What is the main cause of Legionnaires’ disease?

About 90% of cases of Legionnaires’ disease are reported to be caused by Legionella pneumophila serogroup 1 but is that because the confirmatory test most commonly used is a urinary antigen test specific for Legionella pneumophila serogroup 1.

Public Health England provide regular statistics for Legionnaires’ disease, the 2016 version includes this chart:

cause of LD 2014 2016

Clearly in 2016 only 29.9% of cases are confirmed as Legionella pneumophila serogroup 1.. The most common method of testing, the urinary antigen test as seen in the chart below, was subject to a recent PHE document that stated “PHE has been made aware of potential discrepant results between clinical and reference laboratory Legionella pneumophila urinary antigen testing. The proportion of referred urines that are confirmed as positive has reduced. Between January – March 2017, only 63.2% of urinary samples reported as positive locally were confirmed by the reference laboratory, compared to 81.3% during the same period in 2016.”

This shows that the urinary antigen test was 81.3% “correct” at the time of the chart above but has reduced in its “confirmed positives” lately.

Diagnostic test for LD

Undoubtedly the common view is that Legionella pneumophila serogroup 1 is the main cause of Legionnaires’ disease but have we built this information on a urinary antigen test that only tests for Legionella pneumophila serogroup 1 when in the chart above 69% of cases of Legionnaires’ disease are from an unknown Legionella pneumophila serogroup. This is not the same as saying it is not Legionella pneumophila serogroup 1 but it is a big percentage of unknowns.

In the end you may argue it doesn’t matter as in the UK we treat all Legionella as the same when trying to eradicate it environmentally. But it is interesting that we have built a picture of a cause that may be reliant on 29.9% of results. Certainly in Australia and New Zealand this certainty would not stand up as Legionella longbeacheae is a major cause of Legionnaires’ disease there with approximately 30% of all cases.

The answer may be to treat all statistics with caution or at least understand what the information is telling us. Whilst there is some difference in how some Legionella species react in the environment the main “tools” used to get rid of Legionella bacteria in engineered systems will work for “all” Legionella bacteria so from a practical prevention perspective the information above is interesting but does not affect our actions.

Where it affects us is in the training we offer end users so be aware of the statistics in case you get a delegate on a training course who has read this post.

If you would like to know more about Legionnaires’ disease and its prevention why not contact Collaton Consultancy Limited on general@collatonconsultancy.com or +44(0)7958 124563.

Nigel Richardson is an expert witness for causation relating to Legionella, he is also an experienced water treatment, Legionella and Pseudomonas consultant and trainer for end users and water treatment companies alike helping companies comply with the relevant laws and guidance.

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