In July 1976, a convention of members of the American Legion -- a veterans' group -- was meeting in Philadelphia at the Belleville Stratford Hotel in honor of America's bicentennial. Soon, 221 attendees would be sickened and 34 dead of an illness it was believed no one had ever seen before. Swine flu was suspected, as were toxic chemicals or terrorism of some sort. None of these proved to be the cause.
The CDC sprang into action and by January of 1977 the culprit was identified: Legionella pneumophila. The bacteria had been living in the warm water of the hotel's air conditioning cooling tower, whence they were aerosolized and spread through the building via air ducts.
Just yesterday, these bacteria made headlines again when they were discovered to have infected at least three and killed one last year at the Luxor Hotel and Casino in Las Vegas, where the bacteria were dwelling in "water samples" -- most likely the plumbing or A/C system. Although this sounds scary, you have to put things in perspective: Legionnaire's Disease, or legionellosis, already sends up to 18,000 people a year to the hospital in the United States, and doctors may overlook the diagnosis in others. But most of these infections do not take place at high-profile pyramidal Las Vegas casinos.
L. pneumophila, a gammaproteobacterium like E. coli or Salmonella, is actually quite an interesting organism in and of itself. It is related to no other respiratory pathogen, and in fact, its usual host is not a bird or a pig or a human. It's an amoeba. Legionella live within and, in fact, parasitize free-living amoebae in nature*. We are accidental hosts.
Their preferred home in amoebas may explain why they like invading our macrophages so much. Macrophages are amoeba-like immune cells that wander through your body at will -- often slithering between tissue cells -- looking for invading microbes to engulf and digest, just as amoebas feed. Legionella bacteria get taken up by our macrophages and then parasitize them -- and in turn us -- just as they do wild amoebas.
If the infection progresses (which in many healthy people it does not), the consequences for a human shake out in one of two delightfully-named ways. The first is Pontiac Fever (I actually have this, but it's because I drive a suh-weet Pontiac), a flu-like syndrome that usually resolves on its own. It was named for an outbreak in Pontiac, Michigan, that pre-dated that the Philadelphia outbreak and was identified ex post facto. The second option is Legionnaire's Disease, which also begins with flu-like symptoms before progressing to pneumonia accompanied by sky-high fevers of up to about 107F. This option kills somewhere between 5 and 30% of the people who develop the symptoms. People sickened by the bacteria are typically middle-aged or older, and often have weakened immune systems.
In addition to water towers used in industrial air-cooling systems, the bacteria-parasitized amoebae can also hang out in shower heads, medical respiratory devices, spas, hot tubs, large central air conditioning systems, fountains, swamp coolers, ice machines(!), misting equipment, domestic plumbing, swimming pools, and humidifiers (another reason you should use only boiled or distilled water in them). The bacteria (and of course, their amoeba hosts) also thrive in freshwater ponds and creeks.
Legionella get into the air after an infected water source is disturbed mechanically. Any fine droplets produced may evaporate quickly, leaving the bacteria suspended and easily inhaled. The bacteria thrive between 77 and 112 F, so one simple strategy for keeping water free of the disease is to maintain a temperature hotter or colder than that.
It used to be thought the Legionella bacteria couldn't travel far by air. But the investigation into an epidemic in Pas-de-Calais, northern France in 2003-2004 traced 86 confirmed cases and 18 resulting deaths to a cooling tower in a petrochemical plant, and found that some of the victims lived as far as 7 km from it. Once airborne, those bacteria can really move -- or at least can survive a long time as the wind moves them. Perhaps it is a natural dispersal strategy for reaching new amoeba hosts.
*In fact, the first amoeba-parasitizing giant viruses were found by scientists investigating a local pneumonia outbreak and looking for Legionella in amoebas found in an area water tower. They found large particles inside amoebas in the cooling tower they assumed were bacteria. It was only when the "bacteria" failed to respond to bacteria-specific molecules (PCR primers) that the scientists realized they had a new (and entirely harmless to humans) super-giant virus on their hands.