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| Legionella and Building Services |
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The primary chain of causation is useful for discussing the epidemiology of Legionnaires’ disease. Controlling any link in the chain will presumably decrease the occurrence of infections. Source Legionella live in the natural environment and water supplies, normally in low concentrations and little can be done to prevent the ingress of legionella into potable water systems. Amplification Factors that induce the growth of legionella, include:
Another factor that is often overlooked is time - how old is the system? How long has stagnant water been standing for? Water systems in some older buildings have been found to be over fifty years old; corrosion and fouling in pipework and fittings is often significant, providing conditions that may be extremely difficult to control. Stagnant water found standing for only a few days in dead legs and conventional showers has been shown to present a reasonably foreseeable potential risk to health. Removing or reducing any of these factors will reduce the potential risk of infection. Dissemination Cooling towers and showers are primary examples of devices that produce aerosols (fine mists) that can carry legionella bacteria. Aerosols are microscopic water particles having negligible falling velocity when in suspension in air. Potentially harmful aerosols are large enough to suspend legionella bacteria and small enough to be inhaled deep into the lung. Typically, sized between 0.5µm and 5µm. Raindrops are typically 100µm in diameter, drizzle 100µm. Evaporated water particles, which cause the steaming effect in bathing areas, are too small to suspend legionella. Harmful aerosols can stay airborne for hours, traveling hundreds of meters and maintaining the ability to cause infection. High speed photography has revealed that when a water drop hits a hard surface the drop rapidly changes into an expanding disc of water that rises to form a thin coronet-shaped conical film. This continues to expand in size until it becomes progressively thinner, eventually collapsing into an aerosol.
It is worth noting that showers produce significant amounts of aerosols, often in confined spaces occupying hosts. Whereas, cooling towers also produce significant amounts of aerosols, ideally distributed upwards, away from personnel and into an open atmosphere. Contact with aerosols should always be avoided where possible; unfortunately this is not always possible. Host The primary mode of transmission of legionellosis is inhalation of legionella in aerosols of breathable size. Transmission occurs occasionally via other routes, including direct inoculation of surgical wounds with contaminated potable water during the placement of surgical dressings and aspiration of contaminated water by persons recovering from specific types of surgery. It is important to recognize highly susceptible occupancies, such as in:
There are other legionella infections, the most common being Pontiac fever. This is the acute non-pneumonic form of legionellosis that gets it name from the explosive epidemic of a flu-like illness that occurred in Pontiac, Michigan, 1968. Why Legionnaires’ disease should manifest itself as Legionnaires’ disease, with a lower attack rate and a significant mortality, or as Pontiac fever, characterised by a very high attack rate (95%+) and no mortality, has not yet been fully explained. 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 a significant association between the seropositivity and a flu like illness, for staff tested during the Legionnaires’ disease outbreak at the Stafford District General Hospital, 1985. Incidence of disease A number of studies indicate that legionella cause up to 5% of community-acquired pneumonias and as such the number of reported cases grossly underestimates the scale of the problem. Some of the reasons why: a substantial proportion of patients may not become hospitalized; some patients infected with legionella may not develop pneumonia; incorrect diagnosis; incidents not reported. Article by: |
