Legionnaires’ disease

2019 annual report on Legionnaires’ disease

In 2019, Denmark saw a total of 269 notified cases of Legionnaires’ disease (legionella pneumonia). Hereof, 163 (61%) were men. The median age was 69 years for men (range 27-98 years) and 71 years for women (range 37-95 years).

A vital status report shows that 22 patients (8%) died within 30 days (median 12 days; range 3-29 days) from known onset or admission due to Legionnaires' disease. Thus, the mortality rate was slightly lower than in 2017 and 2018 (approx. 11%, both years).

A total of 217 (81%) patients had probably become infected in Denmark, including 22 at a hospital or nursing home, see Table 1 for more infection categories.


The distribution by region and province for patients probably infected in Denmark in the 2016-2019 period is presented in Table 2. The table shows that the province of Funen had the highest detected incidence in 2019 with 9.6 cases per 100,000 inhabitants.


Community-acquired infection in Denmark

Among 171 cases of community-acquired infection in Denmark (Table 1), 75 were culture verified, including 38 (51%) that were Legionella pneumophila serogroup 1. Eleven (6.4%) of the 171 cases died in connection with their Legionnaires' disease . In 47 cases, it is known that a suspected water supply in the person’s home or elsewhere had been tested. In 12 of these cases, there was agreement between the typing results of isolates from patients and water/environmental samples. In 11 cases, there was a match with environmental isolates from water from the patient’s own residence, whereas in one case there was a match with isolates from an occupational stay at a hotel. In another 12 cases, no match was established between isolates from the suspected source of infection (mainly samples from the patient’s home) and the patient isolate. In 20 of the 47 tested cases, a patient isolate was not available for comparison with water sample isolates. Even so, in 19 of these cases, Legionella pneumophila was detected in the tested water systems. Only three of the 47 tested water systems were negative for cultivable Legionella pneumophila, which underlines that Legionella may be found in most water systems.

Transmission during travels in Denmark

Four of the cases are believed to have become infected while travelling in Denmark. Three of the cases were notified to the European Centre of Disease Prevention and Control (ECDC). In three cases, Legionella pneumophila was detected following infection tracing including water samples. In one of these cases, the patient isolates matched those from the water supply of a hotel. Even though only a limited number of travel-associated Legionnaires’ disease cases with infection in Denmark is recorded both among Danish and international tourists, the findings show that it is important that camping areas, hotels and other places of accommodation have a functional Legionella control scheme in place.

Institutionally acquired infection in Denmark

A total of 22 cases of presumed or certain infection via institutions were notified, 12 from hospitals and ten from nursing homes. Eight of these 22 patients (36%) died in connection with their Legionnaires' disease, which is fewer than in 2017 (57%) and slightly fewer than in 2018 (40%). Ten cases were culture verified; in five of these cases, water samples were tested. The results from the tests confirmed that three of the cases were due to infection from the institutions’ water (one nursing home and two hospital departments).

Infection acquired during travel abroad

A total of 52 cases were probably infected during travels abroad, none of whom died. A total of 47 of these cases were notified with the ECDC, 15 of whom were part of international clusters counting two or more cases. For the remaining five travel cases, no information was available about accommodation or the affected people had stayed at private, non-notifiable places. The most frequent travel destinations were: Italy with 11 cases, Thailand with 10 cases, and Spain and the United Arab Emirates with four cases each. Italy thus remains the country where more Danes become infected, whereas Turkey (two cases) and France (no cases) still account for a lower share of cases (see the 2018 annual report). The share of people infected abroad (52/269; 19%) was slightly lower than normally (approx. 25%), and in 2020 we expect the share to decline to a very low number owing to travel restrictions imposed by the corona crisis. A total of 18 of the 52 cases were culture verified, 16 (89%) hereof were Legionella pneumophila serogroup 1. 

Laboratory-confirmed cases

Legionnaires' disease may be diagnosed by culture, urinary test (LUT), increasing antibody titer in serum immunoglobulin, or by PCR. Detection of Legionella spp. with no specification of pneumonia is considered a doubtful case of Legionnaires' disease, as the clinical significance of this is unclear.

Legionella pneumophila was isolated by culture from 111 patients and, for the first time ever in Denmark, Legionella parisiensis was isolated from one patient. The distribution by serogroup and subgroup is presented in Table 3. In addition to serogrouping, the sequence type (ST) of the clinical Legionella pneumophila isolates was also established by whole-genome sequencing. A total of 48 STs were detected (from 102 isolates with a valid ST result), including four new sequence types. As in previous years, the most frequently occurring sequence type was ST1 (belonging to serogroup 1) with 24 results (24%), whereas the remaining 47 were distributed on just 78 isolates.


Thus, in 2019, 112 from a total of 269 cases (42%) were culture verified. In all, 241 patients had a Legionella-positive PCR (90% of all recorded patients) of which 26 were reported as Legionella spp. and were therefore negative to or not tested for Legionella pneumophila. It was possible to culture Legionella pneumophila from four of these 26 samples, and an additional four patients tested LUT positive (a total of five). Thus, a minimum of eight of the 26 (31%) cases had a Legionella pneumophila infection. In one patient only, it was possible to verify by culture that the patient had a non-pneumophila infection, as Legionella parisiensis was cultured. It remains unknown which legionella species caused the remaining Legionella spp.-positive findings.

In 2019, a total of 69 LUT-positive patients (26%) were detected, compared with 129 (47%) in 2017 and 113 in 2018 (45%). LUT primarily detects serogroup 1 cases, whereas approx. 40% of all culture-verified cases are caused by other serogroups. LUT is particularly important for diagnosing of travel-associated cases, which are mainly caused by serogroup 1. Thus, in 2019 a total of 42% (22 of 52) travel-associated cases were LUT-positive, whereas this only applied to 22% (47 of 217) of the cases who were presumably infected in Denmark. Furthermore, LUT can verify the PCR diagnosis and serves as an important supplement to PCR. Thus, in 2019, a total of eight PCR-negative patients were LUT-positive.

In 2019, 110 patients (41%) were PCR positive only, and therefore did not have an officially verified diagnosis (in accordance with the ECDC’s case definition. For comparison, the corresponding share was only 22% in 2017 and 35% in 2018.

This report is also described in EPI-NEWS 20/20.