From how often one thinks about and tests for Legionella, one might have expected it to appear in the exam papers sooner. Certainly other rarer (tetanus) and more exotic or tropical (leptospirosis) have been the subject of SAQs, and so it was only a matter of time before this classical ICU organism had made its way into the CICM exams as the subject of Question 13 from the second paper of 2023. The college wanted to know about both L.pneumphila and L.longbeachae, which seemed like a lot (two organisms in one SAQ?).
These rods are
Legionella are legion. Jomehzadeh et al (2019) mention that there are about thirty species that have been cultured from patients, let alone in the environment, and so it suddenly no longer seems like such a burden to have to focus on two common organisms (L.pneumophila and L.longbeachae). Water is their main reservoir, and only L.longbeachae is an exception, as it likes to live in organic potting mix where the heat of decomposition sustains a temperature and moisture content favourable for growth. So, from the perspective of Legionella, your lungs are basically compost, or some kind of welcoming warm pond. Under normal circumstances, these rods cannot reproduce in the extracellular environment, and would usually make their way into a protozoan where they live intracellularly as parasites. The preferred host is probably some kind of freshwater amoeba like a Hartmanella or Naegleria , as that is where free-range Legionella tend to hang out, though if pushed they will also colonise some kind of random ciliate. The relationship with the protozoan is strictly parasitic and the bacterial cells end up in a phagosome which they disable and subvert for their own personal needs, continuing to reproduce until they ultimately kill the host. And where the amoeba content of the normal human lung is thankfully very small, Legionella gladly invade and parasitise human macrophages, as the next most amoeba-like thing.
Is there much difference between Legionella pneumophila, the beast that got its name from the convention of the American Legion in Philadelphia (1976), and Legionella longbeachae, discovered a year later from with pneumonia in Long Beach, California, USA?
The way this is likely to play out in CICM exams is via the vehicle of the "outline your assessment" question, which typically requires a "history, examination, investigations" sort of structure. The "history" would surely contain some of the risk factors and the modes of transmission, and the investigations would yield some characteristic biochemistry findings like hyponatremia, but the examination would be fairly vanilla, as nothing about the physical features shouts "Legionella" at the examiner. The money is in the history and bloods.
Risk factors (from Jomehzadeh et al, as well as the CDC website) include:
That's right, the CDC website specifically implicates hot tubs. In the absence of hot tubs, Legionella will reluctantly continue to party in other bodies of water, including manmade ones, such as showerheads, decorative water features, aquariums, cooling systems, and water heaters. The main reason the healthcare facilities or "travel with overnight stay" are implicated is because these facilities typically have that magical combination of lax maintenance and large complex water distribution/cooling/heating systems. Those water systems also help these bacteria gain access to your lungs by facilitating the generation of aerosols, which is the main mode of transmission. Inhalation of aerosolised water is the main mechanism, but you can also aspirate the Legionella-rich water directly into your lungs if you happen to be immersed in it, and there is at least one case of person-to-person transmission (involving a lot of coughing and a small poorly ventilated room).
This watery route is mostly a reference to the transmission of L.pneumophila. The other major species in Australia, L.longbeachae, is mostly acquired from compost, potting mix and manure, where there are plenty of amoebae to parasitise. Those soil amoebae often also have the delightful ability to form protective cysts, which therefore also protect the Legionella, and allow both organisms to survive dessication and storage. Thus, as most normal people do not aerosolise their compost, they can still inhale these bacteria while gardening, as dried dusty particles of potting mix are an essential part of the hobby. Thus,
It is apparently a rather reluctant human pathogen and the CDC website mentions that only about 5% of people who are exposed to a source will become infected.
There are two main syndromes which can present with distinct patterns of illness:
Legionellosis proper takes about 14 days to incubate, whereas Pontiac fever only takes about 60 hours to get started.
It is generally said that one cannot make distinction between Legionella and any other sort of pneumonia, but a few general points can be made:
Which brings us to:
Agulló-Ortuño et al (2006) took about 170 patients with Legionella infection and compared them to non-Legionella pneumonias. The biochemical differences were:
But these are generic and one would not commit to any diagnosis on the basis of a suspicious history and vaguely supportive biochemistry. Proper laboratory diagnosis of Legionella consists of:
Question 13 from the second paper of 2023 called for "four methods for diagnosis" which was a simple list-em-up for only two marks, meaning that all the extra detail was probably not necessary. It is not inconceivable that at some point in the future, somebody may ask about the advantages and disadvantages of these tests, and then they would be able to refer to this excellent table in the CDC page. For most people it is enough to remember that the urinary antigen does not detect fully 50% of the total Legionella burden in the Australian population.
So, what might you treat this with? Normal thinking would trend towards something that treats intracellular pathogens, like a macrolide or a fluoroquinolone. That is indeed what eTG recommends for the Australian Legionella sufferer:
And these recommendations mostly just upgrade from oral to IV for disease of increased severity, with a 7-10 day duration of therapy. The Sanford Guide and UpToDate do something similar, though UpToDate authors recommend not to rely on tetracyclines for L.longbeachae as it is apparently resistant, and Sanford people stand out with the range of alternatives and helpful tidbits (eg. where they point out that there is no additive effect from using dual therapy).
Well. This is not a transmissible disease that requires isolation, as you literally need to be breathing the same uncirculated air as the patient for a number of whole days before you get infected. However, as the usual sources of infection are highly visited public places, notification of the authorities becomes an essential part of the management process. Thus: