Pandemic, Epidemic, Endemic and Legionella
Pandemic is a word most of us had never used before early 2020. Now barely a day goes by without us using it. That's because we're in one.
Before COVID-19 was a pandemic, it was an epidemic, and before that it was probably endemic. What do these words mean? Do they have any relevance to Legionella control?
What is a Pandemic?
Pandemic comes from two Greek words 'pan' meaning 'all or everything' and demos meaning 'population'. So pandemic means in every population. Global is the quick word for describing it. Historically there haven't been too many disease pandemics recorded.
In the middle ages the 'great plague' (bubonic plague, Yersinia pestis)is estimated to have killed almost a third of the known global population. Poor understanding of disease transmission and little or no effective treatments had devastating results. Limited travel options and poor communication meant the plague ran it's course for around 300 years. Add that to your lockdown woes and worries! These days the infection can be easily treated with antibiotics.
The 1917-18 'Spanish 'flu' (Influenza H1N1) killed over 20 million people; more than 20 times the fatalities of the Great War in a single year. War is hell - but disease is worse! Again poor understanding of disease transmission and little or no treatments added to the grief. If only it was as simple as not spitting? The annual 'flu shot has significantly reduced the burden of disease from 'pandemic flu' that circles the globe each year.
Without going into the myriad of opinions currently available of how or why - we are now in another virus pandemic. This virus is more adaptable than 'flu, there are more of us, and we love to travel! Treatments are better but like the annual 'flu cycle - we're going to have to live with COVID-19.
What is an Epidemic?
The 'epi' in epidemic from the Greek meaning ' around', or 'over'. So epidemic means over or around a population. So epidemic means an outbreak of disease within a defined area or population.
What is an outbreak? A working definition is 'an increase in the expected number of cases of disease in a location or population within a given time period'. That is more cases than usual, in one place at one time. An epidemic is usually declared to be over when no more cases are observed after a full incubation period of the disease has elapsed. This is an important point to remember when looking at Legionnaires' disease outbreaks.
Things like food poisoning (Salmonella) occur as outbreaks. A number of people attend a food premises or eat a food product in the same time period and get sick. It's not hard to see the link between this definition and contact tracing - people, place and time. Many epidemics resolve almost as quickly as they started. Occasionally they continue to spread and the risk of becoming a pandemic increases, as we are experiencing now.
What is endemic?
'En' in Greek means 'inside' or 'within' so endemic means within a population. Some diseases are always with us and 'lurk' in the population. Many of these have a 'baseline' that we can predict. In a 'normal year' we can expect a number of cases of disease will occur as a matter of course. A number of diseases survive in the environment and cause problems for us through accidental exposure. If these diseases are transmissible then it can trigger an outbreak
Bubonic plague survives 'endemically' in animals (rodents) in the environment. If human environments tempt infected animals to move in (eg rats in dirty cities) then the disease can 'jump' to humans and spark an epidemic.
What has this got to do with Legionella?
That's a very good question - as the politicians love to say (why don't they answer them?). The good news is Legionella infections do not cause pandemics. The disease is almost always linked to a single source, so there are no options for the global population to get exposed.
Legionella is certainly a source of epidemics. Most famously cooling towers and spa pools cause outbreaks. Like the food poisoning scenario outbreaks centre on a point source that people are exposed to in a specific time period. Legionnaires' disease (LD) epidemics have been described as 'explosive'. That is they go from nothing to a big problem in a short time period.
The chart above covers a typical 'point source' outbreak and Legionnaires' disease outbreaks globally tend to follow this pattern. Things would look very different if Legionnaires' disease was transmissible between individuals!
This outbreak pattern works for Legionnaires' disease outbreaks from cooling towers, spa pools, etc. The chart above shows the 2000 Melbourne Aquarium outbreak. It's a bit crowded but note the following.
If we count the first day of exposure (red diamond, April 11) as day 1 the first case (pink diamond, April 17) appears at day 6. This is mid way through the usual LD incubation period of 14 days.
By day 5 (April 21) there are 37 cases; an incubation period into the outbreak and it has reached a peak.
