Legionella 2020 Annual Report

Legionella 2020 Annual Report

A total of 278 cases of Legionnaires’ disease (LD) were recorded in 2020, 263 of which were considered to have become infected in Denmark, Table 1. 2020 recorded both the highest number of Legionnaires’ disease cases ever with presumed infection in Denmark and the highest ever incidence, Table 2. More cases were recorded in April, May and from July to December than the average for the three preceding years, Figure 1; although some variation was seen in the number of monthly cases from year to year, Figure 2. Of the 278 cases in 2020, 165 were men (59%) aged 26-96 years (median 66 years). Among women, the cases were aged 25-99 years (median 74 years).



A total of 15 patients became infected during travels abroad (nine in Europe, five in Asia and one on the American continent), and 241 were known to have become infected in Denmark, including 209 cases locally/in their homes, 25 at an institution or a hospital, and seven while traveling within Denmark, Table 1. The remaining 22 cases were presumably also infected in Denmark, as no information about travel activity was provided. Thus, a total of 263 (95%) persons probably became infected in Denmark. This constitutes a 25% increase compared with the average number recorded between 2017-2019.

This increase may possibly be associated with an increase in test activity for persons tested for L. pneumophila by PCR in 2020 compared with previous years, as many admitted patients with suspected COVID-19 were also tested for legionella infection. LD is one of several differential diagnoses of severe (hospital-requiring) COVID-19 and very much affects the same age groups. Thus, in 2020, approx. 37% more patients were tested for LD by L. pneumophila PCR than in the preceding year.

An increased test activity was seen during the entire pandemic, particularly in the months of March-April and again from November-December 2020, which coincides with the periods when more people were admitted with COVID-19, Figures 3 and 4. It remains unknown to which degree the increase in the number of tested patients may be linked to the observed increase in the number of recorded LD patients, but this may be one of several factors explaining the increase.



Ref.: https://www.dst.dk/da/Statistik/covid-19-hurtige-indikatorer

A total of 39 patients with LD died within 30 days of known onset or admission with LD, Table 1. Of these, 23 were men aged 53-90 years (median 77 years) and 16 were women aged 59-94 years (median 79 years). Thus, the mortality rate was 14%, which is slightly higher than previously recorded (around 10%).


The high mortality rate and the low male/female ratio (1.5:1) may be linked to the fact that in 2020, only few cases were associated with travel. The mortality rate for travel-associated cases is generally lower (<5%) than for non-travel-associated cases. The male/female ratio for travel-associated LD is generally around 2.5:1, whereas the corresponding ratio for people who have become infected in Denmark is lower, approx. 1.5:1.

The recorded cases of Legionnaires’ disease who had presumably become infected in Denmark were distributed geographically with 122 cases on Zealand, 44 on Funen and 96 in Jutland; see Table 2.


The increase in 2020 compared with previous years was particularly distinct for the areas of North Zealand, South Jutland and Copenhagen subs. Funen remained the area with the highest incidence recorded, but registered a slight decline compared with 2019. Copenhagen City remained the area with the lowest incidence recorded in Denmark. No obvious explanation was found for the relatively large increase observed in North Zealand, South Jutland and Copenhagen Subs., as no outbreaks or clusters were recorded.

Community-acquired infection in Denmark

Among the 209 cases of community-acquired infection in Denmark, Table 1, 65 were culture verified, including 40 (61.5%) that were L. pneumophila serogroup 1 (21 Pontiac and 19 non-Pontiac). In 35 cases, Statens Serum Institut has knowledge that a suspected water supply in the person’s home or elsewhere was tested. Consistency/Matching between the typing results from patients and water/environmental samples was found only in seven cases; all from the patients’ residence. In six cases, no match was detected between isolates from patients and water samples from the patients’ residence.

In 16 cases, no patient isolate was available for comparison with water sample isolates. Only three of the water systems tested free of L. pneumophila, corresponding to a 91% positive rate, which is roughly in line with the general approx. 80% positive rate of colonised Danish water systems. One community-acquired cluster was detected with two cases residing in the same block of houses in East Zealand. The two cases are linked to a new case established in 2021 and to three cases from 2018. Of these, four of the cases are culture-verified with detection of L. pneumophila serogroup 1 subgroup Philadelphia ST1. Water samples from the properties have tested negative to cultured Legionella.

Transmission during travel in Denmark

Seven cases are believed to have become infected while traveling in Denmark during stays at a rented holiday home, hotel or camping site. Two cases were associated with stays at the same hotel.

One of these cases was culture verified, and the same type (L. pneumophila serogroup 1 subgroup Philadelphia ST1) was detected in the water system of the hotel.

Institutional and hospital-acquired transmission in Denmark

A total of 25 cases with presumed or confirmed infection from healthcare institutions were notified; nine from hospitals and 16 from other institutions such as nursing homes. Thirteen cases were culture verified, six cases of serogroup 1 OLDA/Oxford ST1, one case of serogroup 3 ST993, two of serogroup 5 subgroup Cambridge ST80, two of serogroup 6 ST728, one with no serogroup ST1916 and one case of L. jamestownensis, all of which were so-called common environmental types. Isolates were matched to water samples in three hospital-acquired cases and in three cases acquired at nursing homes.

Infection acquired during foreign travel

A total of 15 persons (5.4%) were probably infected while traveling abroad, which is a steep decline from previous years when the corresponding proportion was 20-25%. Nine of the 15 cases had travelled abroad in January and February 2020; the remaining cases had travelled in the period from June to October. Thailand and Spain were the most frequently recorded travel destinations with four and three cases, respectively. Two cases formed part of international clusters counting two or more cases. Legionella pneumophila serogroup 1 was cultured from six of the patients, serogroup 3 from a single patient.

Laboratory-confirmed cases

Legionella pneumophila was detected by culture in 92 patients. L. longbeachae was detected in one patient; and for the first time ever in Denmark, L. jamestownensis was detected in a patient. 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 established by whole-genome sequencing. A total of 39 different ST were detected, including two new sequence types. The most frequent ST was ST1 (belonging to serogroup 1) with 25 isolates (27%). The distribution was largely identical to previous years, but a slight decline was seen in the share of subgroups belonging to serogroup 1 Pontiac, which is associated with the lower number of travel-associated cases in 2020, as these cases have more frequently become infected with these subgroups.

Additionally, a slight decrease was seen for serogroup 3 and an increase was recorded for serogroup 6. As in previous years, non-serogroup 1 comprises of approx. 40% of all cases and combined with serogroup 1 non-Pontiac, approx. 70% of all culture-verified cases are caused by types that are considered less infectious. In other European countries, the inverse proportion is generally seen. Thus, in 2019, 94 of a total of 278 cases (33.7%) were culture verified. In all, 254 patients had a Legionella-positive PCR (91.4% of all recorded patients).

The L. pneumophila urine test (LUT) was positive in 62 cases (22%), which is a continuation of the decline seen in recent years, EPI-NEWS 20/2020, but this may probably be explained by the low number of travel-associated cases in 2020. Lower-airway samples from 11 LUT-positive patients were PCR negative. Therefore, we still recommend LUT testing of urine samples on suspicion of Legionnaires’ disease.

This annual report is also described in EPI-NEWS 18/2021.