To best understand the ‘summer risk’ in relation to the incidence of Legionnaires’ disease, it’s useful to refer to the Legionella surveillance reports issued by Public Health England (PHE), which succinctly summarise the monthly position (total reported and total confirmed cases of Legionnaires’ disease) whilst offering year-on-year comparison data.
If we’re to analyse the data reported by PHE then a ‘bell curve’ or ‘inverted U’ can be observed during the months of July and November representing an increased number of reported and confirmed cases during this time period – see Figure 2 & 3 below.
Whilst Legionella bacteria live preferentially in the environment and therefore are ubiquitous within water systems (in planktonic or free-floating cell form), these bacteria may cause a problem when provided with ‘favourable growth conditions’. Moreover, when water temperatures are between 20-45˚C and when water is slow moving or stagnant and nutrient-rich, then these conditions are often precursors for biofilm formation. The architecture of ‘mature’ biofilm is often dense and can provide good protection to microorganisms which reside within it.
Legionella in planktonic or free-floating form are often adequately controlled with temperature (ensuring that hot water remains hot and cold water remains cold – in accordance with HSG 274 Part 2 recommendations) but when adequate temperature control has not been achieved – leading to water temperature falling within the aforementioned ‘risk range’ (20-45˚C) and subsequent biofilm formation, then Legionella may gain greater protection from mainstay control measures such as temperature, therefore increasing the risk of waterborne infection and subsequent disease from non-compliant water systems/estates.
Turning our attention back to the ‘summer months’ of the year; compliance to water temperatures detailed within the guidance notes can sometimes be difficult – considering the impact of higher ‘ambient temperatures’ on cold water systems, which often increases the temperature of cold water supplied to premises. Whilst this may in-part help to explain the increase in Legionnaires’ disease cases throughout the warmest part of the year, we must also consider the impact of ‘holiday season’ and that a significant proportion of reported and confirmed cases of Legionnaires’ disease are reported under the ‘travel abroad’ demographic as well as ‘community acquired’. Nosocomial or hospital-acquired Legionnaires’ disease is also noteworthy as although cases from this demographic represent an overall minority, the mortality rates associated with this group are approximately three times higher compared to cases reported from non-healthcare groups such as the community or from travel (UK and abroad).
In closing, this data supports the theory that Legionnaires’ disease may affect the well and unwell alike. Be it the fact individuals travel overseas to somewhere sunny and warm and stay in hotels, villas, apartments, equally those who prefer to holiday in the UK during the summer time. Not to forget that particular concern is in high-risk populations such as those within healthcare environments, whereby predisposing factors (immunocompromise) may lead to a poorer prognosis.
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Editors Note: The information provided in this blog is correct at date of original publication - July 2019.