Legionnaires Disease
Legionellosis is the collective term used for infections caused by Legionella pneumophila and other bacteria from the family Legionellaceae.
Legionnaires' disease is a form of pneumonia which can affect anybody, but which principally affects those who are susceptible because of age, illness, immunosuppression, smoking etc. and can be fatal. It is caused by the bacterium Legionella pneumophila and related bacteria. Legionella may also cause less serious illnesses which are not fatal or permanently debilitating but which can affect all people.
The Legionellaceae family is large with over 40 species identified so far. The species Legionella pneumophila causes about 90% of all cases and outbreaks. Sixteen different serogroups of Legionella pneumophila have been described although, Legionella pneumophila serogroup 1 is most commonly associated with cases of Legionnaires' disease in the UK.
On average, there are 200-250 reported cases of Legionnaires' disease each year in the United Kingdom (UK). However, it is thought that the total number of cases of the disease may be generally underestimated. Infections which originate in the UK are often sporadic, for which no source of infection is traced. Studies show that most cases of legionellosis have been attributed to hot and cold water services in a variety of buildings, cooling towers and whirlpool spas where there is inadequate or inappropriate Maintenance and Hygiene Management.
Other sources have been identified including humidification systems, industrial coolants and respiratory therapy equipment. Legionnaires' disease was first identified following a large outbreak of pneumonia among people who attended an American Legion Convention in Philadelphia in 1976. A previously unrecognised bacterium was isolated from lung tissue samples which was subsequently named Legionella pneumophila.
Infection is attributed to inhaling legionella, either in tiny droplets of water (aerosols small enough to penetrate deeply into the lung), or in droplet nuclei (the particles left after the water has evaporated). There is also evidence to suggest that the disease may also be contracted by inhaling legionella bacteria following ingestion of contaminated water by susceptible individuals. Person to person spread of the disease has not been documented.
Legionella bacteria are common and can be found naturally in ground soil, environmental water sources such as rivers, lakes and reservoirs although usually in low numbers. Legionella bacteria can survive under a wide variety of environmental conditions and have been found in water at temperatures between 6°C and 60°C. Water temperatures in the range of 20°C to 45°C seem to favour growth although the organisms do not appear to multiply below 20°C but will not survive for long periods above 60°C.
They may however, remain dormant in cool water and multiply only when water temperatures reach a suitable level. Temperatures may also influence virulence; legionella bacteria held at 37°C have greater virulence than the same legionella bacteria kept at a temperature below 25°C. If there is a means of creating and transmitting water droplets, (i.e. by operating showers and opening tap outlets etc.) people in the vicinity are potentially at risk.
Symptoms are similar to pneumonia following an incubation period of between 2 to 10 days (usually 3-6 days). Some patients also exhibit other symptoms such as diarrhoea, sickness, stomach upsets and malfunction of the central nervous system. Mild cases may recover without antibiotic therapy and in large outbreaks the mortality rate varies between 10 to 25% although with prompt antibiotics the figure is much closer to 12%. In survivors recovery is usually complete. Certain groups of people are more known to be at higher risk of contracting Legionnaires' disease. For example, men appear more susceptible than women, as do those over 45 years of age, smokers, alcoholics, diabetics and those with cancer or chronic respiratory or kidney disease. Legionella pneumophila is also responsible for a short feverish form of the illness without pneumonia, known as Pontiac fever. Its incubation period is typically between 2-3 days. Another species of legionella, L.micdadei, is responsible for a similar form of the illness without pneumonia called Lochgoilhead fever after an outbreak in Lochgoilhead, Scotland. The incubation period can be up to 9 days. A high percentage of those exposed to this agent tend to be affected. However, there have been no recorded deaths associated with either Pontiac or Lochgoilhead fevers. The disease is usually diagnosed by a combination of tests.
The organisms may be cultured from the patients sputum, bronchial washings or lung tissue. Alternatively, tests are used to measure the presence of antibodies in the blood and, increasingly, tests are available to measure specific antigens in the patients urine. Legionella pneumophila serogroup 1 can be further sub divided to distinguish between strains most commonly associated with Legionnaires' disease. Additionally, 'genetic fingerprinting' methods such as Restriction Fragment Length Polymorphism (RFLP) and Amplified Fragment Length Polymorphism (AFLP) can be valuable tools in the investigation of outbreaks. Such methods of typing can sometimes provide a means of linking the organisms isolated from patients to the sources of cases of outbreaks.
Other sources of nutrients can include, commonly encountered organisms within the water system itself such as algae, amoebae and other bacteria. The presence of sediment, sludge, scale and other material within the system, together with biofilms, are also thought to play an important role in harbouring and providing favourable conditions in which legionella bacteria may grow. A biofilm is a thin layer of micro-organisms which form a slime on surfaces of a water system in direct contact with water. Biofilms, sludge and scale can protect legionella bacteria from temperatures and concentrations of biocide that would otherwise kill or inhibit these organisms if they were freely suspended in the water. It should be remembered that cases of Legionnaires' disease have already occurred among staff in the workplace (i.e. offices, factories, shops and hospitals etc.) and also among visitors (including delivery drivers, members of the public, hotel guests, patients and even passers by). As legionella bacteria are commonly found in a variety of environmental sources they may eventually colonise manufactured water systems and be found in cooling tower systems, hot and cold water systems and other plant which either use or store water.