Somewhere around day 14 (April 26) the cooling towers responsible are disinfected. From this point the case numbers decline to zero within the next 14 days - a single incubation period.
The outbreak is declared over two weeks after the last case - a further incubation period.
A quick summary of that is the outbreak peaks begins after one incubation period, peaks during the second incubation period and resolves in the 3rd and 4th.
As an aside, note the importance of incubation periods. They are useful for managing all sorts of outbreaks. They can determine length of exposure, isolation periods, lockdowns and identifying when the event is over.
For Legionella infections from building water systems it is a different story ......
Endemic Legionnaire' disease
Sporadic cases of Legionnaires' disease turn up every year. These are single cases often where the source is not identified or where the exposure site is domestic (shower, hot tub). In a normal year (wouldn't we love one of those!) these sporadic cases account for the majority of reported cases. So this represents our background - just under 1 case for every 100,000 of population.
Outbreaks of LD associated with building water systems do not fit the traditional 'epidemic' definition. Explosive outbreaks with multiple cases within a single incubation period have rarely been recorded from potable water systems. Why is this so?
Outlets in a building often function as multiple 'point sources' of Legionella contamination. In the case of a single contaminated shower it is unlikely that it will be used by multiple people in a short time frame. It is even less likely it will be used be multiple susceptible people in a short time frame. On top of that, once the shower has been used it has been flushed by the user, so may not be traceable as a source.
The exception to this is in high risk facilities like cancer treatment, renal transplant and intensive care. Sure enough the data on building water outbreaks tends to be loaded towards disease linked to these sorts of facilities.
So Legionnaires' disease in building water systems tends to look more like endemic disease. Infections in health care settings tend to be sporadic. That is, occasional cases of disease from bacteria lurking in the system at seemingly low levels. In some instances these 'occasional cases' have been linked to multiple infections from a single outlet in a building.
So in health and aged care long slow epidemics occur. Multiple cases over multiple years that are more than an incubation period apart are a published fact. They are also quite hard to track down.
Why do we care?
The endemic nature of Legionella infections in health and aged care tends to hide the extent of the issue. This is because there are seemingly very few cases (until you add them all up) and no outbreaks. Data on the number of cases attributable to health and aged care is very fuzzy but a figure of 30% of all notifications seems to be a reasonable assessment.
I can assure you that there is no calculable number of Legionella in a water outlet that can be equated to a risk of disease. There are simply too many variables and unknowns. I can also assure you that detections of Legionella in building water systems, regardless of the concentrations, can be linked to sporadic (endemic) cases of disease.
Make a well informed assumption that your health care facility is colonised at some level by Legionella. Legionella is endemic to built water systems. Few professionals in the area would dispute this. So - you can assume you are prone to endemic disease - or long slow outbreaks. The only way to manage the potential for cases of disease is by good housekeeping. A water safety plan that includes assessment of your system and an action plan to manage it is your best defence. Your plan may incorporate disinfection - but make the plan first, don't put the cart before the horse!
About Us
All of the areas mentioned above are what we specialize in. Our business delivers industry leading processes, systems, training and support as well as market leading disinfection products that will enable you to manage and reduce risk of waterborne infection. We are an ISO accredited business and hold Systems, Safety and Environmental certification, please feel free to contact us if you would like to talk more.
Things to read
Bentham, R, and Whiley, H. Quantitative Microbial Risk Assessment and Opportunist Waterborne Infections–Are There Too Many Gaps to Fill? 2018, June, Int. J. Environ. Res. Public Health 2018, 15, 1150.
Beauté, J, Plachouras, D, Sandin, S, Giesecke, J and Sparén, P. Healthcare-Associated Legionnaires’ Disease, Europe, 2008−2017, Emerg Infect Dis. 2020 Oct; 26(10): 2309–2318.
Greig et al 2000, An outbreak of Legionnaires’ disease at the Melbourne Aquarium, April 2000: investigation and case–control studies, 2004, MJA ,180 (7).
World Health Organisation 2007. Legionella and the Control of Legionellosis. Chapter 3. Approaches to Risk Management.
