Appendix A: Legionnaires' disease overview

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The term “Legionnaires' disease” was coined in 1976 after a respiratory disease affected many delegates attending a convention in Philadelphia held by the American Legion of Pennsylvania. There were 221 recorded cases of a strange illness that led, ultimately, to 34 documented fatalities. Eventually, the bacteria that was responsible for the disease was isolated, and was later named as Legionella pneumophila, or L. pneumophila. The source of the bacteria was traced to the building water systems. Initially, the cooling towers were implicated, but further investigations revealed that deficiencies in the potable water system were the most probable source.

In 2012, a significant outbreak of Legionnaires' diseaseoccurred in Québec City, resulting in 13 deaths and 170 documented cases of the disease. The source of the infection was traced to a cooling tower in a privately owned building in Lowertown, Québec City.

Two distinct illnesses, Legionnaires' disease and Pontiac fever, have been associated with Legionella bacteria. The milder form is Pontiac fever, an influenza-like illness that is rarely fatal. Legionnaires' disease is a much more serious form, with severe pneumonia-like symptoms, and is fatal in 10–15% of the cases.

The bacterium responsible for Legionnaires' disease belongs to the genus Legionella. There are approximately 35 Legionella species known to produce the disease, and many of these species are commonly found in water bodies such as lakes and rivers. They can survive for several months in a wet environment and multiply in the presence of algae and organic matter. The species Legionella Pneumophila has often been implicated in Legionnaire's disease.

Legionnaires' disease is not infectious, as it cannot be transmitted from person to person. According to the U.S. Occupational Safety and Health Agency (OSHA) it is an “opportunistic” disease that most commonly attacks individuals who have a weakened immune system. Disease transmission normally occurs via inhalation of an aerosol of water contaminated by the bacteria.

Legionella is potentially a serious health concern in many public buildings. The only effective way of preventing such outbreaks is the proper design, operation, and maintenance of components in mechanical systems that are susceptible to bacterial growth and dissemination. Testing is useful for determining the effectiveness of the operation and maintenance program.


The major stages in the outbreaks of Legionnaires' disease are as follows:

1. Proliferation and amplification

This occurs in water systems where the bacteria feed on available nutrients such as organic matter. The most favourable conditions for amplification of the bacteria include a temperature range of 25–42 °C, pH levels in water in the range of 6–8, stagnation of water supply, the presence of amoebae, and the formation of sludge, sediments, and biofilms. Natural products such as rubber and wood favour amplification, while growth is inhibited by metals such as copper.

2. Dissemination

Formation of aerosols such as mists, sprays, and droplets is the primary means of dissemination for Legionella bacteria. Cooling towers, evaporative coolers/condensers, water jet humidifiers, atomizers, spas, and, fountains are all possible sources.

3. Inhalation

Deep inhalation of the Legionella bacteria is the primary source of infection. The attack rate is 2–7%, with an incubation period of 2–10 days. The risk of acquiring the disease is higher for elderly people, smokers, heavy drinkers, and, those with pre-existing lung disease. Statistics indicate that women are three times more likely to get the disease than men.

By inhalation, the small droplets carrying the bacteria are deposited deeply in the lower respiratory tract. Exposure time does not have to be long for the disease to occur. Cases of the disease have occurred simply by walking near a building having a contaminated cooling tower and by washing without showering after a brief exposure.

Survival of the bacteria in aerosols is reported to be at a maximum at 65% relative humidity. The risk of acquiring the infection increases with the number of bacteria deeply inhaled, and therefore, with the proliferation of these bacteria in the water source and the extent to which they are dispersed in aerosols.

If the dose received overwhelms the susceptible host's natural defence mechanisms, the bacterium multiplies intra-cellularly and infection occurs. Although previously healthy people may contract the disease, the persons most at risk are those who have a pre-existing condition that weakens their immune system.

4. Diagnosis

This is the last stage in the recognition of a case of Legionnaires' disease. With proper treatment methods, fatalities occur in a very small percentage of the cases.

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