Get Caught Up: Michigan reports jump in Legionnaires’ disease cases in 2021

State reports 569% increase from July 1-14, 2020 to 2021

A micrograph showing lung tissue infected with Legionnaires' disease
A micrograph showing lung tissue infected with Legionnaires' disease (CDC/William Cherry via Wikimedia Commons)

DETROIT – Michigan health officials are investigating a notable increase in Legionnaires’ disease cases across the state this summer, and excessive rain and flooding appear to be the culprit.


Get Caught Up” is ClickOnDetroit’s Saturday news review to help readers catch up on the biggest stories of the week.


According to MDHHS, between July 1 and July 14, 107 cases of Legionnaires’ disease have been reported in 25 counties. While increases in cases are often related to environmental factors, including heat and rainfall, this represents a 569% increase from referrals from the same period in 2020 (16 cases) and a 161% increase from referrals from the same period in 2019 (41 cases).

Confirmed cases include 19 in Wayne County, 17 in Oakland County and in the City of Detroit and 15 in Macomb County.

“Recent weather trends including rain, flooding and warmer weather may be playing a role in the rise of reported legionellosis cases this summer,” said Dr. Joneigh Khaldun, MDHHS chief medical executive and chief deputy director for health. “We want everyone to be aware of Legionnaire’s disease, especially if they may be at higher risk for illness and we ask that healthcare providers remain vigilant, and test and treat appropriately.”

What to know about Legionnaires’ disease

Legionella bacteria causes two forms of legionellosis respiratory infections. Legionnaires’ disease is an infection with symptoms that include fever, cough and pneumonia. A milder form of legionellosis, Pontiac fever, is an influenza-like illness without pneumonia that resolves on its own.

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MDHHS and local health departments are working to inform healthcare providers of the increase in cases and share information regarding testing and treatment. While legionellosis cases are most common in the summer and early fall when warming, stagnant waters present the best environment for bacterial growth in water systems, this increase is higher than expected for Michigan for this time of year. MDHHS is requesting that providers keep this in mind with patients that present for care with pneumonia and existing risk factors for this infection. To date, no common sources of infection have been identified.

Transmission to people occurs when mist or vapor containing the bacteria is inhaled. Legionnaires’ disease does not spread person to person.

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Risk factors for exposure to Legionella bacteria include:

  • Recent travel with an overnight stay.
  • Recent stay in a healthcare facility.
  • Exposure to hot tubs.
  • Exposure to settings where the plumbing has had recent repairs or maintenance work.

Most healthy individuals do not become infected after exposure to Legionella. Individuals at a higher risk of getting sick include the following:

  • People over age 50.
  • Current or former smokers.
  • People with chronic lung disease.
  • People with weakened immune systems from diseases, such as cancer, diabetes or liver or kidney failure.
  • People who take immunosuppressant drugs.

Individuals with any concerns about Legionnaires’ disease or exposure to the Legionella bacteria should talk to their healthcare provider.

Legionella bacteria are found naturally in freshwater lakes and streams but can also be found in man-made water systems. Potable water systems, cooling towers, whirlpool spas and decorative fountains offer common environments for bacterial growth and transmission if they are not cleaned and maintained properly. Warm water, stagnation and low disinfectant levels are conditions that support growth in these water systems.

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About the Author:

Ken Haddad is the digital special projects manager for WDIV / ClickOnDetroit.com. He also authors the Morning Report Newsletter and various other newsletters. He's been with WDIV since 2013.