Legionella: Health Effects, Occurrence and Sampling

Health effects of Legionella

In 1976, in Philadelphia, USA, over 200 attendees of the US-American Legion, developed pneumonia. The disease was later called “Legionnaires’ disease”. The causative agent, a Gram-negative bacterium, was named Legionella pneumophila. Legionella pneumophila causes 85-90% of all cases of Legionella infections (legionellosis). There are over 40 species of Legionella.

Legionella pneumophila can cause very severe infection of the respiratory system. However, Legionnaires’ disease epidemics are rare but the disease is fatal if untreated. The disease may develop within 2 to 13 days (average 5-6 days).

Another form of legionellosis is Pontiac fever, named after an outbreak in 1968 in Pontiac, USA. This form of disease, caused by a number of Legionella species, is milder than Legionnaires’ disease. Pontiac fever develops within 48 to 72 hours and the illness may clear in 2-5 days. No fatal cases have been reported in relation to Pontiac fever. This disease mainly appears as epidemics. Pontiac fever is believed to be a reaction to inhaled Legionella antigens rather than an infection.

Disease transmission

There is no evidence for transmission of legionellosis from person to person or by ingestion. Legionella infection occurs when people inhale the bacterium via fine water droplets as aerosols from the environment. Indoor transmission of legionellosis has been reported via contaminated hot water supplies in hospitals, hotels and other public buildings, respiratory therapy equipment, jacuzzis, spas and air-humidifiers.


Legionella bacteria are part of the natural aquatic bacterial population of lakes and rivers. They are present in all types of fresh water, including tap water. Legionella multiply in water, using other microorganisms like bacteria, algae and protozoa. Their concentration in fresh water is influenced mainly by the temperature. They are isolated more frequently and in higher concentrations from warm water (30 to 50 °C.). However, Legionella also survive at much lower temperatures indoors as well as outdoors. At temperatures above 60 °C Legionella can’t survive.

Sampling Of Legionella

Sampling of Legionella in indoor air or water on a routine basis is not recommended. However, sampling is recommended to:

  • determine the source of outbreaks of legionellosis
  • check the effectiveness of maintenance practices and control measures for hot water supplies and humidified ventilation systems
  • guarantee the safe use of hot water supplies and humidified ventilation systems.

When investigating the water services within a building for Legionella, the condition of pipes, the joining methods used, the presence of lagging, sources of heat, and the standard of protection afforded tanks should be noted, as well as disconnected fittings, ‘dead-ends’, and cross-connections with other services.Water Sampling
Water samples should be collected in sterile autoclavable plastic containers. The samples should be taken from:

  • the incoming supply;
  • tanks;
  • an outlet close to, but downstream of, each tank;
  • the distant point of each service;
  • the water entering and leaving any fitting under particular suspicion.

Surface Sampling
Using swabs, surface samples should be taken from shower heads, pipes and taps. Also, sludge, slime or sediments within building water services or humidifiers can also be collected, particularly where accumulation occurs.

Sample Handling and Storage
Samples should be stored at room temperature (20 ± 5 °C.) in the dark and should be processed within 2 days. That means the samples should be sent to the laboratory within 24 hours. It is also important to confirm with the lab that they have the necessary media before sampling is done.

Air sampling
The presence of Legionella in indoor air can be investigated using Reuter Centrifugal Sampler (RCS) or the Andersen sampler. Regardless of the sampler used, the recommended sampling agar at present is BCYE-agar.


  1. Flannigan, B., R.A. Samson, and J.D. Miller (Editors). Microorganisms in home and indoor work environments: diversity, health impacts, investigation and control. 2001. London, UK: Taylor & Francis (ISBN: 0-415-26800-1).
  2. Wanner, H-U, AP Verhoeff, A Colombi, B Flannigan, S Gravesen, A Mouilleseux, A Nevalainen, J Papadakis, and K Seidel. 1993. Biological Particles in Indoor Environments. Indoor Air Quality and Its Impact On Man. Brussels: Commission of the European Communities. Report No. 12.

For more information on indoor bacteria, please visit http://www.moldbacteria.com/ or call 905-290-101.