The world's first known victim of a Legionella infection was Ray Brannon. The 61-year-old died on July 27, 1976, having contracted the disease three days earlier at a meeting of some 2,000 Air Force veterans in the ballroom of a Philadelphia hotel. Another 182 participants fell ill, 29 of them fatally. Thus, the American Legion became the eponym of these waterborne bacteria.
It was not until a quarter of a century later that an obligation to report legionellosis was established in Germany. Since then, statistics have been regularly published in the Epidemiological Bulletin of the Robert Koch Institute. Although the number of infected persons is low, the actual figures are probably much higher. Between the years 2015 and 2019, the number of reported cases almost doubled – from approximately 800 to 1,500 (RKI 2021). Only in 2020 was there a temporary decline, due to the extensive travel restrictions resulting from COVID-19.
Taking under-reporting into account, the Robert Koch Institute estimated in 2015 that there could be as many as 30,000 infections per year. This figure is now likely to have increased to around 60,000. Many diseases are brought in by travelers, but clinics also contribute to the spread. Mortality rates vary from 5% for travel-related infections to 17% for hospital-related infections. Despite several high-profile outbreaks, such as the recent one in Poland, most infections occur unnoticed. Buildings that are not subject to inspection, such as owner-occupied private homes, pose a particular risk.
According to Zacharias et al. (2023), the known reasons for the high rate of unreported cases of 97% include a lack of or inadequate diagnostics, hot water temperatures that are too low, low water consumption, standing water in unused pipes, and the diagnostic method itself.
Given various factors, a further increase in the number of cases of the disease may be expected. As a result of global warming the temperature of cold water is often above 20 °C. In addition, the retention period of water in the mains is increasing as a result of decreasing water consumption. Higher energy prices are causing consumers to lower the temperature of their hot water. At the same time, significant investments in the existing infrastructure are currently unforeseeable.
One positive aspect is that a new limit value has been established in the German Drinking Water Ordinance [TrinkwV]. In accordance with Section 51 TrinkwV, measures to reduce the Legionella concentration are now required as soon as the technical action level for Legionella spec. of 100 CFU (colony-forming units) per 100 milliliters of drinking water is reached – and not only when this value has been exceeded.
Immunocompromised, mostly elderly people are particularly affected by Legionella infections. The mortality rate in nursing homes and hospitals is about 10%. For this reason, the German Society of Hospital Hygiene [DGKH] recommends a limit value of less than 1/100 ml, especially for high-risk wards (DGKH 2018). Incidentally, this limit value also applies to other facultative pathogens. Compliance with these limits is often only possible through the employment of Point-of-Use sterile filters. Since the infective dose is very likely to be significantly lower for this group of patients than for immunocompetent people, reliable protection is particularly relevant.
The high number of unrecorded cases results in uncertainty, economic losses and considerable costs. The task now is to continue to track down the Legionella pathogen through increased attention and to prevent infections. Possible solutions are already available.
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