Why Is Legionnaires’ Disease More Commonly Reported in the Summer?

The number of reported cases reported to the Centers for Disease Control and Prevention (CDC) has been steadily rising since 2000. Although many cases likely go undiagnosed, health departments across the United States reported nearly 8,500 cases of Legionnaires’ disease (LD) in 2018.1 Data shows the incidence of Legionnaires’ disease peaking in the summer months and early fall.2 In July 2018, a Legionnaires’ disease outbreak in Michigan was partly attributed to warm weather.3

Are Legionella bacteria more prevalent during warm temperatures?

Science says that yes, Legionella bacteria are more prevalent during warm summers. The CDC states that Legionella bacteria grow best within a certain temperature range (77°-108°F). In warmer times of the year, water pipes that carry cold water may reach a temperature that allows Legionella bacteria to amplify.4 When the water becomes aerosolized, the odds increase that someone will inhale contaminated water droplets with higher levels of Legionella bacteria.

Additionally, the nutrients and biofilm that allow Legionella bacteria to proliferate are also better able to grow in warm temperatures. Where nutrients are adequate, microbes generally grow more rapidly at warmer temperatures than at colder temperatures. Therefore, warmer temperatures facilitate the growth of opportunistic pathogens, such as Legionella bacteria, in the biofilm.5 The more biofilm grows, the more likely Legionella bacteria will grow.

Bottom Line: Although Legionella bacteria testing should be done year-round to assess water quality, keep a close eye on Legionella bacteria results at your facility during the summer months.

Be sure to contact Barclay Water Management, Inc. for all your Legionella bacteria management needs!

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