Spotlight on Legionella

Control of Legionella in Cooling Towers

Legionnaires’ disease is a serious, sometimes lethal pneumonia, caused by the bacterium Legionella pneumophila, which breeds in water.

The largest and most common source of Legionnaires' disease outbreaks are cooling towers (heat rejection equipment used in air conditioning and industrial cooling water systems) primarily because of the risk for widespread circulation.

Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for controlling the growth and proliferation of Legionella within cooling towers.

Airborne Spread

Recent research in the Journal of Infectious Diseases1 provides evidence that Legionella pneumophila can travel at least 6 kilometers 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, northern France, in 2003-2004. There were 86 confirmed cases during the outbreak, of which 18 resulted in death.

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 kilometers from the plant.

Various microbial control methods have been used to control the spread of Legionella pneumophilia but it requires an approach to biocide usage that takes into account the ecology of Legionella. Ongoing industry research focuses on identifying types and dosage regimes of biocides that control not just the traditionally plate-grown legionella, but Legionella that has been propagated through the bacterium’s environmental host.

1 Source: Nguyen TM, Ilef D, Jarraud S, et al. (January 2006). "A community-wide outbreak of legionnaires’ disease linked to industrial cooling towers--how far can contaminated aerosols spread?" The Journal of Infectious Diseases 193 (1): 102–11. doi: 10.1086/498575. PMID 16323138.