Legionella species? Do we care?

Australian Legionella testing laboratories report detection of Legionella pneumophila Serogroup 1 (Lp1), L.pneumophila Serogroup 2-14, and Legionella species (L. spp) in compliance with Australian Standard 3896. Frequently it is assumed that the most important result is Lp1 as it is the organism of prime concern. Some sources suggest that Lp2-14 or L.spp are not a direct health risk – but an indicator of conditions suitable for growth of Lp1. Is this the case? Maybe not!



Cooling Towers

An interesting fact is that of all Legionella species Lp1 accounts almost exclusively for all cooling tower associated outbreaks of disease. This is so much the case that a community outbreak of disease that is not Lp1 suggests that a cooling tower is not the source. Other serogroups and species are commonly found in cooling water systems – but for some reason very rarely cause disease from these installations.


Also of interest is that within the sub-strains of Lp1 the Pontiac strain is the most common cause of disease. Some sub-strains of Lp1 do not appear to have the ability to cause disease at all.

Spa Pools

Once we look at other water systems the position changes. Disease caused by use of spa pools includes Lp 2-14 and other L. spp. Admittedly Lp 1 predominates still, but there is a significant range of other species and strains that have been implicated in disease (see table below). This is probably a consequence of the nature of exposure. A person sat in a contaminated spa has a much greater likelihood of inhaling a large dose of bacteria compared to a person a hundred meters from a cooling tower.


Health and Aged Care

Once we move into health and aged care premises the balance shifts even further in favour of other Legionella species causing disease.  Lp1 still dominates but at least another 15 species of Legionella other than L.pneumophila caused serious disease in these facilities. The only reported causes of disease linked to building water systems are Lp2-14 group serogroups 2, 4, 6, and 12. This is probably a function of the exposed population. By their nature health and aged care facilities have high concentrations of people who are unwell and susceptible to infection. This gives greater opportunity for the less infectious Legionella species to cause disease.


Extracted from WHO Legionella and Prevention of Legionellosis (2007)

So when a test report is returned with a positive result for L. sp. Do we care? Yes! firstly because of the argument that conditions are ripe for Legionella pneumophila to move in; and secondly because depending on the exposure of a susceptible population it may indicate a real risk of disease. Taking into account the route of exposure and nature of your susceptible population is an important part of developing water safety and risk management plan. This will assist in interpreting test results and establishing acceptable tolerances and control measures.

You can read more about the new national guidelines for preparing risk management plans at https://www.hindmarshplumbing.com.au/riskassessment/

Further Reading

World Health Organisation (2007) Legionella and Prevention of Legionellosis

Lealth, aged care, risk assessment, hospital