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Legionella Species: Prominence and Implications for Industry

The identification of Legionella Anisa as the most prominent species over and above Legionella pneumophila, was the surprising finding from a review of over 70, 000 water samples conducted by the Legionella Control Association (LCA).

In this In Focus segment we examine the findings from a LCA led review into the prevalence of different Legionella Species detected in 70, 000 water sample results from three laboratories in the UK over a 2-year period. We discuss how ‘Legionella Species Detected’ results are considered in terms of risk and the implications of the LCA findings for industry.

LCA Findings
In January 2022 the Legionella Control Association (LCA) published a news story ‘Post Lockdown Positivity: Emerging Legionella Species Data’ which presented key findings from the analysis of over 70, 000 Legionella positive water sample results collected over a 2- year period. The data was gathered from three different UK based laboratories, each of which used the MALDI-ToF confirmation methodology which reported down to ‘species’ level.

The key findings were;

  • Over 53% of the results were L. anisa,
  • Over 32% of the positives were L. pneumophila (both Sero Group 1 and Sero Group 2-15),
  • Nearly 1% of positives were for L. rubrilucens,
  • Over 6.5% of the results did not confirm a species type,
  • There were over a dozen other species identified in results that accounted for <1% of the data set (1).

The exercise was triggered following a webinar hosted by the LCA in July 2021, in conjunction with HSE, PHE and Local Authorities aimed at raising awareness of increasing legionella positivity rates post lockdown. The data at that time demonstrated that the average positive rate in the UK had increased by around 2% following the lockdowns in response to COVID 19. Consequently, members raised the question as to whether this increase in positivity rates could be attributed to specific Legionella species.

What the findings tell us
The fact that over 53% of the samples confirmed as L. anisa from a dataset of 70,000 positive results, compared to 32% L. pneumophila was an unexpected finding. It had been a long-held industry opinion that L. pneumophila was the more prevalent species of Legionella, and therefore being of greater concern and representing the biggest risk. This approach may have been based on data (Joseph, 2002 2) which found “70% of Legionella infections are caused by L. pneumophila serogroup 1, 20–30% are caused by other serogroups, and 5–10% are caused by non-pneumophila species

The data from the LCA shows that L. anisa is more prominent in water systems than L. pneumophila; so why are we not finding L. anisa is clinical cases as much? The answer to this may be simple. The Urinary Antigen Test (UAT), that is commonly used to confirm Legionnaire’s disease, has a bias towards L. pneumophila; meaning that L. anisa would not be detected using this test. Moreover, a lot of the UAT’s only look for L. pneumophila Sero Group 1, not even the full range of pneumophila. (Shimada 2009 4).

We have a clinical confirmation test that is biased towards L. pneumophila, yet this species of Legionella is only present in around 3 in 10 positive samples, this means that as many as 7 in 10 UAT could be providing negative results when the patient is infected with Legionnaire’s disease. While the Polymerase Chain Reaction (PCR) method does detect these additional species, it is not the routine diagnostic tool used.

Legionella Species and the guidance
With over 50 species and 70 serogroups having been identified, what does the guidance say about how the risk from differing Legionella species should be considered? The Health Technical Memorandum (HTM) 04-01 Part A(5) and Part B (6) along with the Approve Code of Practice for Legionella (ACoP L8 4th Edition) (7), all suggest that we treat each Legionella species detected as an equal. This can be inferred given that the guidance makes no reference to a hierarchy of risk between specific Legionella species. The tendency to treat different species differently has evolved and been adopted by industry over time, possibly on the back of empirical evidence such as Joseph (2002).

Implications of the data
Traditionally, we have seen the response to any Legionella Species other than L. pneumophila be treated as less of a risk to human health; and even then Sero-Group 1 was always felt to be a higher risk than Sero-Groups 2-15. Yet over 25 of the Legionella Species are known to be pathogenic to humans(3). It is important to note that both L. anisa and L. rubrilucens are both confirmed Human Pathogens and therefore can cause Legionnaire’s disease.

The findings of the LCA data review suggests that this commonly held approach is incorrect and could have dangerous consequences. Whilst the extent of the data set reviewed was extensive, the LCA do state that this data is in its infancy and further research needs to take place before any significant changes are considered or undertaken. However, it is clear to see from this data that the prominence of species of Legionella, other than L. pneumophila, should be considered when undertaking a sampling and analytical plan.


  1. 1. https://www.legionellacontrol.org.uk/news/121/
  2. 2. Joseph C (2002a). Surveillance of Legionnaires disease in Europe. In: Marre R et al., eds. Legionella, Washington DC, ASM Press, 311–320.
  3. 3. https://specialpathogenslab.com/legionella-species/
  4. 4. Shimada T (2009) Systematic Review and Meta analysis Urinary Antigen Tests for Legionellosis in CHEST Journal Volume 136, Issue 6 1576 – 1585
  5. 5. Health Technical Memorandum 04-01 Safe water in Healthcare Premises Part A: Design, installation and commissioning (2016) Paragraph 4.22 – 4.24
  6. 6. Health Technical Memorandum 04-01 Safe water in Healthcare Premises Part B: Operational Management(2016) Paragraph 7.45 – 7.49
  7. 7. Approved Code of Practice for Legionella (4th Edition) (2013) Paragraph 58
  8. 8. Health & Safety Guidance 274 Part 2 “The control of legionella bacteria in hot and cold water systems” (2014) Paragraph 2.117

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