I was shocked to read the headline “Legionnaires’ disease incidence quadruples in US.” The CDC announced this week that the number of Legionnaires’ disease cases in the United States has quadrupled from 2000 to 2014. The CDC now estimates that there are approximately 5,000 cases and 20 outbreaks of Legionnaires’ disease reported annually in the United States.

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The disease was discovered 40 years ago this July during one of the largest respiratory disease investigations conducted by the CDC in U.S. history.  Since then, researchers have learned the bacterium Legionella can cause severe pneumonia among people who inhale contaminated droplets of water and is associated with a 10% mortality rate. Those who are most at risk for developing the infection include the elderly and patients who are immunocompromised or have other underlying health conditions.

In a recent MMWR, Laurel E. Garrison, MPH, of the CDC’s Division of Bacterial Diseases and National Center for Immunization and Respiratory Diseases, and colleagues reported that the incidence of legionellosis, including Legionnaires’ disease and Pontiac fever, increased from 0.42 cases to 1.62 cases per 100,000 persons in the U.S., yielding a 286% increase from 2000 to 2014. Most cases of these cases, however, can be prevented with improvements in water system management.

Overall, 415 cases and 65 deaths occurred during the outbreaks (2000 to 2014), all caused by Legionella pneumophila. The most frequent locations of outbreaks were hotels and resorts, accounting for 44% of the outbreaks. Most of the deaths (85%), however, occurred in health care settings.

Although the most common source of exposure to Legionella was potable water, such as water used for showering, outbreaks linked to cooling towers were associated with a larger number of cases.

To improve water management systems, the CDC developed a toolkit called “Developing a Water Management Program to Reduce Legionella Growth and Spread in Buildings: A Practical Guide to Implementing Industry Standards.” The guidance is a simpler interpretation of ASHRAE Standard 188, a document that sets industrial standards for building engineers, which can be utilized by building owners and managers and public health officials. It provides a checklist that will help those working in building safety to identify areas where Legionella can grow and ways to reduce contamination risks. The toolkit was first implemented in Flint, Michigan, where 91 cases of Legionnaires’ disease were detected in 2014 and 2015. There have been no cases of Legionnaires’ disease reported in the area this year.