Source Control
The most common sources of Legionella and Legionnaires' disease outbreaks are cooling towers (used in industrial cooling water systems), domestic hot water systems,and spas. Additional sources include large central air conditioning systems, fountains, domestic cold water, swimming pools (especially in Scandinavian countries and northern Ireland) and similar disseminators that draw upon a public water supply. Natural sources include freshwater ponds and creeks. Many governmental agencies, cooling tower manufacturers, and industrial trade organisations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers.
Recent research in the Journal of Infectious Diseases provides evidence that Legionella pneumophila, the causative agent of Legionnaires' disease, can travel at least 6 km from its source by airborne spread. It was previously believed that transmission of the bacterium was restricted to much shorter distances. A team of French scientists reviewed the details of an epidemic of Legionnaires' disease that took place in Pas-de-Calais, northern France, in 2003–2004. There were 86 confirmed cases during the outbreak, of which 18 resulted in death. The source of infection was identified as a cooling tower in a petrochemical plant, and an analysis of those affected in the outbreak revealed that some infected people lived as far as 6–7 km from the plant.
Several European countries established the European Working Group for Legionella Infections (EWGLI) to share knowledge and experience about monitoring potential sources of Legionella. The EWGLI has published guidelines about the actions to be taken to limit the number of colony-forming units (CFU, that is, live bacteria that are able to multiply) of Legionella per litre:
| Legionella bacteria CFU/litre | Action required (35 samples per facility are required, including 20 water and 10 swabs) |
|---|---|
| 1000 or less | System under control. |
| more than 1000 up to 10,000 |
Review program operation. The count should be confirmed by immediate re-sampling. If a similar count is found again, a review of the control measures and risk assessment should be carried out to identify any remedial actions. |
| more than 10,000 | Implement corrective action. The system should immediately be re-sampled. It should then be "shot dosed" with an appropriate biocide, as a precaution. The risk assessment and control measures should be reviewed to identify remedial actions. (150+ CFU/ml in healthcare facilities or nursing homes require immediate action.) |
According to the paper "Legionella and the prevention of legionellosis," found at the World Health Organization website, temperature affects the survival of Legionella as follows:
- Above 70 °C (158 °F) - Legionella dies almost instantly
- At 60 °C (140 °F) - 90% die in 2 minutes (Decimal reduction time (D) = 2)
- At 50 °C (122 °F) - 90% die in 80–124 minutes, depending on strain (Decimal reduction time (D) = 80-124)
- 48 to 50 °C (118 to 122 °F) - Can survive but do not multiply
- 32 to 42 °C (90 to 108 °F) - Ideal growth range
- 25 to 45 °C (77 to 113 °F) - Growth range
- Below 20 °C (68 °F) - Can survive but are dormant, even below freezing
Other sources claim alternate temperature ranges:
- 60 to 70 °C (140 to 158 °F) to 80 °C (176 °F) - Disinfection range
- 66 °C (151 °F) - Legionella die within 2 minutes
- 60 °C (140 °F) - Legionella die within 32 minutes
- 55 °C (131 °F) - Legionella die within 5 to 6 hours
- 20 °C (68 °F) to 45 °C (113 °F) - Legionella multiply
- 20 °C (68 °F) & below - Legionella are dormant
Control of Legionella growth can occur through chemical or thermal methods. The least expensive and most effective control method is keeping all cold water below 25°C (78°F) and all hot water above 51°C (124°F). Copper-silver ionization is a heavy metal, systemic toxin that destroys biofilms and slimes that can harbor Legionella over the long term. To date no copper-silver system has had EPA approved efficacy studies resulting in final EPA approval as a biocide. Chlorination with chlorine dioxide or monochloramine are extremely effective oxidizing biocides. Ultraviolet light is an excellent treatment but it does not leave a residual in the bulk water system. Thermal eradication is a short term marginally effective and expensive method. Ozone is extremely effective oxidizing biocide for cooling towers, fountains and spas treatment.
Read more about this topic: Legionella
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