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Animation: A recent history of Legionnaires' disease in Michigan

In 2014 and 2015, Genesee County saw the largest outbreak of Legionnaires' disease in at least a decade. The outbreak coincided with the city of Flint's switch from Detroit city water to water from the Flint River (and the subsequent lead exposure crisis).


The Michigan Department of Health and Human Services has focused on the water system of a Flint hospital, McLaren, in its response to the outbreak. More than half of those sickened had been patients or visitors there. McLaren, however, says the outbreak came from a tainted Flint municipal water supply during that period when the city was using the Flint River.

Legionnaires' disease is a severe form of pneumonia caused by a waterborne bacteria called legionella pneumophila. It was first discovered in 1976, after attendees of an American Legion convention in Philadelphia got sick. You can contract Legionnaires' disease by breathing in droplets of contaminated water.


Legionella is found in most freshwater, but low numbers of bacteria don't usually cause disease. A problem occurs when the bacteria grow and multiply. Legionella grows best under certain conditions. For example, it prefers a temperature range of 77 – 108 degrees Fahrenheit. It also does better in water with depleted levels of disinfectant (like chlorine).


Large buildings with complex plumbing are particularly susceptible to legionella growth. In 2016, the Centers for Disease Control and Prevention released a guide for managing water systems to combat the growth of legionella in high-risk facilities (including hospitals). During a 2016 investigation by the state and the CDC, McLaren Flint was not following some of the CDC's recommended practices. For example, temperatures in the hospital's hot water system sometimes weren’t high enough to keep the legionella from growing.


According to some scientists, conditions in the Flint municipal water system were good for legionella to thrive. The lead crisis occurred because no corrosion control agents were added to the water, allowing lead to leach from old pipes into the water supply. Another effect of improper corrosion control is excess iron in the water, as well as chlorine depletion. Iron actually feeds the growth of legionella, and chlorine levels that are too low won’t kill it.



MDHHS keeps track of cases of Legionnaires' disease in the state, and they've provided us with their data. We've turned it into maps and graphs that show the distribution of Legionnaires' disease in Michigan over the last 10 years. While a few counties have seen spikes over the years, the 2014/2015 increase in Genesee County is the only situation in the last decade that the state of Michigan considers an "outbreak."


"There have been a couple of instances over the past few years, decade or so, where we've seen a higher number than normal of legionella cases, as well as deaths in various counties,” said Angela Minicuci, a spokeswoman for the Michigan Department of Health and Human Services, “(but) nothing to the degree in which we saw in Genesee county in 2014 and 2015.”


Minicuci said the state has seen an overall increase in Legionnaires’ cases since 2013, which could be the result of increased testing.


"In 2013 we went from seeing baseline about, just under 200 (cases) across the state to over 200 and beyond the last few years, and I think a lot of that does have to do with that increased awareness and testing and better understanding of Legionnaires’ disease."


Cases have increased nationally, as well.


"The number of cases reported to CDC has been on the rise over the past decade,” the CDC website says. “This could be due to a true increase in the frequency of disease due to a number of reasons (e.g., older U.S. population, more at-risk individuals, aging plumbing infrastructure) or increased testing for Legionnaires’ disease."


Correction: An earlier version of this story's graphics indicated that 91 people were officially sickened as a part of the 2014/2015 Genesee County Legionnaires' disease outbreak. That was incorrect. The official tally is 90. The graphics have been updated to reflect this.

Kaye is an alumnus of Michigan Tech's environmental engineering program. She got her start making maps for the Traverse City-Based water news organization Circle of Blue, and, since then, she's been pretty devoted to science communication and data visualization.
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