In 2010, the European Union member countries with the highest incidence of legionellosis were, in increasing order, Slovenia with 2,83 cases per 100,000 people, followed by the Netherlands (2,81), Spain (2,48), Denmark (2,40) and France (2,38) with 6 cases in total.
Eight years later, more than 11,000 cases have been reported in the European Union. Infected individuals were mainly located in Italy (26%), France (19%), Spain (13%) and Germany (13%). The number of Legionnaires’ disease cases for 2018 represents an increase of approximately 16% compared to 2017, where 9 259 cases were recorded.
How to explain this strong upward trend and the unequal distribution by country compared to their respective population within the EU?
Inadequate control standards
The lack of prevention and self-monitoring tests is partly responsible for legionella growth in hot water and cooling towers water systems. For example, in September 2018, in the region of Brescia, Italy, it was only after a pneumonia outbreak that the Italian health authorities ordered checks on the cooling towers of three companies in the region. 9 out of 10 samples were found to be contaminated with legionella. Thus, control is often performed too late, in response to established pathological cases.
A 2005 study already showed that domestic hot water systems in Italian hotels were heavily colonized by Legionella. 75% of the buildings surveyed and 60% of the samples contained more * per liter, of which 87% was Legionella pneumophila. This means that there are few controls and preventive measures in place in some countries, while in others, such as the United Kingdom, Germany or France, oversight measures are stricter.
Older populations are most vulnerable
Countries with the highest proportion of elderly people (aged 65 and over) are Italy (22.3%), Greece (21.5%) and Germany (21.2%). However, the elderly or immunodeficient individuals are most at risk of developing the acute symptoms of Legionnaires’ disease, which can be fatal. Young, healthy people have a lower risk of developing legionellosis, which contributes to fewer cases in countries with lower average ages.
Climate is a decisive factor in legionella proliferation
Southern countries have an additional factor associated to the disease. Legionella grow most rapidly in warm environments, i.e. in the 25-45 °C range. In countries where the temperature regularly exceeds 25 °C, there is an increased risk of contamination since “cold” water can easily reach the temperature threshold allowing Legionella growth.
A study suggests that higher heat wave occurrences and intensity attributed to climate change could explain to some extent legionellosis progress, particularly in France, the Netherlands and Germany.
Heterogeneous distribution of at-risk facilities in the European union
Cooling towers and sanitary hot water systems are major breeding grounds of Legionella. A much higher rate of industrialization in Western Europe generally implies a larger number of cooling towers (ACT). Furthermore, Southern Europe has a higher demand for public and domestic cooling devices such as air conditioners and mist cooling systems, with Italy, Spain, Greece and France together accounting for the majority of EU sales.
Legionella infections remain under-diagnosed
Regarding the notified rates of Legionnaires’ disease, it is important to keep in mind that the disease is not systematically reported in every country, especially in cases of spontaneous recovery. On the other hand, the symptoms of legionellosis are very similar to the flu during the first stages, therefore both diseases are often confused.
For example, according to the European Center for Disease Prevention and Control (ECDC), legionella is largely underdiagnosed in Greece while countries such as the United Kingdom, The Netherlands, France and Italy have introduced mandatory reporting measures that explain, in part, their top incidence rankings. Thus, until very recently, the legionellosis contracted in Greece was mainly diagnosed in tourists home countries, after their return. Still today, the disease is very rarely reported in the Greek population. In addition, data on Legionella are almost non-existent for some countries such as Romania or Estonia.
Multiple parameters converge to a single key priority: prevention
As shown above, comparisons of currently available data on legionella in the European Union are to be interpreted with caution.
Distribution of legionnaires’ disease recorded cases by country depends on a multitude of factors: the regulatory framework (obligation to declare the disease and/or to conduct preventive tests), demographic trends, climate, economic and development stage of the country (industrialization, types of water infrastructure, heating and air conditioning …) as well as diagnosis rate.
The overall growth of legionellosis cases recorded in Europe and around the world is driven by various factors such as the increased frequency of heat waves, aging population, constant expansion of water distribution networks and installations involving aerosol spraying but also a higher reporting rate of Legionnaires’ disease as a result of massive awareness efforts by national health authorities.
Fighting Legionnaires’ disease requires implementing simple prevention measures, such as conducting regular self-monitoring tests, anticipating the risk and act quickly in case of contamination. Fortunately, innovative, cost-effective and easy to use on-site solutions are now available, that provide reliable, confidential results.
*CFU/L : Colony-Forming Units per liter of water
 Borella P, Montagna MT, Stampi S, et al. Legionella contamination in hot water of Italian hotels. Appl Environ Microbiol. 2005;71(10):5805–5813. doi:10.1128/AEM.71.10.5805-5813.2005 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1265926/
 Beauté J, Zucs P, de Jong B, on behalf of the European Legionnaires’ Disease Surveillance Network. Legionnaires’ disease in Europe, 2009-2010. Euro Surveill. 2013;18(10):pii=20417.
European Commission, COMMISSION STAFF WORKING DOCUMENT Review of available information Accompanying the document Communication from the Commission to the European Parliament, the Council, the European Economic and Social Committee and the Committee of the Regions on an EU Strategy for Heating and Cooling, Février 2016, https://ec.europa.eu/energy/sites/ener/files/documents/1_EN_autre_document_travail_service_part2_v6.pdf
Legionella Colonization of Hotel Water Systems in Touristic Places of Greece: Association with System Characteristics and Physicochemical Parameters Maria A. Kyritsi 1 , Varvara A. Mouchtouri 1, Antonis Katsioulis 1,2, Elina Kostara 1 , Vasileios Nakoulas 1 , Marina Hatzinikou 1,2 and Christos Hadjichristodoulou 1,2,*
Featured image: Modified from: European Centre for Disease Prevention and Control, https://ecdc.europa.eu/en/legionnaires-disease/surveillance/atlas, accessed on July 3, 2019