Legionnaires' disease, Annual Report 2018

Legionnaires’ disease

Legionnaires’ disease, Annual Report 2018

Notified cases

The criteria for confirmed Legionnaires’ disease (LD) are pneumonia and a concurrent positive culture and/or a Legionella urine test and/or a titre increase in serum immunoglobulin against Legionella pneumophila serogroup 1, whereas the criteria for probable Legionnaires’ disease is pneumonia and a concurrent PCR positive for Legionella pneumophila or Legionella species, and/or a titre increase for other serogroups or species.

In 2018, a total of 266 cases of confirmed or probable LD were notified. The median age was 69 years (range 21-93 years); 67 years for men and 71 years for women. A total of 175 (66%) cases were men. A total of 205 cases are considered to have become infected in Denmark, which is approx. the same share as in 2017 (n=210), when the total number of detected LD cases peaked (n=278; EPI-NEWS 45/18). The median age for LD cases that had become infected in Denmark (205) in 2018 was 71 years, and 135 (66%) were men. A vital status report showed that 29 patients (11%; in line with the level observed in 2017) died within 30 days (median 8 days; range 0-24 days) after the date of LD onset or their admission due to LD.

The number of cases is listed by infection categories in Table 1.


Table 2 shows the number and incidence per 100,000 of notified cases of LD acquired in Denmark in 2018 by region and area, compared with the incidences for 2015, 2016 and 2017.


The incidence in 2018 varied somewhat for LD cases who had become infected both in Denmark and abroad. Figure 1 presents the incidence of infected people for these categories, by regions and areas. In 2018, a separate study was conducted examining LD incidence in the 2015-2018 period in which the variations between regions and areas are described in detail. For further information, please see EPI-NEWS 19/19.

The trend for travel-associated cases is more difficult to assess because of the low number of cases. Some areas enjoy a relatively low occurrence of travel-related cases (e.g. Funen and the City of Copenhagen), whereas others have recorded a relatively high occurrence in relation to the total number recorded (e.g. North Jutland).


The intra-annual LD variation was slightly different in 2018 compared to 2017. Specifically, more cases were seen in the last part of 2018, particularly in December (n=28), a month which in other years contributes relatively fewer cases, Figure 2.


Community-acquired infection in Denmark

Among 163 cases with presumed or known community-acquired infection in Denmark, 62 cases were culture-confirmed. Seventeen of the 163 cases died in connection with their LD. For 34 cases, it is known that a suspected water supply in the person’s home or elsewhere had been tested. In 14 of these cases, there was agreement between the typing results of isolates from patients and water/environmental samples. In 13 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 water from the shower in a sports facility. In 13 of the 34 tested cases, no patient isolate was available for comparison with water sample isolates. In 12 of these cases, Legionella pneumophila was detected in the tested water systems. Thus, in all but one of the 34 tested locations, Legionella could be cultured, which simply underlines that Legionella can be detected in most water systems. 

Infection during travel in Denmark

Six of the cases are believed to have become infected while travelling in Denmark. Three of the cases were reported to the ECDC, as information about travels in the incubation period could be obtained, and these were publicly available travel destinations like hotels or camping areas. In three cases, infection tracing was conducted and water samples taken. At these three locations, Legionella pneumophila was detected, and in two of the cases, the patient and environment samples matched. The locations were a camping area and a hotel. Even though only a small number of travel-associated LD with infection in Denmark among both Danish and international tourists is recorded, the findings show that it is important that camp sites, hotels and other places of accommodation have a functional Legionella control scheme for their water systems in place.

Institutionally acquired infection in Denmark

A total of 20 cases of presumed or certain infection via institutions were notified, 12 from hospitals and eight from nursing homes. Eight of these 20 patients died in connection with their LD, which is a somewhat lower mortality (40%) than was recorded in 2017, when mortality was 57% (16 of 28). Even so, the mortality remains high compared with previous years (30%). Eight cases were culture-confirmed; in only three of these cases, water samples were tested. The results from the tests confirmed that two of the cases were due to infection from the institutions’ water (a nursing homes and a hospital department).

Infection acquired during foreign travel

A total of 61 cases were probably infected during travels abroad, two of whom died. In all, 49 of the cases were notified with the ECDC as, in the remaining cases, no information about accommodation locations could be obtained or the cases had stayed in private lodgings. Fifteen of these cases formed part of international clusters counting two or more cases in various countries.

The most frequent travel destinations were: Italy with ten cases, Germany with nine, Spain with seven, Turkey and The United Arab Emirates with five each (Table 3). In all, cases of infection were recorded from 22 countries. The share of cases infected abroad (61/266; 23%) was thus roughly in line with the one recorded in 2017 (24%). This share remained relatively constant (approx. 25%) in the preceding years, regardless of the total number of LD cases recorded. It should be noted that The United Arab Emirates remains a country where infections are relatively frequently acquired, whereas France follows a trend towards a lower number of cases.


Cases with unknown category of infection

A total of 16 cases could not be related to an infection category, as information about disease onset or stays in the incubation period could not be obtained. Two cases had become infected abroad (Turkey and Spain), but the majority of the remaining cases had presumably become infected in Denmark and not in hospitals/institutions.

Laboratory-confirmed cases

LD can be diagnosed by culture, Legionella urine test (LUT), titre increase in serum immunoglobulin or by PCR. Legionella pneumophila was isolated by culture from 90 patients. No other Legionella species were detected by culture from patients in 2018. The distribution by serogroup and subgroup is presented in Table 4. The serogroup distribution is very similar to that observed in previous years when serogroup 1 comprised approx. 60% and serogroup 3 approx. 20% of all clinical isolates. In addition to serogrouping, the sequence type (ST) of the clinical Legionella pneumophila isolates was also established by whole-genome sequencing. In all, 45 different STs were detected.

In 2017, a total of 52 different STs were detected among 130 clinical isolates, i.e. the same relative share. As was the case in previous years (EPI-NEWS 45/2018), the most frequent ST was ST1 (belonging to serogroup 1) with 21 isolates (23%). ST87 (serogroup 3), which in 2017 comprised a 15% share (19 cases), comprised only 4.5% (4 cases) in 2018. In contrast hereto, ST93 (also serogroup 3) was the second-most frequently detected ST in 2018 with a total of 8 cases (9%).


In 2018, fewer patient isolates were cultured than in 2017 (133/278 patients; 48%); both quantitatively (90) and relatively (90/266 patients; 34%). This was so even though almost the same number of PCR-positive patients was identified in the two years (216 in 2018 and 219 in 2017). But there was a clear trend towards the samples being more weakly positive in 2018 than in 2017. In 2017, the SSI received 87 samples that tested positive to Legionella pneumophila by PCR. On average, the samples had a Ct value of 31.0, whereas 110 samples from 2018 had an average Ct value of 34.2. At Ct values above 35, it is often not possible to culture Legionella from the samples. We do not know why relatively many samples had higher Ct values (a lower bacterial load) in 2018 than in 2017, but this may have contributed to the low culture success rate in 2018.

In 2018, a total of 113 patients tested LUT positive (42%) compared with 129 (47%) in 2017. There were both fewer culture-verified and fewer LUT-positive patients in 2018 than in 2017. Hence, increasing from 22% (n=62) in 2017 to 35% (n=94) in 2018, the share of patients positive by PCR alone (of whom three also had positive serology) rose. These cases cannot be considered confirmed cases.