Legionellosis

Key facts

• The bacterium L. pneumophila was first identified in 1977, as the cause of an outbreak of severe pneumonia in a convention centre in the USA in 1976.
• The most common form of transmission of Legionella is inhalation of contaminated aerosols produced in conjunction with water sprays, jets or mists. Infection can also occur by aspiration of contaminated water or ice, particularly in susceptible hospital patients.
• Legionnaires’ disease has an incubation period of 2 to 10 days (but up to 16 days has been recorded in some outbreaks).
• Death occurs through progressive pneumonia with respiratory failure and/or shock and multi-organ failure.
• Untreated Legionnaires’ disease usually worsens during the first week.
• Of the reported cases 75–80% are over 50 years and 60–70% are male.

It is often categorized as being community, travel or hospital acquired based on the type of exposure.

Worldwide, waterborne Legionella pneumophila is the most common cause of cases including outbreaks. Legionella pneumophila and related species are commonly found in lakes, rivers, creeks, hot springs and other bodies of water. Other species including L. longbeachae can be found in potting mixes.

The bacterium L. pneumophila was first identified in 1977, as the cause of an outbreak of severe pneumonia in a convention centre in the USA in 1976. It has since been associated with outbreaks linked to poorly maintained artificial water systems, particularly cooling towers or evaporative condensers associated with air conditioning and industrial cooling, hot and cold water systems in public and private buildings, and whirlpool spas.

The infective dose is unknown, but can be assumed to be low for susceptible people, as illnesses have occurred after short exposures and 3 or more km from the source of outbreaks. The likelihood of illness depends on the concentrations of Legionella in the water source, the production and dissemination of aerosols, host factors such as age and pre-existing health conditions and the virulence of the particular strain of Legionella. Most infections do not cause illness.

The cause

The causative agents are Legionella from water or potting mix. The most common cause of illness is the freshwater species L. pneumophila which is found in natural aquatic environments worldwide. However, artificial water systems which provide environments conducive to the growth and dissemination of Legionella represent the most likely sources of disease.

The bacteria live and grow in water systems at temperatures of 20 to 50 degrees Celsius (optimal 35 degrees Celsius). Legionella can survive and grow as parasites within free-living protozoa and within biofilms which develop in water systems. They can cause infections by infecting human cells using a similar mechanism to that used to infect protozoa.

Transmission

The most common form of transmission of Legionella is inhalation of contaminated aerosols. Sources of aerosols that have been linked with transmission of Legionella include air conditioning cooling towers, hot and cold water systems, humidifiers and whirlpool spas. Infection can also occur by aspiration of contaminated water or ice, particularly in susceptible hospital patients, and by exposure of babies during water births. There is no direct human-to-human transmission.

Distribution

Legionnaires’ disease is believed to occur worldwide.

Extent of the disease

The identified incidence of Legionnaires’ disease varies widely according to the level of surveillance and reporting. Since many countries lack appropriate methods of diagnosing the infection or sufficient surveillance systems, the rate of occurrence is unknown. In Europe, Australia and the USA there are about 10–15 cases detected per million population per year.

Of the reported cases 75–80% are over 50 years and 60–70% are male. Other risk factors for community-acquired and travel-associated legionellosis include: smoking, a history of heavy drinking, pulmonary-related illness, immuno-suppression, and chronic respiratory or renal illnesses.

Risk factors for hospital-acquired pneumonia are: recent surgery, intubation, which is the process of placing a tube in the trachaea, mechanical ventilation, aspiration, presence of nasogastric tubes, and the use of respiratory therapy equipment. The most susceptible hosts are immuno-compromised patients, including organ transplant recipients and cancer patients and those receiving corticosteroid treatment.

Delay in diagnosis and administration of appropriate antibiotic treatment, increasing age and presence of co-existing diseases are predictors of death from Legionnaires’ disease.

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