21 cases have been reported, of which 8 have been identified with the same sequence type (i.e. same bacteria), all the infected individuals “residing in, working in, or having visited North West or South West London.” The further 13 cases are under investigation to determine if they are also part of the outbreak.
The UKHSA has added “Epidemiological, microbiological, and environmental investigations are ongoing to determine whether there are any common links and to identify any potential common sources. No common exposure locations have been identified through case interviews.”
GOV.UK, through original Health Protection England guidance “Investigation of Legionnaires' disease: cases, clusters and outbreaks” defines:
- A cluster – 2 or more cases with onsets of symptoms that are close in time, within days or months depending on the category of exposure, close in space and/or share an epidemiological link according to the exposure-specific definitions.
- An outbreak - An outbreak is declared when 2 or more cases meet the criteria for a cluster (depending on category of exposure) and are close in time (onset of illness within 28 days from the onset date of the previous case) and have strong epidemiological and/or microbiological evidence of a common source of infection.
NW and SW London are a big areas, so geography is playing a part here, particularly in identifying and eliminating any source or sources. We know from a 2004 outbreak in Pas-de-Calais, Northern France, most of the 86 people who contracted Legionnaires’ disease, of which 21 died, lived within 6 kilometres of the source, however, one individual was 12 kilometres away. Similarly, an outbreak in the Sarpsborg/Fredrikstad region of Norway in 2005 (56 cases and 10 deaths) saw infected individuals as far as 20km apart (with no common places visited).
What is also unusual about the London outbreak is timing, as typically numbers of reported Legionnaires’ cases in the UK are low over the Winter period and peak during the Summer. I have highlighted several times previously where climate is having an effect on Legionella and this could certainly be a factor here too. The year so far has been warm and wet, conditions that could very well facilitate the wider dissemination of viable Legionella organisms in the atmosphere. This coupled with the potential seeding of other risk systems and so the outbreak organism “leapfrogging” from one area to another is a further possibility.
We are seeing globally increasing cases of Legionnaires’ disease and climate I believe is a contributing factor to some of the changes/anomalies we are seeing. What I also believe is that well managed, well maintained water systems pose much less of a risk, wherever they are.