Legionella
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 Legionella sp. under UV illumination.
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Legionella adelaidensis |
Legionella is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophilia [1]. Legionella are common in many environments, with at least 48 species and 70 serogroups identified. The side-chains of the cell wall carry the bases responsible for the somatic antigen specifity of these organisms. The chemical composition of these side chains both with respect to components as well as arrangement of the different sugars determines the nature of the somatic or O antigen determinants, which are essential means of serologically classifying many Gram-negative bacteria.
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[edit] Pathogenesis
Legionella have been known for some time to live within amoebae in the natural environment and members of the genus Francisella. Legionella species are the causative agent of the human Legionnaires' disease and the lesser form, Pontiac fever. Legionella transmission is via aerosols—the inhalation of mist droplets containing the bacteria. Common sources include cooling towers, domestic hot-water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources of Legionella include freshwater ponds and creeks. Person-to-person transmission of Legionella has not been demonstrated.[2]
Once inside a host, incubation may take up to two weeks. Initial symptoms are flu-like, including fever, chills, and dry cough. Advanced stages of the disease cause problems with the gastrointestinal tract and the nervous system and lead to diarrhea and nausea. Other advanced symptoms of pneumonia may also present.
However, the disease is generally not a threat to most healthy individuals, and tends to lead to harmful symptoms only in those with a compromised immune system and the elderly. Consequently, it is actively checked for in the water systems of hospitals and nursing homes. In the United States, the disease affects between 8,000 to 18,000 individuals a year.
[edit] Controlling potential sources of Legionella
Common sources of Legionella include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems, domestic hot water systems, fountains, and similar disseminators that draw upon a public water supply. Natural sources include freshwater ponds and creeks.
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 in northern France in 2003–2004. There were 86 confirmed cases during the outbreak, of whom 18 perished. 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.[3]
Several European countries established a working group known as the European Working Group for Legionella Infections (EWGLI)[4] to share knowledge and experience about monitoring potential sources of Legionella. That group has published guidelines about the actions to be taken to limit the number of colony forming units (i.e., the aerobic count) of micro-organisms per mL at 30°C (minimum 48 hours incubation):
| Aerobic count | Action required |
|---|---|
| 10,000 or less | System under control. |
| more than 10,000 up to 100,000 |
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 100,000 | 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. |
Temperature affects the survival of Legionellae as follows:
- 70 to 80 °C (158 to 176 °F) - Disinfection range
- At 66 °C (151 °F) - Legionellae die within 2 minutes
- At 60 °C (140 °F) - Legionellae die within 32 minutes
- At 55 °C (131 °F) - Legionellae die within 5 to 6 hours
- 50 to 55 °C (122 to 131 °F) - They can survive but do not multiply
- 20 to 50 °C (68 to 122 °F)- Legionellae growth range
- 35 to 46 °C (95 to 115 °F) - Ideal growth range
- Below 20 °C (68 °F) - Legionellae can survive but are dormant
The above data can be confirmed in an online article by Reliance World Wide.[5]
Control of Legionella growth can be through :
A. Chemical Treatment
1. Adding the required Biocide
B. Non-Chemical Treatment
1. Using Merus-Ring (www.merusonline.com)
[edit] Guidelines for control of Legionella in cooling towers
Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers. Below is a list of sources for such guidelines:
- Centers for Disease Control and Prevention - Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
- Cooling Technology Institute - Best Practices for Control of Legionella
- Association of Water Technologies - Legionella 2003
- California Energy Commission - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
- Marley Cooling Technologies - Cooling Towers Maintenance Procedures
- Marley Cooling Technologies - ASHRAE Guideline 12-2000 - Minimizing the Risk of Legionellosis
- Marley Cooling Technologies - Cooling Tower Inspection Tips {especially page 3 of 7}
- Tower Tech Modular Cooling Towers - Legionella Control
- GE Infrastructure Water & Process Technologies Betz Dearborn - Chemical Water Treatment Recommendations For Reduction of Risks Associated with Legionella in Open Recirculating Cooling Water Systems
[edit] References
- ^ Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology, 4th Edition, McGraw Hill. ISBN 0-8385-8529-9.
- ^ Winn, W.C. Jr. (1996). Legionella (In: Baron's Medical Microbiology, Barron, S. et al, eds., 4th Edition, University of Texas Medical Branch. ISBN 0-9631172-1-1. (via NCBI Bookshelf)
- ^ Long-range transmission of Legionella
- ^ European Working Group for Legionella Infections
- ^ What is Legionnaires' Disease?
[edit] See also
[edit] External links
- CDC Divison of bacterial and Mycotic Diseases: Legionellosis
- Directors of Health Promotion and Education page on Legionellosis
- European Working Group for Legionella Infections
- Recent outbreaks of Legionnaires' Disease
- Legionnaires' disease Outbreaks
- Images of Legionella bacteria:
- http://www.repower.ch/INimg/Newsletter/legionella.jpg
- http://newsimg.bbc.co.uk/media/images/38922000/jpg/_38922367_legionella203.jpg
- http://www.chemistryquestion.com/images/Question/legionella.jpg
- Images
- Support groups:
