How severe is Legionnaires disease (legionellosis) within the general population?

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Fortunately the disease is very selective in attack and infects only 2 to 5 % of those appropriately exposed to the bacteria. However, Legionella bacteria are believed to be among the top three causes of sporadic, community-acquired pneumonias. The American Society of Microbiology has reported that 15 to 30 % of patients admitted to intensive care units with pneumonia have legionellosis. The CDC has estimated that the disease infects 10,000 to 15,000 persons annually in the US.

But OSHA estimates that over 25,000 cases of the illness occur each year, causing more than 4,000 deaths. The infection rate may be even higher because the disease is difficult to distinguish from other forms of pneumonia. Therefore, some medical professionals believe the infection rate may be as high as 100,000 per year. Average mortality rate for infected individuals is generally in the range of 15 to 20 percent.

Legionnaires' disease has an incubation period of two to ten days. Severity ranges from a mild cough and low fever to rapidly progressive pneumonia and coma. Early symptoms include malaise, muscle aches, and slight headache. Later symptoms include high fever (up to 105°F), a dry cough, and shortness of breath. Gastrointestinal symptoms including vomiting, diarrhea, nausea, and abdominal pain are common. The disease is treated with erythromycin or a combination of erythromycin and rifampin.

Another fever related to Legionnaires' is Pontiac fever, which is a non-pneumonia, flu-like disease associated with, and likely caused by, the Legionella bacterium. This disease has an "attack rate" of 90% or higher among those exposed, and a short incubation period, 1-3 days. Complete recovery usually occurs in 2-5 days without medical intervention. The factors that cause the same organism to produce two illnesses with major differences in attack rate and severity are not known.

The likelihood of contracting Legionnaires' disease is related to the level of contamination in the water source, the susceptibility of the person exposed, and the intensity of exposure to the contaminated water. Disease transmission usually occurs via inhalation of an aerosol of water contaminated with the organism. Aspiration of contaminated water into the lungs may also cause the disease. In the Philadelphia Legionnaires' disease outbreak, the hotel's cooling tower was identified as the likely source of the disease, although domestic water sources were not evaluated.

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The disease has been associated with domestic hot-water systems in a number of outbreaks. In many instances it has been difficult to identify a likely source for aerosolization of the suspected water source. Although transmission of the disease other than through direct inhalation of aerosols may occur, the mechanisms are not clearly understood. The organism requires water, and the disease cannot occur in the absence of a contaminated water source. There is no evidence that the disease can be transmitted from one person to another.

What are the conditions in water systems that promote growth? L. pneumophila bacteria are widely distributed in water systems. They tend to grow in biofilms or slime on the surfaces of lakes, rivers and streams, and they are not eradicated by the chlorination used to purify domestic water systems. Low and even nondetectable levels of the organism can colonize a water source and grow to high concentrations under the right conditions.

Other conditions that promote growth of the organism may include heat, sediment, scale, and supporting (commensal) microflora in water. Common water organisms including algae, amoebae, and other bacteria appear to amplify Legionella growth by providing nutrients or harboring the organism. Because of its ability to remain viable in domestic water systems, it is capable of rapid multiplication under the proper conditions.

Water conditions that tend to promote the growth of Legionella include:

  • stagnation;
  • temperatures between 20?and 50°C (68?- 122°F) (The optimal growth range is 35?- 46°C [95?- 115°F]);
  • pH between 5.0 and 8.5;
  • sediment that tends to promote growth of commensal microflora; and
  • micro-organisms including algae, flavobacteria, and Pseudomonas, which supply essential nutrients for growth of Legionella or harbor the organism (amoebae, protozoa). The common sources of contaminated water are likely water sources that frequently provide optimal conditions for growth of the organisms include:
  • cooling towers, evaporative condensers, and fluid coolers that use evaporation to reject heat. These include many industrial processes that use water to remove excess heat;
  • domestic hot-water systems with water heaters that operate below 60°C (140°F) and deliver water to taps below 50°C (122°F);
  • humidifiers and decorative fountains that create a water spray and use water at temperatures favorable to growth;
  • spas and whirlpools;
  • dental water lines, which are frequently maintained at temperature above 20°C (68°F) and sometimes as warm as 37°C (98.6°F) for patient comfort; and
  • other sources including stagnant water in fire sprinkler systems and warm water for eye washes and safety showers.

It is important to remember that any water system can be a source of disease if the water in it is subjected to conditions that promote growth of the organism. Remember, however, that the primary sources of exposure to contaminated water are heat rejection systems (cooling towers, fluid coolers, etc.) and domestic hot-water systems. Waters such as these must be treated with caution when exposure is unavoidable.