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Legionella has the unusual characteristic of causing two diseases, Legionnaires Disease and Pontiac Fever. Legionnaires Disease is a pneumonia which attacks 2-5% of those exposed, and these represent those who are susceptible. Between 5-15% of those who contract legionella die from it. Factors influencing susceptibility include the elderly and those with suppressed immune systems, heavy smokers and alcoholics, and others with weak lungs or constitutions. Males are over twice as susceptible as females. It incubates in human hosts within 2-10 days and will not abate without medication. Estimates of the number of cases vary from 25,000 to 50,000 a year in the US. There have been over 50 separate outbreaks.
Pontiac Fever attacks 95% of those exposed indiscriminately and causes flu-like symptoms. It incubates within 36-48 hours and passes in 2-5 days. This variant made its first appearance in Pontiac Michigan in 1968 in the Oakland County Health Department where 95 of 100 employees and 49 of 170 visitors became sick. Three CDC investigators were sent into the building and they became sick. They were replaced with three more investigators who also became sick. The problem was traced to an evaporative condenser in the basement. This condenser was vented to the roof within two meters of the make-up air intake.
Pneumonia caused by Legionella is indistinguishable on the basis of symptoms, and therefore many cases may go unreported. Legionella is also considered to be a nosocomial disease.
There are 40 species in the family of Legionella bacteria in the world.
Although 12 of these species have been implicated in human disease, some 90%
of disease cases are caused by Legionella Pneumophila. There are 15
serogroups of Legionella Pneumophila and most cases are associated with sero
group 1. There are five subgroups of this serogroup which have different
degrees of virulence. The specific sub-typing of legionella is useful mainly
in prosecutory litigation.
In the natural aquatic environment, legionella feeds on various nutrients but is most adept in the role of an intracellular parasite on other bacteria. Once it is uptaken by, or insinuates itself in, a larger bacteria it resists bacterial defenses and then multiplies. Legionella derives essential nutrients from the water and from other bacteria. L-cysteine and iron salts are required for growth.
Legionella belongs to a special group of bacterial pathogens that evade host defenses by parasitizing phagocytes. This group also includes Tuberculosis bacteria and salmonella.
Legionella can be found in natural hot springs, such as at Yellowstone, where they have been found in samples dating back to the early 1900's.
It avoids ciliary clearance through the use of its polar flagella. It resists immune system defenses in the lungs by cleavage of immunoglobulins, a characteristic of most bacteria. The primary lung defense at the cellular level includes phagocytes and alveolar macrophages, which attempt to ingest the invader. The result in this case is a highly unusual process. After surrounding and engulfing the legionella, the phagocyte or macrophage fails to digest it. Instead, the legionella parasitizes the phagocyte and begins multiplying as if it were in a bacterial host in its natural environment. After doubling its numbers every 2 hours, eventually hundreds of legionella overload the phagocyte and it ruptures.
Ethromycin, an antibiotic, does not kill the legionella, but prevents it from multiplying intracellularly, giving the immune system a chance to gain the upper hand.
One part of the Causative Chain requires that Legionella penetrate deeply into the lungs, to the alveolar (gas exchange) region. The deposition of airborne particles in the human respiratory tract is governed primarily by the size of the particle as shown by the following rough breakdown:
The diagram below illustrates the range of temperatures within which Legionella survives and thrives. This chart also indicates the approximate temperature range of a number of common systems and types of equipment. It can be seen that spas, showers and cooling towers happen to fall exactly within the optimum growth range of Legionella. It can also be seen that evaporative coolers, cooling coils and hot water systems are outside the range needed for Legionella amplification.
Environmental conditions which promote the growth of Legionella are:
Bacteria die off quickly upon drying out in an aerosol and also from UV exposure on sunny days, but Legionella survives well in aerosols in comparison with other bacteria. Airborne Legionella survive over greater distances when the relative humidity is 65% or greater. The risk of Legionnaires Disease drops off exponentially when one is 0.5 km or more from a cooling tower. The infective range is from 500 to 1000 meters.
Click here to view a cooling tower checklist.
Cooling towers in metropolitan areas are difficult to keep clean and usually operate about 35 deg C. They receive hot water from condensers and spray it over a fill material through which air is forced in counterflow to the water. As condensation tends to occur around dust and particles the result is that evaporative coolers have a filtering effect on the air and collect dirt in the water. Biocides are normally used in these evaporative coolers but excess dirt can inhibit the biocide effectiveness. Also, due to changing operating temperatures and conditions, the levels of bacteria in cooling tower water can fluctuate in a cyclical manner. The application of biocides needs to be timed appropriately.
A distinction needs to be made between evaporative coolers and cooling towers & evaporative condensers, a point about which there is some confusion in the literature. Cooling towers and evaporative condensers have both been implicated in legionnaires disease. Evaporative coolers have not.
The two critical factors for domestic water services are water cleanliness and water temperature. Dirty water provides a food supply for the bacteria. Colonization is most likely between 35-40 deg C (95-104 deg F).
Comparison Chart of Water Disinfection Methods in a Hospital Environment
Legionella is more resistant to chlorine and microbial biocides than other microorganisms. Chlorine is an effective treatment, however, because it destroys the food supply of Legionella. Care must be used in the selection of biocides and they must be used consistently and in appropriate quantities. Refer to the various references on water treament for more information.
Comparison Chart of Water Disinfection Methods in a Hospital Environment.
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