MFF0087: Legionnaires' disease

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Hello and welcome to The Medical Fun Facts Podcast.

It's Monday 9 October 2017 and you're listening to episode 87.

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We're up to the letter L and tonight I've chosen to talk about Legionnaires' disease.

Legionnaires' disease is a type of pneumonia caused by various serogroups in the species Legionella pneumophila. There are other species in the genus Legionella that can also cause a lung infection, these include Legionella longbeachæ and L. micdadei.

Legionella pneumophila exists in freshwater and can contaminate cooling towers of air conditioning plants as well as hot water tanks. While the bacterium is not spread person-to-person, it can travel distances in aerosolised form as water mist and be inhaled which may then progress to infection. Not everyone exposed will become infected and go on to develop the disease. People at most risk are older immunologically compromised sapiens especially those who smoke and have chronic lung disease.

The incubation period can vary between 10 and 20 days. The proportion of people exposed who go on the develop disease varies depending on the study you read. It can be from <1% to up to 5% in the general population and much higher in a hospitalised population.

Some of the more unusual symptoms that may be seen in patients with Legionnaires' disease include blood in the sputum, a slow heart rate and a low sodium concentration in the blood.

The bacterium has a wide tolerance of warm temperatures and can survive in freshwater between 25 and 45 °C. Fortunately, temperatures above 60 °C inhibit the bacterium's growth. Legionella pneumophila lives symbiotically within certain amœbæ and biofilms made up of these amœbæ protect the survival of these pathogenic bacteria.

Legionella pneumophila enters human lungs either by aspiration of contaminated water or by inhalation of contaminated aerosols of water or soil. In the lung, the bacteria are consumed by macrophages which are like the symbiotic free-living amœbæ but unlike the symbiotic amœbæ the bacteria thrive in, the bacteria multiply and cause the death of macrophages. Once dead, the macrophage gives up the bacteria which can go on to infect more macrophages. This has an amplification effect resulting in many more bacteria and many dead macrophages which creates a significant inflammatory reaction as cellular debris has to be cleared by the host cellular immune system. Legionella pneumophila also contains other virulence factors including proteolytic enzymes and toxins. The main host response is cellular and while antibodies are produced the antibodies do not appear to be protective. That is, the antibodies do not provide protective immunity.

Legionnaires' disease was first described when an outbreak of severe pneumonia was described in 1976 in a meeting of the American Legion at a convention in Philadelphia. While, Legionnaires' disease is usually due to Legionella pneumophila, there are other causes of legionellosis. For example, in Australia from time to time there have been outbreaks of serious pneumonia in older Australians who have been working with potting mix in their gardens. The cause has often been Legionella longbeachæ. We've also seen outbreaks of Legionnaires' disease associated with hospitals and a large metropolitan public aquarium.

Diagnosis isn't always straightforward. Bacteria in the genus Legionella are fastidious and easily killed so special culture media like buffered charcoal yeast extract (BCYE) agar, is required and growth can be slow even when using a special atmosphere that must be generated for the bacterium to grow well enough for characterisation. In this day and age, respiratory specimens can be tested by PCR for a relatively rapid, sensitive and specific diagnosis. A popular but less robust test involves looking for Legionella pneumophila serogroup 1 antigens in urine. This test can be undertaken quickly and easily, but it's expensive and not all legionellosis is caused by that specific serogroup. It's commonly requested from critical care areas, usually late on a Friday afternoon when medical laboratory scientists are wanting to go home or go out for a drink. I remember when I was a boy, the only rapid assay was an immunofluorescence test that took hours to set up and it had to be read using a fluorescent microscope in a dark room. It was a test that rarely gave clear endpoints and on more than a few occasions I'd be required as the registrar to stay back late into a Friday evening with the medical laboratory scientist to confirm the result and report it through to the intensive care team. It wasn't uncommon to develop motion sickness in the claustrophobic environment of the dark room cooped up with another person. Good times!

Legionellosis is prevented with good water control mechanisms. For example, water should be kept below or above the 20–50 °C range in which Legionella bacteria thrive. Water stagnation in pipes should be avoided, and if necessary, pipes should be flushed and disinfected before allowing the flow of potable water. Aerosols and biofilm formation should also be inhibited through good design and regular disinfection respectively.

Because of the severity of pneumonia in patients in whom Legionnaires' disease is not suspected early enough, we often find patients in intensive care or high dependency units. Effective antimicrobial agents include many macrolides, tetracyclines and fluoroquinolones.

Questions for listeners

Had you heard of Legionnaires' disease before this podcast?

As you get older are you more careful with potting mix?

Why do bad things always happen on a Friday afternoon?

Please leave your answers in the comments section of the show notes or on the Facebook page or on YouTube.

That's episode 87 in the can.

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I'll catch you next week for episode 88. Something beginning with the letter M. Send me suggestions.

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Thank you, and good night.