CPD PROGRAMME | LEGIONELLA pneumonia infection, and Pontiac fever, an acute u-like illness. Even though the colonisation of a water system by legionella may occur frequently, this alone is not enough to pose a high risk to humans, unless the bacteria population reaches high numbers and becomes dispersed through appropriate aerosolisation to impact susceptible people by inhalation.5 Legionella has evolved to exist and multiply in adverse conditions by being a parasite. (The boxout, Prozotoa and amoebae, provides some associated terminology.) Scheid9 explains that the term free-living amoeba (FLA) is applied to a mixed group of parasitic amoebae that are found in biofilms (which can visually manifest as slime), which are a complex layer of microorganisms that have attached and grown on a surface. Some FLAs have a resistant life stage, a cyst, which largely protects the organism from adverse environmental conditions such as higher temperatures and chemical disinfectants and, therefore, provides great tenacity. The FLA feed on algae, bacteria and fungi, as well as on smaller protozoa, and provide protected hosts for bacteria. The FLAs Acanthamoeba and Vermamoeba vermiformis are microscopic, free-living amoebae that exist in water and soil and are considered10 suitable hosts for pathogenic microorganisms (bacteria that can cause disease). Work by Rowbotham11 identified that these FLA can serve as hosts, reservoirs, vehicles and protectors for legionella (Figure 1). A recent systematic literature review12 by Nisar et al Figure 1: Vermamoeba vermiformis, orange, entraps Legionella pneumophila bacteria (2 to 20m long), green (Source: pixnio.com/ science/microscopy-images/hartmannella-vermiformis) shows that Legionella pneumophila combined with Acanthamoeba and Vermamoeba were extensively found in chlorinated and thermally treated water, which indicated the potential tolerance of Legionella pneumophila and its protozoan hosts to survive under a wide range of disinfection conditions. Not only does legionella acquire necessary nutrients from its host, but the host also protects legionella from toxic and unfavourable environmental conditions. AWT1 notes that disinfection of a water source can lead to the eradication or reduction of legionella populations; however, the majority of legionella does not exist in an independent and solitary state (like a plankton). Instead, legionella more often resides inside a protozoan host and/ or biofilm community. When legionella is released from a host, it is encapsulated within vesicles (small fluid-filled bladders) derived from the hosts cell membrane. There can be more than a thousand legionella within a single vesicle, and legionella dispersed within these vesicles appears to be more virulent than legionella remaining parasitised in a protozoan host. There are several authoritative guides to controlling and managing legionella risk in the built environment, including: CIBSE TM13:2013; the newly revised ASHRAE Guideline 12-2020; Health Technical Memorandum 04-01:2016; and the recently updated BS 8580-1:2019 Water quality Risk assessments for Legionella control Code of practice, which includes significant additions including an increased emphasis on risk assessment and management. These all provide further references to what is an increasingly well-documented topic. The freely accessible Health and Safety Executives (HSEs) approved code of practice, Legionnaires disease: The control of legionella bacteria in water systems (L8), 2013, contains practical guidance on how to manage and control the risks in building systems see www.hse.gov.uk/legionnaires. Part 2 of the complementary technical guidance in HSG274 relates specifically to hot and cold water systems. Practically, temperature control is the traditional strategy for reducing the risk of legionella in hot and cold water systems, as temperatures between 20C and 45C have been identified as the optimal range for legionella growth. HSG274 specifically notes that, if hot water is stored, it should be at least at 60C and distributed so that it reaches a temperature of 50C (55C in healthcare premises) within one minute at the outlets. The WRAS information leaflet Preheated domestic hot water Storage of preheated domestic hot water and possible growth of legionella bacteria notes that legionella bacteria in concentrations of 100,000 colony forming units per litre (cfu.L-1, a measure of viable bacterial cells) and higher are not uncommon at the base of conventional hot water storage vessels where temperatures of 20C to 45C are maintained. For comparison, HSG274 Part 2 recommends review of control measures (and possible disinfection) where legionella concentrations are in the range 100 to 1,000cfu.L-1, and corrective action (followed by retesting every few days) where they exceed 1,000cfu.L-1. An example of simple, practical, system design to reduce the risk of legionella accumulation is shown in Figure 2, where a direct-fired storage water heater installation includes a destratification pump that is automatically controlled to provide a daily legionella protection cycle, by circulating water from top to bottom in the cylinder of water at 60C. Coincidentally, the main wholesome hot water (DHW) recirculation pump can be activated to provide a circulation of hot water around the distribution circuit. Although this should be undertaken at times when occupants will not be drawing hot water as water at 60C presents a scalding risk local thermostatic mixing valves should be provided if this is considered an unacceptable risk. (TMVs will, themselves, present further requirements for legionella management.) In the recent review paper13 by Whiley, H et al, they reported that up to 25% of Legionella pneumophila cells can survive heat treatment of 70C, but all of these were in a viable, but non-culturable, state. This could be significant in future assessments of legionella control, as being non-culturable means it will not grow on the growth media that are used in culture methods currently employed to determine the efficacy of disinfection protocols. In older, more complex buildings, it may be practically impossible to maintain the required temperatures. In the coverage14 of legionella protection by the US Centers for Disease Control and Prevention (CDC), it highlights that a building wholesome water system with extensive dead-legs, low disinfectant residuals, tepid hot water temperatures, minimal water flow, and an established legionella biofilm might promote substantial legionella growth and dissemination in weeks or months. 44 July 2020 www.cibsejournal.com CIBSE July 2020 p43-46 CPD Hamworthy 165 v2.indd 44 19/06/2020 15:59