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Legionella in the shade of Corona: 12.000 water samples taken in Covid19-times evaluated

In the proceedings of the dominating covid19 pandemic industrial, commercial and private buildings had to be shutdown to avoid the spread of the corona virus. The German Robert-Koch-Institute (RKI) warned not to neglect the health hazards coming from drinking water installations in terms of legionella growth if the facilities are out of operation for a longer period of time. We took a look at our own set of drinking water data all gathered in corona-times - with surprising results.

In its epidemiological bulletin of October 29th, 2020 the RKI published case figures of the Legionnaire's disease since the beginning of the Corona Pandemic until End of Juli 2020. The researches tried to find out if the corona-related lockdown measures including limited accessibility or shutdown of public and commercial buildings has had an impact on the numbers of Legionnaire's disease regularly reported.

The timeline of cases of Legionnaire's diesase acquired in the community (private/professional environment) shows that from January to March the numbers were slightly higher than usual probably as a result of a smaller number of business trips and vacation journeys. All in all an unusual increase in cases could not be detected.

In respond to the RKI-findings we thumbed through our own data set asking ourselves if the temporary lockdown measures lead to higher Legionella occurrence in the commercial facilities we drew water samples from.

The frist set of drinking water data consists of 13,410 samples we contracted in the period from June to October 2020. We could actually take 12,101 samples we were commissioned to. The success rate of 90.24% is close to the average annual rates from prae-corona times.

In 9.76% cases samples could not be taken. 460 samples we failed to draw due to technical problems and defects of the installation or sampling valve, in 505 cases tenants would not open the door for us to let us take samplers from their apartment. The reluctance to grant us access to the private premises is with 3.77% just one percent higher than in the years 2018 and 2019. Therefore, Corona does not have an significant impact on the tenant's behaviour.

344 times an explanation for the fail-to-draw was missing on the sampling protocol.

881 samples exceeded the (German) threshold value for Legionella of 100 colony forming units (CFU) per 100 ml drinking water. That equals a positive rate of 7.28% and is sightly higher than last year's average (5.93%).

The second set of drinking water data we assessment presented far more interesting results: Just before re-opening their buildings for public access many plant operator's and house or business owners contracted us for - what we call - restart-samplings. So, we took 931 samples from drinking water installations that were either rarely used during lockdown-phase or not in use at all.

136 samples exceeded 100 CFU per 100 ml, which equals a contamination rate of 14.61%. That is significantly higher than the annual average from 2013 to 2019 (5.93%). High concentrations of more than 1,000 CFU were found in 38 sampling. That is a percentage of 4.08% of all samples and therefore twice as high as usual. 9 samples showed a extremely high concentration aboe 10,000 CFU per 100 ml indicating an imminent health threat.


Last but not least we would like to share an interesting finding concerning the temperature regime of the 881 samples contamined with legionella bacteria: in 315 cases the temperature while sample taking was below 45°C, in 462 cases the maximum temperature (tmax) was below 55°C - a direct violation of the German technical rule for safe operation of drinking water installations DVGW W 551 that demands 55°C as a minimum operational (constant) temperature at each circulation point.

If you have further questions on the drinking water data set presented here do not hesitate to contact our Chief Operating Officer Marcus Pikarek, preferably via email:

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