But what is Legionnaire’s Disease?
Legionnaire’s disease is an acute bacterial infection of the respiratory tract caused by the bacterium Legionella pneumophila. It was discovered in 1976 when 234 delegates of the American Legion Convention at the Bellevue-Stratford Hotel in Philadelphia developed pneumonia. It was a public health nightmare; no one knew the cause of this mysterious outbreak of severe pneumonia as none of the usual suspects were to blame! Sadly 34 of the veteran soldiers died. Scientists took lung tissue from the veterans who had died and inoculated guinea pigs and eventually, after much investigation, a new bacterium was discovered as the cause. It was called Legionella pneumophila after the American Legion veterans who had first been affected by it.
L. pneumophila is a relatively uncommon cause of severe community acquired pneumonia (CAP) but it can also cause a milder “flu-like” upper respiratory tract infection called Pontiac fever. Pontiac fever is a self-limiting problem and doesn’t require any specific antibiotic treatment.
How is Legionnaire’s disease diagnosed?
Legionnaire’s disease can be difficult to diagnose because the causative bacterium is difficult to grow in the laboratory. Bacteria that are very difficult to grow in the microbiology laboratory are often termed non-culturable. L. pneumophila is often put in the group, however strictly speaking it is actually a Gram-negative bacillus. It is difficult to grow because it lives inside other cells and is hard to get it out of these cells in order to get it to grow. Even once extracted from these cells it is difficult to grow, requiring up to 10 days incubation on special agar called buffered-charcoal yeast extract (BCYE).
Because it is so difficult and time consuming to grow other simple and cheap methods have been developed to aid in diagnosing infection with L. pneumophila, the most commonly used of which is the urine antigen test. This is a bit like a pregnancy test which takes 15 minutes to diagnose Legionellosis from a sample of the patient’s urine. It is a great test! However there are a few potential drawbacks to the test:
- It only detects L. pneumophila serogroup 1– not a major problem as serogroups 2-14 only very rarely cause human disease
- It can cross react with other bacteria – it is possible to get a positive test with acute Campylobacter infection, patients who test positive should have gastroenteritis ruled out as a differential diagnosis
- It is relatively expensive – of the order of about £20 which is expensive for a usual microbiology test
Any positive L. pneumophila antigen test should be confirmed by a molecular method. This has the advantage of not only confirming that this is definitely legionellosis and not a cross reaction from another bacteria, but also provides a genetic profile to the bacteria which can be used to track down where the patient acquired the L. pneumophila as the source potentially poses a public health risk.
How is Legionnaire’s Disease treated?
The treatment of choice for Legionnaire’s Disease is a fluoroquinolone antibiotic such as Levofloxacin. Macrolides e.g. Clarithromycin are also active but the mortality from Legionnaire’s Disease is lower with fluoroquinolones (0.5% versus 10%). Treatment should be continued for 3 weeks to ensure all of the intracellular bacteria have been eliminated.
The travel history is important because exposure to the bacterium is more common in buildings where the water system is not flushed regularly, or where the cold water, due to heat gain from the environment, is able to warm up above 20oC. This is commonly the case in hot summer destinations, where apartments or hotels have been vacant during the winter, often leaving the water system to stagnate and grow bacteria. Then the season starts and holiday-makers come along, turn on the shower and flush all the bacteria out. A possible peril of the cheap pre-season holiday deal! Practical tip: run the empty shower on hot for 5 minutes before using it if you suspect it might not have been used for a while.
What are the public health implications of Legionnaire’s Disease?
As of 2010 legionellosis is a notifiable disease in the UK, and Public Health England will investigate all cases to try and find out where the source was. It is important to track down the source so that further cases of infection can be prevented. There are strict laws that apply to the control of L. pneumophila in the water systems of both public and private buildings. Ultimately manslaughter charges can be brought against those responsible if they fail to protect people from L. pneumophila and the person dies. One of the worst outbreaks of legionellosis in the UK occurred in Barrow in 2002 see the report here.
The control of L. pneumophila in water is actually not technically difficult, but in practice it can be little complicated requiring Microbiologists to understand plumbing as well as medicine:
- Keep hot water hot (>50oC)
- Keep cold water cold (< 20oC)
- Don’t store water for long periods, flush the system regularly so that the bacterium does not have time to grow
- Keep pipes clear of lime scale and organic material which would provide a nice protected place for the bacterium to grow
- Make sure you know where all of those pipes go! Ensure there are no dead legs or blind loops that can contain stagnant water
In the UK the Health and Safety Executive is the best source for information on the control of L. pneumophila http://www.hse.gov.uk/legionnaires/