Risk Assessment for Legionnaires Disease

Legionella Pneumophila is a respiratory infection that strikes susceptible individuals who are exposed to it.  Infection usually occurs by inhaling aerosols, fine sprays or other microscopic droplets of water.

Legionella Pneumophila are small enough to pass deep into the lungs.  They are deposited in the alveoli, which are the small pockets in the lungs.  Ingestion of Legionella Pneumophila does not always cause immediate illness.  There can be a 2 – 10 day incubation period.  Most reported cases occurred in the 40 – 70 year old age group.  Smokers, people with cancer and chronic respiratory diseases are at a higher risk.  Fatality rates are 15% General Population, 40% Hospital Environment.

Legionella Pneumophila are rod shaped bacteria which are commonly found in all pools of water whether they be a natural or man made aquatic environment.

Conditions that favor the amplification of Legionella Pneumophila growth are:

  • Stagnant water conditions and/or system design configurations that produce stagnation, such as side-arm and dead-leg piping
  • Warm water temperatures between 20 – 50°C (68 – 122°F)
  • Optimal growth is at temperatures between 35 and 45°C (95 – 113°F)
  • Bulk water pH in the range of 5.0 to 8.5
  • Sediment, scale, deposits, biofilm – support not only Legionella growth, but also that of the very important supporting  microbiota for Legionella
  • Microbiota, including algae and many bacteria that supply essential nutrients for growth of Legionella
  • Certain amoebae and other protozoa that harbor Legionella as endosymboints – allowing them to thrive, resist harsh environmental conditions (including biocides) and to significantly amplify.

Legionella Pneumophila uses free living amoebae as well as ciliated protozoa.  From these amoebae they can infect the cells within human lungs under certain circumstances.

Symptoms

Initial symptoms of Legionnaires Disease include high fever, chills, headache and muscle pain.  Dry cough soon develops leading to more difficulty breathing.  Some patients develop diarrhea or vomiting and can become confused or delirious.  Pontiac Fever is a common but less serious infection caused by Legionella.  Symptoms are similar to influenza.

Legionnaires Disease Prevention Plan – Cooling Towers

The scientific knowledge of Legionella and Legionnaires disease has progressed to a point where prevention of disease is a realistic goal.  While 100% guarantees are never possible, minimizing the risk of disease has very real benefits in terms of reducing liability, while promoting public health and safety, and should be a priority item for all building owners and managers.  The strategies necessary to achieve this goal must start with an overall awareness of Legionella, its ecological niches in building water sources (e.g. cooling towers), and the potential for transmission to humans.  A comprehensive plan for minimizing the risk of Legionnaires disease should include:

Assessment of all cooling towers and their location in relation to susceptible populations

Regular maintenance of cooling towers in accordance with manufacturer instructions and/or standard operating procedures

  • use of effective dual alternating biocides via automatic feed
  • drift eliminators checked for efficient operation

Monitoring the levels of viable Legionella on a regular basis in order to evaluate the effectiveness of the maintenance program.  There is no specific infectious density known for Legionella.  In fact legionella is frequently present in water supplies without causing disease.  However, not testing will tell you nothing about your system – until a case of Legionnaires Disease occurs.  We recommend that your company be proactive with Legionella testing and implement an action plan based on results.

Disinfection of cooling towers as necessary in order to eliminate the Legionella (or reduce to background levels)

Total Bacteria in Cooling Towers

The total bacterial numbers, by themselves, are not informative about the status of Legionella in a cooling tower.  Levels above 200,000/ml are sometimes used to indicate general biofouling in the tower water.  Biofouling may lead to interference with heat transfer in the tower, but is not directly related to a health concern.  Following disinfection of a cooling tower, the total bacterial numbers often show a temporary dramatic increase (>200,000/ml), but drop back to regular levels once the microbial ecosystem has been re-established.

Cooling Tower Remedial Action Criteria – Viable Legionella

Very High Risk Situation (levels >5,000 per ml)

  • significant potential for causing an outbreak of Legionnaires disease
  • immediate disinfection of the tower is indicated
  • post-treatment monitoring to determine effectiveness of the disinfection
  • alterations in the current biocide program

High Risk Situation (levels 1,000 – 5,000 per ml)

  • high level of health concern – Legionella at a level associated with outbreaks of Legionnaires disease
  • immediate disinfection of the tower is indicated
  • post-treatment monitoring to determine effectiveness of the disinfection
  • alterations in the current biocide treatment program
  • schedule regular monitoring for Legionella in the future

Moderate Risk Situation (100 – 999 per ml)

  • moderate level of health concern – Legionella approaching the level associated with outbreaks of Legionnaires disease
  • disinfection of the tower is usually indicated as a precaution against any future Legionella amplification
  • post-treatment monitoring to determine effectiveness of the disinfection
  • review current biocide treatment program
  • schedule regular monitoring for Legionella in the future

Low Risk Situation (10 – 99 per ml)

  • low level of health concern – Legionella well below the level associated with outbreaks of Legionnaires disease
  • disinfection of the tower is generally not recommended unless located near susceptible populations
  • review the current biocide treatment program
  • schedule regular monitoring for Legionella in the future

Very Low Risk Situation (<10 per ml)

  • very low level of health concern – background levels of Legionella
  • review location of cooling tower in relation to susceptible population
  • review the current biocide treatment program
  • schedule regular monitoring for Legionella in the future

Legionella – Total Bacteria Ratio

In nature, Legionella are ubiquitous, but constitute a very small percentage of the total bacterial population (i.e. < 1%).  Occasionally in cooling towers, the percentage of Legionella in the tower water increases dramatically, sometimes approaching 100%.  In many of these situations, the level of Legionella is also in the high range.  The tendency for Legionella percentages to increase above 1% would suggest that the Legionella have become more resistant to one or both of the current biocides, and that there is an increased risk of amplifying to higher numbers.  In these situation (regardless of the actual level of Legionella), an alteration to different biocides should be considered.  Recommendations to disinfect would be the same as above.

FA – Positive Bacteria

This number reflects both living and dead Legionella detected by immunofluorscence microscopy.  The test also frequently detects non-Legionella cross-reactive bacteria (i.e. false positive reaction).  Thus, an FA test should never by used as the sole basis of a risk assessment.  Rather, the results give some indication of the recent history of Legionella in a cooling tower. For example, a high level of FA – positive bacteria (in the absence of viable Legionella) suggests the accumulation of dead Legionella from prior colonization.  The Legionella may have been killed by chemical biocides, or other conditions in the tower.  Re-sampling of such a high FA tower in order to confirm the Legionella status would be recommended.

Legionella Risk Assessment

SKASOL performs Legionella Risk Assessments that include a review of your current Legionella control program including microbiocide dosages, overall biocide usage, frequency of addition, types of biocides used and recommendations on their effectiveness. In addition, we assess the design and location of the cooling system and identify potential exposure concerns. For more information, call (800) 839-1000. (M-F 8am – 5 pm PST) If you prefer, send us an e-mail at INFO@SKASOL.COM

Legionella Testing Services

SKASOL offers Legionella testing for our clients. For more information on monitoring your cooling towers for legionella and other bacteria, call (800) 839-1000. (M-F 8am – 5 pm PST) If you prefer, send us an e-mail at INFO@SKASOL.COM