Legionnaires' Disease in Denmark 2023
Legionnaires Disease in Denmark 2023
In 2023, 313 cases of Legionnaires’ disease (LD) were registered among residents in Denmark. Of these, 198 were men (aged 31-98 years, median age 70 years) and 115 were women (aged 30-96 years, median age 73 years). The median age was somewhat lower for both genders in 2023 than in 2022, where it was 73 and 76 years, respectively. Of all LD cases, 35 (11.2%) died, including 21 men (10.6%) and 14 women (12.2%), within a month after onset or hospitalisation. Mortality in 2023 fell to normal levels after several years of higher mortality rates at 14.5% in 2020/2021 and 19.5% in 2022. The proportion of elderly people remained high with 179 LD cases ≥ 70 years (57.2%) and was only slightly higher in 2022 at 62.7%, while mortality in this age group was significantly lower in 2023 (16.2%) than in 2022 (27.2%). Except for cases infected in institutions/nursing homes, mortality in 2023 was lower for all routes of infection than it was in 2022 (Legionnaires´ disease in Denmark 2022), Table 1.
In 2023, the male-to-female ratio was 1.7:1, which is slightly higher than in 2020-2022, when travel activity was low. The proportion of travel-associated cases (abroad) is now almost at the same level as before COVID-19 with 57 cases (18.2%). The male-to-female ratio is always higher for travel-associated cases than for cases infected in Denmark. In 2023, the male-to-female ratio was 3.75:1 (45:12) for travel-associated cases, which is also higher than the usual ratio of about 2.5:1. Travel-associated cases usually have very low mortality, as was also the case in 2023, in which no one died within a month after onset or hospitalisation, Table 1.
Most LD cases were seen in the months of July through October, with between 28 and 30 Danish-infected cases per month. July had the highest number of travel-associated cases with 15 cases. In many countries, there is a clear seasonal variation with fewer cases in winter/spring and many cases in summer/autumn. This seasonal variation is not as clear in Denmark; for example, there were 20 and 21 Danish-infected cases in January and February 2023, respectively.
Infection in Denmark
In total, 256 of the registered cases in 2023 were assumed to have been infected in Denmark (81.8%), Tables 1 and 2. This is a slight increase in the number of Danish-infected cases compared to 2022 with 248 cases and at a higher level compared to the years before COVID-19 (2017-2019) with an average of 210 Danish-infected cases per year in this three-year period.
The overall distribution by province in 2023, Table 2, was approximately the same as in 2022, but with an increase in Central Jutland and especially in East Jutland from 14 cases in 2022 to 26 cases in 2023. North Zealand had the highest incidence in the country in 2023 (9.1 per 100,000), as was also the case in 2022. No outbreaks or clusters have been detected to explain these increases or differences.
Community-Acquired Infection in Denmark
Of the 202 cases of community-acquired infection in Denmark, Table 1, 71 were culture-verified (35.1%), of which 45 (63.4%) with L. pneumophila serogroup 1 (SG1) (17 Pontiac and 28 non-Pontiac) and 14 with SG3 (19.7%). The remaining 12 were with other L. pneumophila serogroups, one with L. longbeachae, and one with L. sainthelensi.
Statens Serum Institut (SSI) is aware of 40 cases where the water supply in the patient’s home or another suspected environmental source was investigated; 36 locations were positive for L. pneumophila and four were negative. For 14 cases, there was a clinical isolate for comparison with environmental isolates. In 10 of these, there was a match between the typing result of the isolate from the patient and from water/environmental samples from the hot water system in the patient’s home (nine cases) and one case from a swimming hall. Three of these cases were infected with a virulent L. pneumophila SG1 subgroup Benidorm sequence type (ST) 42, including the one case from a swimming hall where there were several cases in 2009, EPI-NEWS 41/10 (ssi.dk). Thus, the source of infection could only be verified in 10 out of the 40 investigated cases, and the infection conditions are therefore not definitively clarified for the majority of the LD cases where the infection is assumed to have occurred in Denmark.
Infection During Travel in Denmark
In total, 11 people were likely infected while travelling in Denmark, seven at six different hotels, two during stays in the same rental summer house, one during a stay in a rental apartment, and one during a stay at a campsite. Two cases were from a hotel where there was also one case in 2022 and two cases in 2021, and the outbreak continued in 2023 (Annual Report 2021, 2022). Six cases were culture-verified, five with L. pneumophila SG1 non-Pontiac and one with L. longbeachae. For three cases infected while travelling in Denmark, there was a match between typing results for patient and environmental isolates, including from a hotel and for the two cases infected during a stay in the same rental summer house.
