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Legionnaires disease and its relevance to indoor air quality

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For this blog entry we are covering the topic of Legionnaires' disease following the outbreak in Edinburgh last month and more recently the outbreak in Stoke-on-Trent in England. Scotland's Health Secretary Nicola Sturgeon reported that the number of confirmed and suspected cases of Legionnaires' disease in Edinburgh had risen to 40. Air Quality professionals suspected the source of the Edinburgh outbreak was environmental contamination and early indications suggest it was the result of a contaminated cloud emitted from a cooling tower in the south west of the city. The source of the outbreak in Stoke-on-Trent has not yet been identifed.

Legionella is a Gram negative pathogenic bacteria whose species such as L. pneumophila are the causal agents of legionellosis. Legionellosis encompasses Legionnaires' disease and Pontatic fever, responsible for 8,000 to 18,000 hospitalisations annually. Legionnaires' disease is a more severe form of the infection characterised by pneumonia and fever. Pontatic fever produces symptoms characteristic of acute influenza and lacks the pneumonia like symptoms of Legionnaires' disease. Legionella is not contagious.

Should the condition be left untreated, respiratory failure, shock and multi-organ failure can occur. Legionellosis represents a risk factor to both healthy and “at risk” populations such as the immunocompromised and immunosuppressed. As a result centres with patients with immunosuppression or transplant units should consider routine testing for Legionella in their water systems. Given its broad range of infectivity, it is a cause for concern for healthcare and facility managers.

Legionella is frequently isolated from natural and artificial aquatic systems. In natural water sources, Legionella may be present but in concentrations that are non-culturable. As a result, Legionella may enter artificial aquatic systems from naturally occurring sources undetected, where they may rapidly proliferate to dangerous levels under favourable conditions. Artificial aquatic systems associated with outbreaks of legionellosis include water cooling towers, architectural fountains, drinking fountains, air conditioning devices and “dead-legs” in plumbing systems. Outbreaks are also common in leisure facilities, such as saunas and hot tubs. Other reports have associated Legionella with dental unit water lines, posing a risk to healthcare workers and patients. In such instances preventive measures should be taken to ensure decontamination of such environments.

The growth and survival of Legionella in such environments is facilitated by its broad range of optimum growth temperatures and pH. Legionella shows high tolerance to both acidic and basic conditions, surviving in environments ranging from pH 2.7-8.3. Similarly, its mesophilic nature allows it to survive in temperatures ranging from 20˚C to 50˚C. Coupled to the ability of Legionella to survive a range of environmental pressures, is its ability to interact with the naturally occurring microflora. Species of Legionella have been shown to use protozoa as intracellular replicative niches, affording protection from the external environment and providing protection against thermal destruction. Furthermore, the ability of Legionella to form biofilm-bacterial like “plaques” enables adherence to surfaces, provides resistance to chemical and physical removal and may affect the rate of water flow by obstruction. The culmination of this may result in further aquatic stagnation, facilitating bacterial proliferation.

Transmission of Legionella from aquatic systems to a host typically occurs via spray or aerosol transfer. An aerosol of Legionella may be formed when water droplets from a spray carrying Legionella evaporates, leaving small airborne particles small enough to be inhaled. These particles may then disseminate through the respiratory tract where they colonise and cause infection. Of note, the transfer of Legionella  by aerosol transfer has been reported to disperse Legionella up to 3.2 kilometres from its aquatic reservoir. This puts individuals at risk of infection who may not have come into direct contact with the facility containing the contaminated aquatic system.

Given the versatile nature of this microorganism, an informed approach must be taken to ensure its complete elimination from an aquatic system. Current disinfection techniques use a range of UV, filtration and chemical-based approaches. UV disinfection is a proven effective technique that is simple and safe to use. However its use is site dependant, given it is only effective at point of use, and offers no systemic control. Point-of-use filters have the advantage of being easy to install, and may be used in both hot and cold systems. Like UV however, they are only suitable at point of use. Continuous circulation of water through the system at 55˚C is a simple method of preventing Legionella growth. However it is not suited for cold water systems. As a result, the approach used to remove Legionella from an environment must factor in the nature and use of the environment. This is best illustrated by oxidizing agents such as hypochlorite which have a high rate of disinfection, but may cause corrosion of plumbing systems.

For managers of healthcare facilities, it should be noted that a new standard for the prevention of Legionellosis has been released by the American society of heating refrigerating and air-conditioning engineers (ASHRAE). The new measures outlined by ASHRAE 188 aims to reduce the instances of Legionellosis. It is therefore advisable for healthcare managers to familiarise themselves with this new standard to safe guard their facility against outbreaks and associated legal enquiries.

Related links
Q&A: Edinburgh Legionnaires' disease outbreak
Legionnaires' disease in Stoke-on-Trent: Four more cases
CDC: Legionella (Legionnaires' Disease and Pontiac Fever)

ASHRAE 188 Prevention of Legionellosis Associated with Building Water Systems (pdf)
World Health Organisation, Legionella and the prevention of legionellosis (pdf)
Risk of exposure to Legionella in dental practice (pdf)
Nosocomial Legionnaires' Disease Caused by Aerosolized Tap Water from Respiratory Devices
Surveillance of hospital water and primary prevention of nosocomial legionellosis: what is the evidence?