Legionella pneumophila is a Gram-negative bacilli from the family Legionellaceae that has only one genus, Legionella. There are about thirty known species existing around the world, and at least nineteen of them can cause pneumonia in humans.
The disease, now called Legionnaires disease, or legionellosis, is a pneumonia-like illness that was virtually unknown until an incident in Philadelphia in 1976 when about 5,000 Legion members were attending a convention. About two weeks later, many of those who attended that convention came down with flu-like symptoms. Over the next week, twelve died. During the ensuing weeks, 170 were hospitalized with pneumonia, with twenty-nine dying as a result. An extensive investigation revealed the culprit to be a bacterium isolated from the air-conditioning ducts of the hotel that hosted the event.
This was not a new disease, however. After months of detective work, researchers found the same organism had caused outbreaks in a Washington DC hospital in 1965 and, ironically, in a health department office in Pontiac, Michigan, in 1968. It was also found in a frozen guinea pig that had been injected with the blood of a patient who had died in 1947 from an unknown cause. Today, there are still about 1,300 cases reported annually, with a mortality rate of 5-30%.
The organism thrives in wet, dark, dank environments similar to puddle scum. L. pneumophila is most frequently found in air-conditioning units where water is allowed to stand during seasons when not in use. It has also been found in shower heads and water taps, and can survive in room temp tap water for more than a year. Other areas where it has been isolated are as follows: streams, ponds, mud, hot tubs, humidifiers, misting devices, decorative fountains, etc. Chlorination does not remove it if concentrations are high. Person to person contact does not seem to be involved, but it is only contracted from breathing in spores.
The progression of the disease resembles lobar pneumonia and usually begins two to ten days after exposure. Some early symptoms include diarrhea, weakness, headache, muscle aches, malaise, anorexia, and a dry cough. Fever increases, along with prostration and difficulty breathing. Some cases can also show liver and kidney involvment, but the primary location is the lungs. The duration of the disease lasts five to sixteen days, with the vast majority of the cases involving older people and those with suppressed immune systems where age is not a factor. The immunosuppressed individuals can be those with cancer or liver disease, those who smoke or drink excessively, and those who have had transplants or suffer from an autoimmune disease. Erythromycin is the antibiotic of choice but rifampin is also used.