Legionnaires' disease

Legionnaires' disease is an acute respiratory infection caused by a common bacteria that results in a serious case of pneumonia. It first became a well-known disease in 1976 when a serious outbreak occurred among a large number of people attending an American Legion convention. Researchers eventually discovered that the bacteria can be easily found in nature wherever there is warm and moist stagnant water, and that it is transmitted by breathing it in.

A mysterious outbreak

During July 21 to 24, 1976, over 4,000 members of the American Legion met in Philadelphia, Pennsylvania, to attend their fifty-eighth annual convention and to celebrate the nation's two hundredth birthday. When the meetings were over, the attendees and their families returned home, but not all was right. On July 27, only three days after the convention, one of the legionnaires who had been in Philadelphia died from a pneumonia-like illness. On July 30, a physician in Bloomsburg, Pennsylvania, realized that the three patients he was treating for a similar condition had all attended the convention in Philadelphia. That same day, a nurse in the nearby Chambersburg Hospital noted a similar condition in three patients who had gone to the same convention. However, it was not until August 2 that state officials were able to put together the illness with its victims' whereabouts and to realize that there was some undeniable connection between this serious febrile (pronounced FEH-brile) or feverish illness and the legionnaires' convention. By that date, eighteen legionnaires had already died. Federal officials at the Center for Disease Control (CDC) were notified and became immediately involved in what was now a mysterious and spreading outbreak.

Words to Know

Antibody: A protein produced by certain cells of the body as an immune (disease-fighting) response to a specific foreign antigen, or any substance that the body considers foreign, such as a bacterial cell.

Bacteria: Single-celled microorganisms that live in soil, water, plants, and animals that play a key role in the decay of organic matter and the cycling of nutrients. Some are agents of disease.

Pneumonia: Any of several diseases caused by bacteria or viruses in which the lungs become inflamed.

"Philly Killer"

By this time the media realized that it had a major story on its hands, and from then on health officials had to work under the close watch of radio, television, and newspaper reporters. Still, no one, including the CDC, was able to pinpoint the immediate cause of this disease. Since it was now directly connected to the Philadelphia legionnaires' convention, the media referred to it as the "Philly Killer," "Legion Malady," "Legion Fever," "Legion Disease," and finally the name that took hold, "Legionnaires' disease." As this name became used regularly, some members of the American Legion thought it was bad for the organization since it might suggest that they were somehow responsible for the disease. Other members thought the opposite and considered it a sort of tribute or honor to those who had already died. Despite their opinions, the name for the disease stuck, and it is still called that today by non-scientists.

Probably the main reason for the name sticking was that researchers could not identify the organism causing this disease and therefore had nothing to name it. Technically, the CDC simply called it "Respiratory Infection-Pennsylvania." For several months as they studied the disease, this name persisted until an April 1977 CDC report made reference to "Legionnaires' disease." From then on, even after the bacterium that caused the disease was identified and named, the press and even some in the medical community would refer to it by its popular name.

Discovery by CDC

It was not until nearly six months after the first outbreak that the cause of this disease was identified positively by the CDC. On January 18, 1977, the CDC announced that their investigators had isolated the cause of Legionnaires' disease. Using a piece of lung tissue taken from one of its dead victims, researchers finally were able to demonstrate that a bacterium they would name Legionella pneumophila (pronounced leejuh-NELL-uh new-mo-FEE-lee-uh) was the culprit. By then however, a total of 221 people had contracted the disease and 34 of them had died. Isolating the actual bacterium enabled the CDC not only to learn how it had spread and how to fight it, but it showed researchers that this was a complicated organism responsible for many past unexplained outbreaks.

Course of disease

The Legionella bacterium was an unusual and complicated germ because it was found to cause two diseases, one very serious known as Legionnaires' disease and another milder form called Pontiac Fever. Although Pontiac Fever is caught by 95 percent of the people exposed to it, most of them simply experience flulike symptoms that pass in two to five days. Legionnaires' disease is much harder to catch, with only two to five percent of those exposed actually contracting it. But once contracted, usually by at-risk individuals who are more susceptible, it will not go away without medication and it kills between five and fifteen percent of the people it infects.

Source of infection

CDC researchers named the species of bacteria Legionella pneumophila because the second word means "lung-loving" in Latin. This bacteria is actually very common in the natural world and only causes trouble when it gets into people's respiratory systems. It finds our lungs to be an especially comfortable place because they have conditions the bacteria prefer—they are warm and moist. Legionella are found to exist naturally in stagnant water, and in the Philadelphia case, the CDC traced the outbreak source to the hotel's air conditioning system whose condenser was vented very close to its air intake system. This meant that the large air conditioning system, which had not been cleaned for some time, had the common Legionella germ growing in it, which people then inhaled after the organism had gotten into the air intake pipes.

