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History and overview

Legionellosis is a serious and sometimes fatal form of pneumonia. It is caused by the bacterium Legionella pneumophila and other legionella species. These bacteria are found naturally in the environment and thrive in warm water and warm damp places. They are commonly found in lakes, rivers, creeks, hot springs and other bodies of water. They can also be found in soil and potting mix.

The bacterium Legionella 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 dose of legionella necessary for infection is unknown, but the infective dose for susceptible humans can be assumed to be low, as patients have been known to be infected after exposure of only a few minutes to the sources of some outbreaks, and at up to 3.2 km from the source of others. Infection depends on the water contamination level by bacteria, the effectiveness of formation and dissemination of bacteria through air, host factors and the virulence of the particular strain of Legionella.

The disease and its effect on people

Legionellosis is a generic term describing the pneumonic and non-pneumonic forms of infection with Legionella.

The non-pneumonic form is an acute, self-limiting influenza-like illness usually lasting 2-5 days. The incubation period is from a few and up to 48 hours. The main symptoms are fever, chills, headache, malaise and muscle pain (myalgia). No deaths are associated with this type of infection.

Legionnaires’ disease has an incubation period of two to ten days (but up to 16 days has been recorded in some recent well-documented outbreaks). Initially, symptoms are fever, loss of appetite, headache, malaise and lethargy. Some patients may also have muscle pain, diarrhoea and confusion. There is also usually an initial mild cough, but as many as 50% of patients can present phlegm. Blood-streaked phlegm or hemoptysis occurs in about one-third of the patients. The severity of disease ranges from a mild cough to a rapidly fatal pneumonia. Death occurs through progressive pneumonia with respiratory failure and/or shock and multi-organ failure.

Untreated Legionnaires’ disease usually worsens during the first week. In common with other risk factors causing severe pneumonia, the most frequent complications of legionellosis are respiratory failure, shock and acute kidney and multi-organ failure. Recovery always requires antibiotic treatment, and is usually complete, after several weeks or months. In rare occasions, severe progressive pneumonia or ineffective treatment for pneumonia can result brain in sequelae.

The death rate as a result of legionnella is dependent on: the severity of the disease, the appropriateness of initial anti-microbial treatment, the setting where legionella was acquired, and host factors (i.e. the disease is usually more serious in patients with immuno-suppression). The case fatality rate may be as high as 40 - 80 per 100 in untreated immuno-suppressed patients and can be reduced to 5 - 30 per 100 through appropriate case management and depending on the severity of the clinical signs and symptoms. For persons able to develop an immune response the death rate is usually within the range of 10 - 15%.

The cause

The causative agents, legionellae, are freshwater bacteria that are found in aquatic environments worldwide but artificial water systems sometimes provide environments conducive to the growth of Legionella bacteria. These bacteria survive within or between the cells as parasites of free-living protozoa and within biofilms which develop in water systems where bacteria survive. They can cause human infections by infecting other human cells utilizing a similar mechanism to that used to infect protozoa. L. pneumophila is the species most frequently isolated from patients with either community, travel-associated or hospital-acquired legionellosis.


Legionnaires’ disease is believed to occur worldwide.

How the disease is transmitted

Legionella organisms can be spread by aerosols such as wind. Infection results from inhalation of contaminated water sprays or mists. Infection can also occur by inhalation, particularly during outbreaks in hospital. The bacteria live in water and colonize hot and cold water systems at temperatures of 20 to 50 degrees Celsius (optimal 35 degrees Celsius). They contaminate air conditioning cooling towers, hot and cold water systems, humidifiers, whirlpool spas and other water-containing devices. There is no direct human-to-human transmission.

Extent of the disease

The incidence of community-acquired Legionnaires’ disease varies widely according to the setting investigated and the diagnostic methodology applied. Since many countries lack appropriate methods of diagnosing the infection or surveillance systems capable of monitoring the situation, the real magnitude of the problem is unknown. In 2003, 34 countries (population: 467.76 million) out of the 36 in the European Working Group for Legionella Infections reported a total of 4578 cases, meaning an average rate across Europe of 9.8 per million population. Based on findings from Denmark where a high level of testing for legionella in patients with pneumonia is developed, a more realistic incidence would be closer to 10 000 cases a year for the same 36 countries.

Risk factors for community-acquired and travel-associated Legionellosis include: male, over 50 years, smoker, a history of heavy drinking, pulmonary related deaths, immuno-suppression, and chronic debilitating illnesses.

Risk factors for hospital-acquired pneumonia in the host 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 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.


There is no vaccine currently available for Legionnaires’ disease.

Patients with the non-pneumonic form of infection do not require any antibiotic treatment and the symptomatic approach is sufficient. Patients with Legionnaires’ disease always require antibiotic treatment, following laboratory confirmation of diagnosis.

The public health threat posed by legionellosis can be addressed via preventive measures. Although it is impossible to eradicate the source of infection, it is possible to reduce the risks substantially. Prevention of Legionnaires’ disease depends on good maintenance of possible sources, including regular cleaning and disinfection and the application of other physical (temperature) or chemical measures (biocide) to minimise growth. Some examples are: the regular cleaning and disinfection of cooling towers together with frequent or continuous addition of biocides; maintaining an adequate level of a biocide such as chlorine in a spa pool along with a complete drain and clean of the whole system at least weekly; keeping hot and cold water systems clean and either keeping the hot water at 60°C and the cold below 20°C or alternatively treating them with a suitable biocide to limit growth. Applying such controls particularly in hospitals, industrial sites, hotels, leisure centres, etc will greatly reduce the risk of legionella contamination and prevent the occurrence of sporadic cases.

Such control and prevention measures must be accompanied by proper vigilance on the part of general practitioners and community health services for the detection of cases.

Sources: US Department of Health; The World Health Organization

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