Infectious risk posed by Legionella pneumophila outside of serogroup 1 in hospitals

Because hospitalized patients are particularly fragile, the fatality rate when the disease is contracted in this setting can reach 40 to 80% in an untreated immunocompromised patient.[1] Added to this danger is the constraint of waiting about ten days to obtain a definitive diagnosis. Moreover, the distribution by serogroup is very specific.

Which serogroups are the most virulent?

Legionella is a pathogenic bacillus comprising 49 species. Among these, Legionella pneumophila is implicated in most human infections.

This species itself is divided into 16 serogroups.

Worldwide, serogroups 1, 4, and 6 are the most virulent.

However, this proportion varies depending on:

• Geographic location
• Types of facilities
• The exposed population

For example, a 2002 study[2] based on 1,335 cases of legionellosis caused by Legionella pneumophila in Europe showed that 36% of hospital-acquired legionellosis in European hospitals involved L. pneumophila outside serogroup 1.

This percentage varies around the world and can, for instance, reach 71% in Denmark, Finland, and Sweden.

Testing for nosocomial legionellosis cannot be limited to a urinary test

The search for a soluble urinary antigen is a perfectly reliable method if patients are infected by serogroup 1 legionella.

However, according to a 2017 study[3], testing by this method fails in 20 to 50% of cases, owing to the involvement of non–serogroup 1 legionella in nosocomial legionellosis.

In the event of an infection in this kind of environment, it is therefore essential to perform a pulmonary sample from patients and follow up with Petri dish culture testing.

Given the delay in obtaining a laboratory diagnosis, prevention remains the best strategy

Laboratory culture testing is much longer and more complex than urinary tests. The waiting period of more than 10 days for a reliable legionellosis diagnosis delays action.

The best way to protect patients is to address the problem at its source—particularly by reducing the risk of contamination in water systems by legionella, so that the bacteria cannot infect humans.

It is thus essential to carry out preventive testing as regularly as possible. One must search for all legionella serogroups. Samples are collected at the most representative and high-risk points in the water system.