 260 of the recorded cases were infected in Denmark in 2021 (94.2%), Table 1 and Table 2. This is on par with the number recorded in 2020 (n = 263), when the highest number of cases detected to date in Denmark was recorded, Table 2 and EPI-NEWS no. 18, 2021. Some changes were recorded in the incidence in the various areas compared with 2020 and the previous years. In the Copenhagen City area, which has otherwise had a fairly constant low incidence, recorded 2-4 times more cases in 2021 than previously. The incidence recorded in the area Copenhagen Subs. has increased constantly since 2017, and in 2021 the area recorded the highest incidence of any area in Denmark; 7.1 cases per 100,000 citizens. The West and South Zealand area also followed an increasing trend, whereas the Funen area, which has often recorded the highest incidence of any Danish area, witnessed a slight decrease in the number of cases from 2019 to 2021. In the East Jutland area, a declining trend has been observed since 2017, and in 2021, East Jutland recorded the lowest incidence of any Danish area with 1.4 cases per 100,000 citizens. Since 2017, the West Jutland and North Jutland areas have seen rather constant low incidences (generally ≤ 3 cases per 100,000 citizens). In 2021, the mean incidence in the Capital Region of Denmark and the Zealand Region was also somewhat higher (5.6 cases per 100,000 citizens) than the incidence recorded in the rest of Denmark (3.4 cases per 100,000 citizens) as might be expected given the area incidences listed above.


Community-acquired infection in Denmark

Among 202 cases of community-acquired infection in Denmark, Table 1, 63 were culture verified (31%), including 27 (43%) that were L. pneumophila serogroup 1 (14 non-Pontiac and 13 Pontiac) and 21 with sg 3 (33.3%). In 56 cases, Statens Serum Institut has knowledge that a suspected water supply in the person’s home or elsewhere was tested. For 27 cases, a clinical isolate was available for comparison with environmental isolates. Agreement between the typing results from patients and water/environmental samples was found in 13 (48%) cases; in 12 cases from the patients’ residence and in one case from the patient’s place of work. L. pneumophila was detected by culture in all but two of the 56 locations tested.

Infections during travels in Denmark

A total of 13 cases are believed to have become infected while traveling in Denmark during stays at a rented holiday house, a hotel or a camping site. This is the highest number recorded to date in this category. Eight of the 13 cases were notified to the European Centre for Disease Prevention and Control (ECDC). Two cases from 2021 had become infected at a Danish hotel. One of the cases recorded in 2021 had stayed at the hotel only during daytime and is therefore not formally (following the ECDC criteria) considered a travel-associated case, but the case was included in this statement.

Institutional and hospital-acquired transmission in Denmark

A total of 34 cases were notified with presumed or confirmed infection from healthcare settings; 17 from hospitals and 17 from other institutions, i.e. nursing homes. To date, this is the highest number of cases recorded in this category. A total of 15 cases were culture verified; all belonged to less infectious types (environmental types like L. pneumophila sg 1 non-Pontiac (seven cases), sg 3 (six cases) and sg 6 (two cases).

Infection acquired during foreign travel

A total of 16 persons (5.8%) were probably infected during travels abroad, Table 1. This is the same low level as in 2020. All cases were detected in the months from July to November; Italy was the most frequent travel destination, accounting for five cases. Eleven of the cases were notified with the ECDC. The remaining cases were either non-commercial lodgings or information about the lodgings occupied during the travel was missing. One of the eleven cases formed part of an Italian cluster counting two or more detected cases.

Laboratory-confirmed cases

L. pneumophila was detected by culture in 87 patients. L. bozemanii (L. bozemanae) was detected in two patients, and L. anisa was detected in one patient. Thus, in 2021, 90 of a total of 276 cases (35.9%) were detected by culture, which is in line with the level recorded in 2020. The distribution for L. pneumophila by serogroup and subgroup is presented in Table 3.


In addition to serogrouping (sg), the sequence type (ST) of the clinical L. pneumophila isolates was also established by whole-genome sequencing. In all, 38 different STs were detected. The most frequent ST was ST1 (belonging to sg 1) with 21 isolates (24%). The serogroup distribution (Table 3) is slightly different than in previous years, as a considerable decline was observed in the share of sg 1 Pontiac, which normally comprises 30-40% of the culture-verified L. pneumophila cases. The share of sg 3 was higher in 2021 than in the previous years. Whereas sg 3 normally causes every fifth case, in 2021 sg 3 caused every third case. For the first time ever, sg 1 comprised less than 50% of the culture-verified cases. In all, 252 patients had a Legionella-positive PCR (91.3%). L. pneumophila urine test (LUT, which mainly detects sg 1) was positive in 55 cases (20%), which extends the declining trend observed in recent years, and which is also linked to fewer travel-associated cases (typically sg 1) and the lower total number of L. pneumophila sg 1. Lower respiratory samples from eight LUT-positive patients tested negative by PCR. A total of 22 of the 252 PCR-positive patients (9%) were positive to L. species non-pneumophila or L. species, but had no result for L. pneumophila. The majority of these cases (n = 17) were from the Capital Region of Denmark. Additionally, a minimum of 18 patients had L. species non-pneumophila detected by PCR, but in whom the clinical evaluation was not assessed as being consistent with LD.


No obvious, general explanations exist for the changes observed in LD incidence in 2021 in the Danish areas. Even so, one cause contributing to the low incidence observed in East Jutland is that no cases were detected in the town of Randers in 2021, even though Randers has generally recorded one of the highest incidences seen in Denmark.

2021 added another two cases to the largest hotel cluster ever recorded in Denmark. A total of six LD cases have been recorded since 2019 in which infection may be attributed to the hotel in question; a Finish case from 2019, and two Danish cases in 2020 and 2021, respectively. In 2022, one additional case has currently been recorded. The cause of the outbreak is the infectious L. pneumophila sg 1 subgroup Philadelphia ST1, which has also previously caused clusters of LD cases in Denmark.

Since 2019, the share of sg 1 Pontiac cases has followed a declining trend. The low number of travel-associated cases contributes to this trend as travel-associated cases are most frequently affected by subgroups of sg 1 Pontiac. However, the share recorded in 2021 was markedly lower (18.4%) than the 2020 share (29.3%) despite the same low number of cases who acquired LD during travels abroad in both years (5.8% and 5.4%, respectively).

We recommend testing for L. species as well as L. pneumophila by PCR. Some Departments of Clinical Microbiology (DCM) do not test for L. species but only for L. pneumophila by PCR, which means that approx. 10% of LD cases may go undiagnosed. However, in many cases, the clinical significance of positive findings of L. species is unclear/unknown. Some of these findings are probably incidental and not the cause of the patient’s current complaint, as legionella bacteria are very common in our domestic water supply. Additionally, it is important to be aware that irrigation fluids (BAL) and other fluids used in the sampling and processing of the samples may contain Legionella DNA.

The ECDC is changing the LD case definition. In future, detection of L. pneumophila in lower respiratory samples by PCR (or similar) will be considered a confirmed case of LD (in case of clinical pneumonia). However, detection of L. species or Legionella non-pneumophila in lower respiratory samples will continue to be considered only a probable diagnosis (probable case). Detection of Legionella in upper respiratory samples by PCR will not be included in the case definition.

This annual report is also described in EPI-NEWS no. 37/2022.