What is legionnaire’s disease?

Legionnaire’s disease is a pulmonary infection due to a bacterium called legionella. Legionella are present in the biofilm that coats hot water pipes (biofilm is present in limestone deposits), in microorganisms like ameba, in certain parts of cooling towers and evaporative condensers. They multiply when the temperature is between 25 and 40°C.

The contamination of an individual occurs when water-droplets that are contaminated by a sufficient concentration of a pathogenic strain of legionella are inhaled. The size of the droplets must be smaller than 10 µm (1 µm = 1/1000 of a millimeter) for the bacteria to penetrate into the pulmonary air cells.

The disease generally infects fragile individuals such as children or the elderly and also those who have been weakened by another disease (cancer, diabetes, immunodeficiency...).

Legionnaire’s disease has caused numerous deaths in the world since its identification in 1976. Cooling towers have often been designated as being the cause of epidemics not only in France but also in other countries. In France, many recent epidemics show that cooling towers may not be the cause:

- Epidemic in Paris in 1998 (20 cases, 4 deaths) and in 1999 (8 cases, 1 death)

- Epidemic in Ille et Vilaine in 2001 (22 cases, 4 deaths)

- Epidemic in Meaux hospital in July 2002 (22 cases, 2 deaths)

- Epidemic in Sarlat hospital in July 2002 (31 cases, 2 deaths)

- Epidemic in Montpellier in 2003 (31 cases, 4 deaths)

- And in Lens also in 2003 (85 cases, 14 deaths)

 

Number of cases of legionnaire’s disease declared in France since 1988.

Legionella

Legionella are bacteria (bacillus gram negative), aerobic, found naturally in rivers, lakes and sometimes in the soil. The size of legionella is between 0.5 and 0.7 µm in width and [1 or more] µm in length. The concentrations measured in a natural environment are most often very important (from 10³ to 10⁷ UFC/l).

43 species that encompass 65 serogroups of legionella have been identified. The most common form is Legionella pneumophila from serogroup 1 which is responsible for 80% of cases of legionnaire’s disease.

Their discovery is mainly done through a breeding technique. The results are calculated by the amount of unit forming colony (UFC) per liter. The limit for detection is between 50 and 100 UFC/L depending on the laboratory.

Their growth rate is relatively slow and can vary between 3 and ten days depending on the species. Their optimal growth temperature varies between 30 and 40°C. They survive below 2°C and can multiply above this temperature. One must reach 50°C to destroy the bacteria (several hours) and around 60°C, the destruction time gets very short (counted in minutes). The bacteria can sustain different levels of pH.

Survival and development of legionella in water

 

These bacteria can equally infest amebas that possess a resistance towards many disinfectant products.

Iron, zinc and potassium in small amounts can encourage their development.

Legionella seen through a microscope.

Biofilm

Biofilm is the deposit that is found inside water pipelines but also in reservoirs, filters and pumps. It contains minerals from water, by-products of corrosion in the pipeline and microorganisms. Biofilm is a safe sanctuary for legionella where they can reproduce at high levels sufficient to contaminate the water. The contamination of the water occurs when the water’s pressure is high enough to rip the bacteria off of the pipe’s surface.

Cross section view of a steel pipe that shows heavy limestone deposits favorable to the development of biofilm.