What is Legionella?

Background

Legionella is the bacterium that causes Legionnaires disease. Of this bacterium, Legionella pneumophila is the species most commonly associated with disease outbreaks. Legionnaires disease is identified as a pneumonia type of infection of the lower respiratory tract.

The infection is most commonly acquired by the inhalation of airborne droplets or particles containing viable Legionella. Exposure to Legionella can also cause a short feverish illness without pneumonia known as Pontiac Fever.

Research and investigations indicate that the occurrence of Legionella contamination is greatest in water cooling towers, evaporative condensers, hot and cold water services, water spray humidifiers, air washers, spa baths and pools where water is agitated and recirculated. The contamination from a cooling water tower will cover a far larger area than any other likely source.

Sediment, scale and organic materials present in water systems, can provide nutrients and give protection for Legionella. Legionella has been shown to colonise certain types of water fittings, pipework and materials used in the construction of water systems. The presence of these materials may provide nutrients for Legionella and make eradication difficult. Other organisms in water systems such as bacteria, amoeba and algae can provide a suitable habitat and nutrients in which Legionella can survive and multiply.

The formation of biofilms within water systems is undesirable and may also provide harbourage and favourable conditions for Legionella growth. The presence of Legionella in biofilms and in enclosures within protozoa may protect the organisms from any remedial measure employed to eradicate the bacterium.

Legionella is most likely to proliferate in water systems that have a temperature between 20C and 50C. Human blood temperature of approximately 37C is the most ideal temperature for proliferation. Stagnant water within the above temperature range appears to provide the ideal conditions for proliferation of Legionella.

Legionella will survive at temperatures below 20C but is considered to be in a dormant state with no growth activity. The bacterium does not survive temperatures maintained consistently at 60C or above.

For water samples collected and returned to the laboratory, Legionella pneumophila is recovered by propagation of the organism on a specially supplemented nutrient growth medium. Such samples are normally then incubated at around 37C. It may take up to 7 days for colonies of Legionella to appear. Legionella can be recognised by visual examination of the colonies followed by a number of laboratory techniques to identify species and serogroups.

Where did it begin?

Legionella pneumophila came into the public eye in 1976 when an outbreak hit an American Legion convention in Philadelphia./p>

Out of 4,000 convention attendees, a total of 221 cases (5.5%) were infected. By the time the outbreak ended, of the original 221 'pneumonia' cases, 34 deaths (15%) occurred as a result of exposure to this previously unidentified bacteria. The causative agent would later come to be known as Legionella p. pneumophila and was isolated with its own given genus.

This was not the first outbreak. Retrospective studies have shown a number of pneumonia outbreaks, which we now know were caused by Legionella p. The earliest documented outbreak of Legionnaires' disease occurred in 1957 in Austin, Minnesota of the United States of America. 78 people, including 46 employees working at a local meat packing plant, were hospitalized with acute respiratory disease of an unknown cause. Two of the patients died from this outbreak. In July and August of 1964, at least 81 patients at St. Elizabeth Medical Center in Washington, D.C. of the United States of America, developed pneumonia after being exposed to Legionella p. bacteria. 17 of the patients died as a result of this outbreak. Evidence suggests that the bacteria may have originated from extensive soil excavations that were being carried out on the campus of the hospital during the summer months.

Outbreaks of Legionnaires' disease receive significant media attention. However, this disease usually occurs as a single, isolated case not associated with any recognized outbreak. When outbreaks do occur, they are usually recognized in the summer and early fall, but cases may occur year-round.

The CDC (Center for Disease Control) has estimated that the disease infects 10,000 - 15,000 persons annually in the US. OSHA estimates that over 25,000 cases of the illness occur each year, causing more than 4,000 deaths. Still, others estimate as many as 100,000 annual cases.

Key Facts

Infection by Legionnaires' disease occurs when inhaling water mists (aerosols) containing Legionella bacteria. However, to drink such water is not dangerous. These 'water mists' can occur anywhere if water is taken from pipes.

Showers and taps

Showers are considered to be the most common way of spreading the disease. Aerators on water taps also spread water aerosols.

Cooling towers

Many cases of Legionnaires' disease have been attributed to cooling towers inter alia for air-conditioning.

Spas

Spas can spread Legionella bacteria. The water temperature is ideal for the growth of bacteria and the air bubbles give rise to water aerosols.

Other sources

Other sources are humidifiers, ornamental fountains, high pressure washes and vegetable dampers in shop counters. You cannot catch Legionnaires'disease by drinking water.