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Combination of Factors Gives Rise to Legionella: New Detection Methods Support Risk Management
April, 2009
By Pierre Hiernaux
Legionnaires' disease arose as a mystery and has retained that status since its initial identification in 1977. Since then, numerous studies and recommendations have focused on how the disease is transmitted and the appropriate means of controlling it. Yet it continues to cause tragic events despite a better understanding of the phenomenon.
Risk of infection can be intensified by a lack of consensus on the tolerable limits for bacterial population, the choice of detection methods and maintenance procedures for installations. The potential of legal liability for negligence has catalyzed managers to try to assess and control the risk.
Legionnaires' disease is an infection of the respiratory tract caused by Legionella. It occurs in two clinical forms: Legionnaires' disease and Pontiac Fever. The latter is a mild form and complete recovery usually occurs within three to six days. In contrast, Legionnaires' disease is a serious form of pneumopathy that leads to death in approximately 15% of all cases.
Legionella bacteria belong to the Legionellaceae family, which consists of up to 50 species and a total of 70 sero-groups. This bacterium is naturally present in wet environments, colonizing in fresh water and soil.
Its growth depends on multiple factors, such as: the presence of nutrients; biofilms; dead or living micro-organism; debris; and temperature. Its optimal reproduction temperature is between 25 and 45°C. In temperatures below 20°C Legionella bacteria are viable but don’t develop, while they cannot survive in temperatures above 60°C.
Under favourable conditions, Legionella can grow in several artificial environments, such as potable water networks and cooling systems. However, the sole presence of Legionella in water is not enough to cause the disease.
Infection takes place when an aerosol (micro droplets of 5 µm or less in size) formed from contaminated water is inhaled. Inhalation or aspiration of contaminated aerosol brings bacteria to the lungs.
Three conditions must be met for the infection to happen:
- contaminated water containing Legionella;
- dispersion of contaminated water as an aerosol; and
- inhalation or aspiration of aerosol.
The sources of contamination are installations that promote Legionella’s multiplication and dispersal through aerosol. These characteristics are found in all domestic hot water networks and cooling systems that possess dispersion equipment such as:
- Showers
- Cooling towers
- Fountains
- Whirlpool baths
- Humidifiers, atomizers
- Medical equipment generating aerosols
Biocides proven to be effective against bacteria, algae and protozoa combat Legionella by attacking the main factors contributing to their growth such as biofilm formation, corrosion and scale formation. Treatment programs should be validated regularly using several physico-chemical and microbiological indicators to provide relevant information about the treatments’ performance. However, since no correlation exists between Legionella concentrations and the other parameters, they must always be measured separately.
Effective Legionella risk management cannot rely on isolated actions or sporadic massive disinfections. An emerging innovation for the analysis of water in cooling systems and domestic hot water distribution networks uses a gene development technique by real-time PCR (qPCR) to detect and quantify Legionella. In this approach, a DNA-specific fragment to Legionella genus or Legionella pneumophila species is targeted with specially designed primers for these bacteria and then amplified many times with a PCR apparatus until detection and quantification become possible.
Speed is a major benefit of this technique, as the results are available in 24 hours, while the conventional method by culture takes about 10 days. Furthermore it allows the analysis of a portion of bacteria not normally taken into account – viable bacteria that cannot be cultivated.
Legionella is a bacterium that proliferates in systems only within biofilms or inside protozoa (Legionella in that case is highly infectious) and when it is present in water it is under stress when treated with biocides. In these conditions, it can be detected with qPCR, but not by culture.
Even though culture counts remain the benchmark procedure upon which recommendations and reference values published in regulations are based, the qPCR method allows for more regular and frequent monitoring of installations. This gives managers the flexibility to react promptly if there is a drift in Legionella concentrations.
Pierre Hiernaux, PhD, Microbiology, specializes in Legionnella Risk Management with Magnus Chemicals Ltd. For more information, see the web site at www.magnus.ca.
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