Despite risk control policies and awareness efforts by health authorities, Legionnaires’ disease remains under-diagnosed. Furthermore, susceptibility of hospital patients results in a case-fatality rate reaching up to 40 to 80% in untreated immunosuppressed persons. Adding to the illness severity is the constraint of waiting for at least ten days to reach diagnostic certainty as well as a very particular serogroup distribution.
What are the most virulent serogroups?
Legionella is a pathogenic bacillus that includes 49 species, including Legionella pneumophila, which is found to be involved in most cases of human contamination. This species is itself divided into 16 serogroups. Globally, serogroups 1, 4 and 6 are the most virulent, although this varies by geographic location, type of facility, and populations. For example, a 2002 study  based on 1,335 cases of legionellosis contracted in European hospitals showed that 36% involved non-serogroup 1 L. pneumophila. This percentage varies according to the regions and can reach 71% in Denmark, Finland and Sweden.
Screening for hospital-acquired legionellosis should not be limited to urine test
Detection of soluble Legionella pneumophila antigens is a perfectly reliable method if patients have been infected by serogroup 1 legionella. However, screening of legionnaires’ disease using this technique fails in 20 to 50 of cases according to a study from 2017  due to the important role of non-serogroup 1 L. pneumophila in hospital-acquired infection. It is therefore essential to perform a pulmonary sampling in infected patients, followed by a culture test on Petri dish in healthcare environments.
Delays for obtaining laboratory results highlight the need for prevention
Since the laboratory culture method is much longer and more complex than urine tests, the 10-day waiting period to receive a reliable diagnostic of Legionnaires’ disease delays the action. The best way to protect patients is to address the source of problems, by limiting legionella hazards in water systems before it can infect humans. It is therefore essential to carry out preventive tests for all serogroups of Legionella pneumophila as often as possible, at the most representative points and risk areas in water systems.
What are serogroups? Differentiation by serogroups corresponds to the classification of strains of the same species of bacteria according to their antigenic characteristics. These surface antigens give the bacteria specific antibody binding properties which in turn cause different immune responses in the host depending on the serogroup. Therefore, serogrouping makes it possible to distinguish the strains according to their immunological properties.
What are serogroups?
Differentiation by serogroups corresponds to the classification of strains of the same species of bacteria according to their antigenic characteristics. These surface antigens give the bacteria specific antibody binding properties which in turn cause different immune responses in the host depending on the serogroup. Therefore, serogrouping makes it possible to distinguish the strains according to their immunological properties.
 Helbig, J., Bernander, S., Castellani Pastoris, M. et al., Pan-European Study on Culture-Proven Legionnaires’ Disease: Distribution of Legionella pneumophila Serogroups and Monoclonal Subgroups, Eur J Clin Microbiol Infect Dis (2002) 21: 710. https://doi.org/10.1007/s10096-002-0820-3
Ministère de la Santé et des Solidarités Direction Générale de la Santé, Conseil supérieur d’hygiène publique de France Section des maladies transmissibles, Section des eaux, Le risque lié aux légionelles Guide d’investigation et d’aide à la gestion, 1er juillet 2005 https://solidarites-sante.gouv.fr/IMG/pdf/guid2005-2.pdf
 Pierre DM, Baron J, Yu VL, Stout JE. Diagnostic testing for Legionnaires’ disease. Ann Clin Microbiol Antimicrob. 2017;16(1):59. Published 2017 Aug 29. doi:10.1186/s12941-017-0229-6
 World Health Organization, Légionellose, February 2018 https://www.who.int/fr/news-room/fact-sheets/detail/legionellosis