Institutional and Hospital-Acquired Infection in Denmark
There were 34 reported cases of suspected or confirmed infection from institutions in the healthcare system; 12 from hospitals and 22 from other institutions such as nursing homes. The latter is at the same relatively high level as in 2021 and 2022. In total, ten of these cases were culture-confirmed, seven with L. pneumophila SG1 (one Pontiac and six non-Pontiac) and three with other serogroups (3, 5, 6). SSI is aware that water samples were investigated in connection with 18 of the 34 cases. A match was found in five of the cases, three from hospitals, one from a nursing home, and one from another institution.
Infection Acquired During Travel Abroad
A total of 57 people (18.2%) were likely infected while travelling abroad, Table 1, which was more than in 2021/2022 and almost at the level before COVID-19. The United Arab Emirates (Dubai) was the most common travel destination among those infected with LD, with eight cases. This was followed by Greece with seven cases, and France and Turkey with six cases each. From Italy, which has been the most common travel destination in recent years, there were only four cases in 2023. Only 42 of the 57 cases were reported to the European Centre for Disease Prevention and Control (ECDC), partly because the stay was private (e.g., own house), and partly due to a lack of information about the place of stay during the incubation period. Three cases were part of clusters with two or more international cases, including two from Greece and one from Italy. In total, 30 of the travel-associated cases were culture-verified, 24 with SG1 (80%), of which 22 were with SG1 Pontiac subgroups, Table 3. It is somewhat unusual that eight cases (9%) were infected with less virulent types in this infection category, but 54% (22 of 41; Table 3) of all clinical SG1 Pontiac isolates in 2023 still belonged to this infection category. Only seven cases of travel-associated Legionnaires’ disease were LUT-positive in 2023, of which four were diagnosed solely with LUT, one of which was PCR-negative of one tested.
Laboratory-Confirmed Cases
L. pneumophila was detected in 117 patients by culture. L. longbeachae was detected in three patients, L. bozemanae and L. sainthelensi were detected in one patient each. In 2023, 122 of the total 313 cases (39%) were culture-confirmed, which is slightly lower than in 2022 with 125 culture-confirmed cases (43.6%). The distribution by serogroup and subgroup for L. pneumophila is shown in Table 3.
In addition to serogrouping, the clinical L. pneumophila isolates were also assigned a sequence type (ST) using whole genome sequencing. Fifty-six different sequence types were detected. The most frequent ST was, as in previous years, ST1 (belonging to SG1, both Pontiac and non-Pontiac) with 29 isolates (25%), which is the same level as seen previously. The second most frequent ST was ST87 with seven isolates (6%) belonging to SG3. The serogroup distribution was thus approximately the same as in 2022 (Legionnaires´ disease in Denmark 2022), but with somewhat more SG1 non-Pontiac and slightly fewer SG3 cases.
284 patients had positive Legionella-PCR (90.7%). L. pneumophila urinary antigen test (UAT), which predominantly detects SG1, was positive for 62 cases (19.8%), which is almost the same distribution as in 2022. Lower respiratory tract samples from ten UAT-positive patients were negative by PCR. 46 of the 284 PCR-positive patients (16.2%) were positive for non-pneumophila Legionella species (31) or Legionella species (n=15) without a result for L. pneumophila. 36 of the 46 (78.2%) cases were from the Capital Region of Denmark and thus accounted for 28.1% of the total 128 cases (including cases infected abroad) from this region.
It is primarily in the Capital Region of Denmark that non-pneumophila L. species are tested for and detected by PCR, which may mean that LD caused by non-pneumophila L. species is underdiagnosed in other parts of the country. At the same time, a relatively high proportion of patients with weakly positive results for Legionella species are detected, where the clinical picture does not match Legionella pneumonia, and where the clinical significance of the finding is unknown.
Travel-associated cases are generally younger and have better underlying health than cases infected in Denmark and are most often men. Since they primarily have infections with SG1, they will in most cases be UAT-positive, and it can be difficult to obtain a sufficiently good respiratory secretion for PCR. If only PCR is used for the diagnosis of Legionnaires’ disease, there is a risk of overlooking cases of especially travel-associated Legionnaires’ disease, and there may thus be an underreporting of undiagnosed travel cases.
This report is also mentioned in EPI-NEWS 34/2024.