Legionella pneumophila – Flint, Michigan 2014

by Nicolas Puertas, Misghana Kassa, and Sarah Krnjevic


Between 2014 to 2015, Genesee County in Flint, Michigan was subject to an outbreak of Legionnaires’ disease. The county changed its source and treatment of drinking water, and following this switch discovered that the water was contaminated with heavy metals when residents complained about discoloration of tap water and shared symptoms like rashes. Over the course of June 2014 to March 2015, Genesee County reported 88 cases of Legionnaires’ disease and 12 recorded deaths associated with the outbreak. In January 2016, the health departments of the state of Michigan and Genesee County officially announced that 2 clusters of Legionnaires’ disease had occurred in Genesee County between 2014-2015.

Figure 1: Legionella Pneumophila. Source: CDC. This illustration depicts a three-dimensional (3D) computer-generated image of a group of Gram-negative, Legionella pneumophila, bacteria. The artistic recreation was based upon scanning electron microscopic (SEM) imagery [Image]. (2016). Retrieved from https://phil.cdc.gov/Details.aspx?pid=22879

Description of the Disease

Legionnaires’ disease is caused by Legionella pneumophila infection when the immune system is unable to clear the bacteria fast enough. The disease causes pneumonia which induces severe lung damage to the host. The symptoms include: coughing, shortness of breath, fever, muscle aches, headaches, and can also be associated with diarrhea, nausea, and confusion.

L. pneumophila (displayed in Figure 1) is a Gram-negative bacteria that can survive both in the environment and inside of a host. It is a robust bacteria that can live in multiple types of harsh environment for a long period of time. It can withstand temperatures going from 0-68°C and pH from 5-8.5. Legionella species are usually found in biofilms inside of natural and manmade water systems. Biofilms are an aggregation of bacteria with layers of polysaccharides, proteins, and DNA which protect the organism from potentially harmful factors in the environment such as biocides and chlorine.

Source of the Outbreak 

Laboratory tests have demonstrated that L. pneumophila is sensitive to chlorine, which is the case for many bacteria. This chemical oxidizes the bacteria’s cell wall, effectively killing it. For this reason, it is common practice to add chlorine to water in water distribution systems.

The source of the L. pneumophila outbreak was linked to Flint’s new water source, the Flint River, which was contaminated with heavy metals. The water was found to be highly contaminated with lead and iron, which interfered with the water’s disinfection process. When it passed through the water distribution system these metals would react with the chlorine added for disinfection. By occupying the chlorine molecules these metals prevented them from acting as a disinfectant, and more specifically prevented them from killing off any L. pneumophila present in biofilms inside the water system. Additionally, the chlorine induced corrosion when reacting with the metal of the pipes, providing an ideal environment for L. pneumophila growth. These uninhibited biofilms as demonstrated in Figure 2 were the source of the outbreak in 2014.

Figure 2: Pipe cross section with biofilm formation of Legionella. Source: CDC. (2018). What Owners and Managers of Buildings and Healthcare Facilities Need to Know about the Growth and Spread of Legionella  [Image]. Retrieved from https://www.cdc.gov/legionella/wmp/overview/growth-and-spread.html.

Cause of the Outbreak

The outbreak was caused by the change in water source in 2014 from Lake Huron to Flint River. Prior to this change, the number of cases of Legionnaires’ disease was relatively low at 6-12 cases per year in the Flint area. Following the switch, however, the incidence spiked up to 45 cases per year. Residents began complaining about an acrid smell, discoloration of tap water and shared symptoms like rashes. The aspiration of this water highly contaminated with Legionella provided an entry for the bacteria into the lungs of residents of Flint. This would occur if the water was accidentally inhaled from choking or through small droplets sprayed from outdoor fountains or sprinklers. In individuals without a strong immune system, infection and lethal pneumonia would follow, also known as Legionnaires’ disease.

Measures Taken to End the Outbreak

In October 2015, in order to end the outbreak, the county switched back to its original water supply Lake Huron. The incidence of Legionnaires’ disease then decreased to 16 cases in 2016 and 4 cases in 2017. The outbreak ended, although the bacteria has not completely gone from the water system. There are still detectable levels of Legionella in Flint River’s water today (although below the threshold for concern), found in 25 percent of all nationally collected samples in 2018.


This event was recorded as the 3rd largest Legionnaires’ disease outbreak in American history and as a consequence received nation-wide press. Two high ranking Michigan officials belonging to the Department of Health and Human services faced criminal charges for involuntary manslaughter relating to the outbreak of Legionnaires’ disease. The charges stemmed from the delay in alerting the residents of the county about the outbreak. A lapse of communication between stakeholders like public health agencies, water utilities and the general population prevented a swift reaction to the outbreak. Improving communication channels has been identified as a factor in mitigating the spread of future disease. Furthermore, the outbreak highlights problems with aging infrastructure and a lack of corrosive control shared among municipalities across America. After all, it was the lack of corrosion control that triggered the deterioration of the metallic infrastructure which led to the leaching of heavy metals into the water source.  The management of distribution systems and infrastructure upgrades are ultimately central in preventative measures and would benefit from increased funding for public services.


Byrne, B.G., McColm, S., McElmurry, S.P., Kilgore, P.E., Sobeck, J., Sadler, R., Love, N.G., Swanson, M.S. 2018. Prevalence of Infection-Competent Serogroup 6 Legionella pneumophila within Premise Plumbing in Southeast Michigan. mBio. 9 (1): e00016-18. doi: 10.1128/mBio.00016-18 

Centers for Disease Control and Prevention (CDC). 2018. Legionella (Legionnaires’ Disease and Pontiac Fever). Retrieved from: https://www.cdc.gov/legionella/about/signs-symptoms.html#legionnaires

Diederen, B. M. W. 2008. Legionella spp. and Legionnaires’ disease. Journal of Infection. 56: 1–12. doi: https://doi.org/10.1016/j.jinf.2007.09.010

Fraser, D. W., et al. 1977. Legionnaires’ disease: description of an epidemic of pneumonia. The New England Journal of Medicine. 297: 1189–1197. doi:10.1056/NEJM197712012972201

Hersher, R. 2018. Lethal Pneumonia Outbreak Caused By Low Chlorine In Flint Water. Retrieved from https://www.npr.org/sections/health-shots/2018/02/05/582482024/lethal-pneumonia-outbreak-caused-by-low-chlorine-in-flint-water.

Marrie, T. J., Garay, J. R. & Weir, E. 2010. Legionellosis: Why should I test and report? Canadian Medical Association Journal. 182(14): 1538–1542. doi:10.1503/cmaj.082030

Peplow, M. 2018. The Flint water crisis: how citizen scientists exposed poisonous politics. Nature. 559(7713): 180-180. doi: 10.1038/d41586-018-05651-7

Rhoads, W.J., Garner, E., Ji, P., Zhu, N., Park, J., Schwake, D.O., Pruden, A., Edwards, M.A. 2017. Distribution System Operational Deficiencies Coincide with Reported Legionnaires’ Disease Clusters in Flint, Michigan. Environmental Science & Technology. 51 (20): 11986-11995. doi: 10.1021/acs.est.7b01589

Zahran, S., McElmurry, S.P., Kilgore, P.E., Mushinski, D., Press, J., Love, N.G., Sadler, R.C., Swanson, M.S. 2018. Assessment of the Legionnaires’ disease outbreak in Flint, Michigan. PNAS. 115 (8): E1730-E1739. doi: https://doi.org/10.1073/pnas.1718679115


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s