Legionella Control

Section 2

THE CHAIN OF INFECTION

The mere presence of legionellae in a water distribution system does not necessarily imply a human health risk. For human infection to occur, certain conditions are necessary. These conditions are referred to as the “chain of infection” consisting of six links. All the links have to be present for disease to occur (Figure 2.1). The first link, the pathogen, was discussed in Section 1.

Chain of infection

2.1       SOURCES AND RESERVOIRS

Legionellae are natural inhabitants of water, found a wide range of habitats. They are ubiquitous in streams, lakes and rivers. They also survive in dust, soil and mud. In fact, one of the species, Legionella longbeacheae, is so often isolated from potting soil in Australia that soil has been suggested as the natural habitat of this particular species.¹

            

Figure 2.2 Some natural sources of Legionella

Legionellae from these natural environments can be transmitted to man-made water  systems by various means. For example, from raw water, during water treatment, as part of  post-treatment after-growths within water distribution systems, during building and construction activities and during plumbing repair.²

Once established, they can persist in the water supply for long periods of time and are difficult to eradicate. Therefore, their presence must be considered in all aspects of the design, operation and maintenance of buildings. For this to be effective, cooperation between engineers, occupational health practitioners and microbiologists is essential.

Figure 2.3 Man-made sources of Legionella 

Water sources that provide optimal conditions for Legionella growth can be separated into those containing “non-potable” and those that contain “potable” water.

2.1.1    Non-potable water distribution systems

Heat rejection devices like cooling towers, evaporative condensers and HVAC (heating, ventilation and air-conditioning) systems are often implicated as sources of legionellosis. They contain reservoirs filled with warm, recirculating water that makes them ideal for the growth, amplification and dissemination of micro organisms (including Legionella). In a typical water-cooled system air in induced through or blown over, packing material down which water, ciruclating from a pond under the packing, is allowed to fall by gravity, producing a large wetted surface that cools the falling water.

The constant fall of water through the tower, the large area of the basin, fill, pipes and heat exchanger, the warm temperature of the water, the high relative humidity and high organic content within these devices provide conditions that favour contamination by algae, protozoa, fungi and bacteria. The risk is increased further by the open nature of the systems, excessive aeration and the constant addition of fresh water to make up for water lost through evaporation.

In systems that are not regularly cleaned, sludge accumulates in the reservoir and slime adheres to water covered surfaces, resulting in the presence of large concentrations of micro-organisms, including legionellae, on these surfaces. In addition, water temperatures below 60°C, the age and configuration of the system, the pH of the water and the presence of certain metals may also increase the risk of contamination.

Water derived from municipal supplies but subsequently stored in cisterns, or conditioned prior to heating, is considered non-potable due to the deterioration in chemical and bacteriological quality during storage. Colonisation of such non-potable sources inside large buildings, such as hotels, factories or hospitals, may be a major cause of legionellosis.^4,5,6

2.1.2    Potable (domestic) water distribution systems

Legionellae are often present in potable water supplies, especially in the hot water sections of these systems. The organisms may enter potable water supplies from the main source, even from municipal water, and survive standard treatment protocols because most municipal water systems are not routinely screened for the presence of legionellae and the organisms are chlorine tolerant. Once inside the system, they find a suitable environment to multiply and are usually very difficult to eradicate.

Legionella levels can rise from very low to very high within short periods of time. The factors that give rise to these fluctuations are not well understood and often very hard to determine. These factors include the age and configuration of the pipes, the degree of scaling and sediment and the potential for biofilm formation within the system increase the risk of contamination. Water temperatures of 25 – 42°C, stagnation and the presence of certain free-living amoebae capable of supporting the intracellular growth of legionellae are often mentioned as amplifying factors in published reports. The biofilm and scale that form on surfaces in water distribution systems provide nutrients for legionellae and protect them from hot water and disinfectants. Some materials used in these systems, for example neoprene washers, are more readily colonised than others (Table 2.1). Building location may also play a role in the colonisation of potable water with legionellae.

Hot water tanks are often colonised with legionellae, especially at the bottom where a warm zone may develop and scale and sediment accumulate.⁷ Hot water piping, especially deadlegs, presents an additional risk as legionellae thrive in stagnant water.

2.1.3    Soil

Outdoors, the soil may be contaminated through contact with Legionella-polluted water and become a source of airborne bacteria during earth moving operations, such as construction work.

Table 2.1 Materials used in the construction of potable water lines and fixtures

Very good	Copper
Good	Neoprene Other synthetic materials
Reasonable	Steel
Not recommended	Rubber Plastics

2.2       AMPLIFICATION

Legionellae are usually present in low numbers in natural sources. However, certain factors present in man-made reservoirs can promote Legionella growth and amplification. To improve our understanding of Legionella, its potential to cause disease and how to better control the organisms in water systems, we must understand these conditions. The most important factors amplifying Legionella numbers in man-made reservoirs are listed in Table 2.2.