Attacks the susceptible

The fact that this is the only way that people can contract the disease was discovered by the CDC. Unlike many diseases, you cannot "catch" this disease from another person. The Legionella germ must penetrate deep into the lungs. Further, the cilia (pronounced SIL-lee-uh) in most people's lungs usually capture and expel the bacteria. However, for those who are somehow at risk—like smokers, alcoholics, older people with chronic lung problems, or someone with a weak immune system—these short hairlike projections called cilia do not work the way they should. The Legionella can then get in and infect a person. Another unusual

Legionalla pneomophila. (Reproduced by permission of Custom Medical Stock Photo, Inc.)
Legionalla pneomophila. (Reproduced by permission of
Custom Medical Stock Photo, Inc.

thing about this disease is that the infecting bacteria invade the body's white blood cells and multiply inside them. These are the very cells that the body uses to fight such invaders. Normally, these attack white cells called phagocytes (pronounced FA-go-sites) surround and engulf or swallow up a bacterial invader. Although the phagocytes do manage to engulf the Legionella , they are unable to digest it and soon the attacker becomes the attacked. In fact, the Legionella becomes a parasite and actually begins multiplying inside the phagocyte, who now becomes its host. After doubling its numbers every two hours, the Legionella eventually overloads its host, which bursts and spreads even more invading cells throughout the body. The CDC discovered that the antibiotic erythromycin (pronounced eh-RI-throw-MY-sin) is effective. However, it works not by killing the bacteria but rather by stopping it from multiplying in the cells, therefore giving the body a good chance to combat it on its own.

A new "old" disease

Once the real cause of the disease was known and well understood, researchers realized that this was not some new bacterium that had suddenly emerged but one that had been around all the time. It was simply one that science had never identified. With hindsight, they found that an estimated 8,000 to 18,000 people get some form of Legionnaires' disease every year in the United States. Further, they found that the disease occurs worldwide. For example, it is so common in Australia that roughly one-third of the population has antibodies for it in their blood (meaning that they have been exposed to it at some point in their lives and their bodies have developed a way to combat it).

Since Legionella has been found in cooling towers and evaporative condensers of large air conditioning systems, as well as in spas and showers, all of which have temperature conditions that allow it to thrive, it is important to keep these systems clean and well-maintained. Legionella is easily killed by heating water to high temperatures. It dies off quickly if it dries out and it is also killed by simple exposure to the ultraviolet radiation of the Sun. There is no evidence that people can be infected by air conditioners in their cars or by window units in their homes.

Legionnaires' disease is a major bacterial disease that had existed without being detected until 1976. What made it suddenly known to science was the fact that so many people in the same place got sick all at once, attracting a lot of media attention and suggesting that something had infected them. What they had in common was the fact that they all had spent some time in the same convention hall. Remarkably, the CDC eventually found that these people were the victims of a fairly common, natural bacteria that has been invading humans and other hosts for centuries but of which no one had any knowledge.

[ See also Disease ]

User Contributions:

Excellent! I wasn't sure if Legion Fever and Legionnaire's Disease were the same thing. Very informative. Thanks.
Biology student
Thank you so much! I'm doing a research paper on Legionnaires' and this was a great help! And sorry, but I'd rather not share my e-mail.
Stayed at Inn at Roswell NM from eve of 5/16/10 through morn of 5/19/10 [3 nights]; felt a bit off on 5/19 drive [470 mi] back home, with worsening respiratory problems each day, including much coughing, headache, occasional difficulty w/breathing. Your article mentioned that there is no evidence that people can be infected by air conditioners in their cars or by window units in their homes.
Am 80+, in fairly good health. Can one be infected by an under-window air conditioner of the size in many motel rooms -- i.e., the width of double window -- that blows directly across both beds in the room?
this helped a lot on my scince report. thanks. couldnt find anything anywhere but then i found this and it helped more than anything so far
Thanks so much it really helped me on my science project. I couldn't find any information anywhere except for here! Thanks so much!

I am wondering who wrote this article, it is very informative and need to reference it in my paper... PLEASE HELP ME!!!
I wouldn't reccomend legionairres disease. It changes ones life drastically. After 8 months I am still not back to normal. Probably will never be. Well, still alive, the bright side, adapt. Jan

Comment about this article, ask questions, or add new information about this topic: