Overview
Infections
Legionella Bacteria
National Survey

It should always be remembered that Legionnaires' disease is primarily a man-made disease and is therefore potentially preventable.

Prospective clinical studies indicate Legionnaires’ disease (LD) accounts for approximately 5% of the 180,000 cases of community-acquired pneumonias diagnosed in England & Wales each year. Due to the difficulty and costs involved in diagnosing LD, it is estimated that the number of voluntary reported cases underestimates the scale of the problem by over 95%. In Ireland it generally accepted that the reporting of LD cases is noticeably lower than in the UK .

Although cooling towers are often incriminated in large outbreaks, it is the domestic potable supply, and particularly the hot water systems in larger buildings, that are linked with the majority of reported cases of LD. Special attention has been drawn to the under-use of showers as they have been shown to present a widespread potential risk to health, and as such the Health and Safety Executive recommends showers are flushed through at least once a week or self-purging showering devices are fitted.

Why legionella infection should manifest itself as LD with a lower attack rate and a significant mortality, or as Pontiac fever, characterised by a very high attack rate of 95%+ and no mortality, has not yet been fully explained.

Legionnaires' Disease

The first recognised outbreak of LD occurred at a convention in Philadelphia , USA in 1976. 186 American Legionnaires contracted the previously unknown disease, resulting in 29 deaths. LD is an atypical pneumonia that requires specialised laboratory tests to confirm the diagnosis. Consequently the vast majority of cases remain undiagnosed.

Legionnaires' disease can strike the elderly, the healthy and infirm. Those who are highly susceptible are typically the aged, smokers, and those suffering from a pre-existing disease or a depressed immune system. It is important to recognise highly susceptible occupancies, such as those in: Hospitals, Care Homes, Hotels and Cruise Ships, particularly those attracting the elderly.

This is the acute non-pneumonic form of legionellosis that gets its name from the explosive epidemic of a flu-like illness that occurred in Pontiac, Michigan, 1968.

Pontiac fever presents itself as an acute, short-lived, self-limiting illness. It may be this form of the disease which accounts for much of the seropositivity found in the general population (Glick et al., 1978).  There was also a significant association between seropositivity and a flu-like illness, for staff tested during the legionella outbreak at Stafford District General Hospital (Dept of Health and Social Security 1985).

Legionella is a water borne bacteria which is typically sausage-shaped and usually has a thin flagellum that gives it mobility in an aquatic environment. The organism can multiply rapidly in water at temperatures between 20°C - 45°C. As the temperature rises above 60°C the majority of legionella will be killed within 5 minutes. Maintaining water temperatures above 50°C and below 20°C is therefore the main method of controlling legionella in domestic water systems as recommended by the Department of Health, and the Health & Safety Commission.

There are at least 40 different species of legionella with Legionella Pneumophila being the most pathogenic. The greater the number of legionella present in a water system the more likelihood there is of contracting Legionnaires' disease. This is why all of the available guidance on the control of Legionnaires' disease, stresses the importance of maintaining controls that reduce the numbers of legionella in water systems.

A UK survey by the Public Health Laboratory Service (PHLS) found legionella bacteria contamination present in the domestic hot water systems of the following establishments:

• 75% of Business premises contaminated,  
• 70% of Hospitals contaminated,
• more than 50% of Hotels contaminated.

It is also well documented that domestic water systems account for over three times as many cases of legionella infection when compared with water cooling systems.