Table 2.2 Amplifying factors for Legionella in man-made sources and reservoirs

TEMPERATURE	Legionellae can survive over a wide range of temperatures (20 – 60°C) with an optimum growth temperature of 37 – 45°C as illustrated in Figure 2.2
pH	Legionellae can survive and multiply at a pH of 5.0 – 8.5
STAGNATION	The presence of stagnant water in distribution systems increases the risk of Legionella contamination
WATER TREATMENT	The type of water treatment used may affect the numbers of legionellae present in a distribution system
DISINFECTANTS	The type, concentration and persistence of residual disinfectants in the system may affect the numbers and types of legionellae present
CHEMICAL PARAMETERS	The presence of organic carbon and certain metals like zinc and copper may influence the risk of Legionella contamination
RELATIVE HUMIDITY	Legionellae survive best at 65% relative humidity (RH) and are least stable below 55% RH
SLIME, ALGAE AND PROTOZOA	Amoebae, cyanobacteria and flavobacteria have been associated with the growth of legionellae in water distribution systems
CORROSION PRODUCTS	Legionellae numbers are usually highest in sections where corrosion products are present
CONSTRUCTION	Major construction has been associated with outbreaks of legionellosis. It is believed that legionellae are released from the soil during excavations from where they can enter the cooling tower of the building, air intakes or water pipes, or may be inhaled directly. Another possibility is that, during construction, nutrients already present in dust and dirt may become more readily available for the organisms. In new buildings, plumbing should be flushed before use. Renovated buildings may contain stagnant water, that should be flushed out before returning the building to normal use.
WATER PRESSURE	An increase in water pressure may send dirt into a system, providing a food source for the bacteria or may dislodge scale and sediment containing legionellae from pipes.
BIOFILM, SCALE AND SEDIMENT	In hot water systems the concentrations of legionellae are usually highest within biofilms and at the openings of water outlets. Biofilms are widespread in nature, and on most man-made surfaces when submerged in water. Legionellae are not only able to survive in biofilms, but can proliferate and attach to susceptible hosts like protozoa, depending on the type of material on which the biofilm is formed. For example: ·         Legionellae have been found in biofilms forming on plastic surfaces in water piping systems. At a temperature of 40° they were shown to account for approximately 50% of the total biofilm flora; ·         Legionellae are less likely to be present on copper surfaces because copper generally do not support biofouling. If present, the bacteria are usually found in small numbers; ·         Metal plumbing components and associated corrosion products provide iron and other metals needed for Legionella, thereby supporting their survival and growth. Sediment and scale assists in the growth of supporting organisms and free-living protozoa, increasing the risk of contamination by legionellae. For this reason, its presence often indicates a possibility of Legionella contamination. Algal slime also provides a stable habitat for their survival and multiplication.

DISINFECTANTS

After disinfection, municipal water supplies usually travel several kilometers before it reaches the point of use. During this course, disinfectant residuals diminish and there is increasing exposure to potentially biofilm-contaminated piping. Although municipal water systems are required to be disinfected at their points of distribution to conform to existing standards for bacterial disinfection, these standards are based upon the absence of coliform bacteria and do not include any specific testing requirements for Legionella.

2.3       TRANSMISSION

After growth and amplification of legionellae to potentially infectious levels, the next requirement in the chain of infection is to achieve transmission of the bacteria to a susceptible host. Modern technology like cooling towers used to recirculate water for air-conditioning and humidifying purposes and other ventilation systems can cause the formation and distribution of aerosols through which the organisms can spread. Transmission can also occur through direct installation, aspiration or ingestion (Table 2.3).

Table 2.3 Dissemination of Legionella bacteria [relook this table!]

AEROSOLISATION	The most widely accepted theory for Legionella transmission is that the organism is aerosolised from a water distribution system and is inhaled as droplets of respirable size. Aerosolisation may affect persons outside as well as inside a building.
DIRECT INSTALLATION	Respiratory therapy devices have the potential to disseminate legionellae by delivering gases at high temperatures and volumes, and by generating concentrated aerosols, enabling them to bypass the normal defence mechanisms of the respiratory system.
INGESTION	The fact that diarrhoea is one of the symptoms of Legionnaires’ disease suggests the gastro-intestinal tract as the source in some cases. After infection via this route, the bacteria may spread to the respiratory tract, thereby causing Legionnaires’ disease.
ASPIRATION	Another well documented mode of transmission that effectively gets bacteria into the lungs is aspiration (a “choking process” that often occur during drinking, swallowing or dlearing the throat, and during respiratory therapy). Evidence suggests that it may be a more common mode for Legionella dissemination than previously believed although it has not been widely described in the literature. REMEMBER! Never presume that  you cannot get Legionnaires’ disease from drinking water containing Legionella.

2.4       INFECTION AND HOST SUSCEPTIBILITY

Infections caused by Legionella species are collectively known as legionellosis and include Legionnaires’ disease and Pontiac fever. Subclinical (asymptomatic) infections have been reported. Legionellosis occurs worldwide, in people of all ages and race groups, with no evidence of person-to-person spread of infection. It is most common in summer and autumn months. The incidence of legionellosis varies from country to country and from region to region. Recently, an increase in the worldwide incidence of reported legionellosis cases has become evident. This may be explained by the availability of improved diagnostic and testing methods, increased awareness of the symptoms and improved surveillance. However legionellosis, especially sporadic cases, is still not always reported to public health authorities, making it difficult to estimate its true incidence.

The mode of transmission, inoculum size, particle size and host susceptibility influence the severity of infection. Approximately half of the currently known Legionella species are implicated in disease, but L pneumophila is still considered to be the causative agent in about 80% of diagnosed cases. However, this picture might change as the number of available diagnostic tests increases – it is thus important to regard all legionellae a potentially pathogenic until proven otherwise.