The facts about...

Legionnaire's disease

Historical background

In 1976 an outbreak of pneumonia occurred in members of the American Legion (an ex-servicemen’s organisation) attending a conference at the Belview Stratford Hotel in Philadelphia. After much investigative work, the cause was shown to be a bacterium named Legionella pneumophila, an organism which had not previously been recognised. Retrospective studies have since shown that some earlier outbreaks of severe pneumonia of unknown cause were, in fact, due to infection with Legionella.

The organism

Legionella pneumophila causes about 90% of cases of legionella infection. This species has now been subdivided into smaller groups known as serogroups, and although all of them can cause the disease, the commonest cause is serogroup 1. Studies have now revealed a family of related organisms, the Legionellaceae, in which there are over 40 species, some of them not yet named. The named species have been isolated from environmental sources or patients, and their names reflect either the place where they were found or the person who discovered them, e.g. L bozemanii and L. longbeachae. Some of the environmental species have not yet been shown to cause human disease.

Legionellas are widely distributed in nature and their natural habitats are the water of rivers, warm springs, streams, lakes and river banks. In their normal environment legionellas do not cause infections, but transferred to the water supplies and air conditioning units of large buildings they can cause large outbreaks of disease.

The diseases

Legionnaires disease

Most infections with legionellas are respiratory infections, although other infections can occur, such as wound infections due to contact with contaminated water. The commonest result of infection is an acute pneumonia, Legionnaires disease. After an incubation period of two to ten days the patient begins to feel ill with malaise, general aches, a headache, and a dry cough. Within a day or two their temperature may reach 40OC and patients may become confused and seriously ill. The signs and symptoms may be more those of a septicaemia (a reaction to bacteria in the blood) than respiratory illness, but after another day or two it becomes obvious that the patient has severe respiratory infection.

Patients may develop complications such as thrombocytopenia (a deficiency of blood elements required for blood clotting), acute renal failure, or failure of other important organs. The organism may spread via the bloodstream and cause infection elsewhere in the body. Abdominal symptoms such as nausea, vomiting and diarrhoea are quite common. Legionnaires disease is a potentially fatal condition; in the United Kingdom in 1996, the mortality rate was 12%. The disease is treated with antibiotics (such as erythromycin), plus supportive treatment including mechanical ventilation if necessary.

About half the cases occurring in the UK are associated with travel abroad, the destinations most frequently associated with Legionnaires disease are (in descending order) Spain, France, Turkey and Greece which are popular destinations for British holidaymakers. Travel associated cases are now reported by European countries to a central reporting scheme run by the Communicable Disease Surveillance Centre in London, so that outbreaks occurring in one country but involving the citizens of several nations can be detected and acted upon as quickly as possible.

More than two thirds of the cases which originate in the UK are associated with urban areas particularly industrial estates, which may be due to a greater concentration of water cooling towers in those areas. Other infections are associated with travel within the UK, and legionella infection may also be acquired in hospital, although this is now rare.

There are a number of risk factors for Legionnaires disease. The disease is commoner and/or more severe in men, cigarette smokers, elderly patients, those with pre-existing lung disease and patients with deficient immune systems.

Pontiac Fever

This is a mild form of legionellosis that resemble influenza. It is so called because Legionella pneumophila was shown retrospectively to have been the cause of an outbreak in Pontiac, USA, in 1968. The incubation period is usually short, one or two days, and so is the illness. The patients develop a malaise, mild aches, some fever, chills and a headache, but in a day or two it usually resolves and X-ray examinations show no sign of pneumonia.

Legionellas in Water Supplies

Legionellas are commonly found in small numbers in collections of fresh water, but there they probably do no harm. They can survive normal drinking water disinfection, and may enter the mains water supply for homes and industry. Once in the distribution system, they may multiply (particularly if the water temperature is favourable or if the system contains excessive sediment) and susceptible people may be exposed to the organism via sprays generated by water taps or shower heads. This is much less likely to be a problem in domestic water supplies because the turnover of the supply does not allow the water to stagnate and the legionellas to multiply.

Air conditioning systems may constitute the greatest hazard. The organisms multiply in the water of cooling towers in which a cascade of water is used to cool air in pipes or vanes. The water does not come into direct contact with the air, but a great deal of it is lost by evaporation or as spray, and as cooling towers are usually situated in the roofs of buildings the clouds of droplets containing legionellas may be drawn into the air intakes of the building or fall on people passing by. Humidification systems have also been incriminated as a source of infection.

Prevention

Person to person transmission is virtually unknown. Legionellas in the water supply or in air conditioning systems may be almost impossible to eradicate, but a series of control measures is recommended. Hot water should be stored at 60OC, although this may be difficult to achieve in older hot water systems which cannot be relied upon to store all the hot water at a consistent temperature. Cold water should be stored at 25OC or less, but with very large storage tanks this may not be possible at all times of the year. Air conditioning plants, including cooling towers and humidifiers, must be examined and cleaned regularly and all cooling towers must be registered with the local authority. Biocides and anti-scaling compounds may be added to the water. Water fittings and washers should not be made of material which encourages the growth of the organisms, and water distribution systems should be designed to avoid dead-ends which case water stagnation and allow multiplication of the organism.