What is Legionnaires' Disease?

An outbreak of this disease that caused 29 deaths in Philadelphia in 1976, largely among people attending a state convention of the American Legion, led to the name Legionnaires' Disease. Subsequently, the bacterium causing the illness was identified and named legionella. The causative agent - what would come to be known as legionella pneumophila - was isolated and given its own genus.

Legionnaires' Disease is a bacterial disease that may cause pneumonia. The majority of cases are reported as single (isolated) cases but outbreaks can occur. Respiratory transmission of this organism can lead to infection, which is usually characterised by a gradual onset of 'flu-like’ symptoms. Patients may experience fever, chills, and a dry cough as part of the early symptoms. Patients can develop severe pneumonia that is not responsive to penicillin or amino glycosides. Legionnaires' Disease also has the potential to spread into other organ-systems of the body, such as the gastro-intestinal tract and the central nervous system. Accordingly, patients with advanced infections may experience diarrhoea, nausea, disorientation, and confusion. Pontiac fever is also caused by L. pneumophila but does not produce the severity of the symptoms found in Legionnaires’ Disease. The ‘flu-like’ symptoms are still seen in Pontiac fever patients, but pneumonia does not develop and infection does not spread beyond the lungs.

It is a naturally occurring organism and can be found in low levels in the water supply. It is probable that it comes from earth that has contaminated a pipeline during repair work, and only becomes dangerous in high concentrations.

The time from infection to start of the illness (the incubation period) is between two and ten days.


Reporting organisations

The European Surveillance Scheme for Travel Associated Legionnaires' Disease has 31 participating countries. The role of the scheme is to collect information on cases of Legionnaires' Disease occurring in travellers, so that sites associated with more than one case can be detected and the authorities in the relevant country informed. Since the ‘European Directive on Package Travel’ in 1996, the surveillance co-ordinator for England and Wales has informed the Association of British Travel Agents (ABTA) and the Federation of Tour Operators (FTO) of any English cases that are associated with accommodation likely to be used by tour operators.

A national surveillance scheme was set up in England and Wales in 1979 and since then an average of about 200 cases are reported each year. This figure is thought to be dramatically under-reported. In 1998, 25 deaths in England and Wales were reported as being caused by legionella.


How is it transmitted?

Legionnaires’ Disease is caused by infection with legionella bacteria. It is contracted from small droplets of water, which are contaminated with legionella bacteria, and which have become suspended in the air. Aerosol droplets that allow transmission of legionella are found in 'wet' air conditioning systems, whirlpool spas and showers. Legionella can grow in any water system that is not properly maintained. This means that if a source of infection is not detected and appropriately dealt with it can continue to infect people indefinitely.


The effect of temperature on Legionella