Legionnaires’ Disease in a Post-COVID-19 World

Legionella and Covid, Patient on OxygenIn 2021, regional health departments nationwide  alerted the public of the sudden increase of Legionnaire’s Disease in their local areas, many concentrated in the Northeast US. Michigan’s Health Department recorded a nearly 600% increase in Legionella cases compared to 2020 and a 261% increase from 2019 (LeBlanc). Similarly, the Chicago government reported an increase of three times from the 2020 and 2019 recorded cases (Duncan).

Public data available through Google Trends supports this observation. Google Trends is a collection of search interest in a specified search term over a selected course of time. Google Trends reports this data as “Interest Over Time”, where the number of searches for that term is relative to the overall number of searches done on Google in that time frame (Google). This means the higher the number reported, the greater popularity that term has relative to all searches performed on Google at that time point. From November 2020 to November 2021, the search term “Legionella” had an upward trend of search interest (Figure 1), and in the past five years, it also shows an increase in search interest (Figure 2). Legionnaire’s Disease search term interest does not indicate any trend. This may be due to Legionella outbreaks being caught before infection can occur, or that national search term interest increases significantly when the outbreak is at an epidemic scale or tourist destination.

Recognition of Legionnaires’ Disease is a modern occurrence, with the bacteria discovered after the 1976 American Legion outbreak in Philadelphia, PA. Since that time, the CDC has reported an increase in reported cases of Legionnaires’ Disease that has started to escalate. The CDC reports that between 2000 and 2018, the rate of reported cases grew by nearly nine times (CDC). 

This growth may partly be due to increased awareness and testing for the uncommon infection (CDC). Yet with this increased awareness also came new policies and regulations, such as ASHRAE 188-2021, to further reduce infection (ANSI/ASHRAE). The matter was further complicated by the onset of the COVID-19 pandemic in 2020.

Several sources point the blame in the recent surgency to policies associated with COVID-19 shutdowns. Many buildings, and with them their water systems, sat unused for months, giving Legionella bacteria an optimal environment to proliferate. While this likely is a contributor, it does not explain the long-term trend of Legionella cases increasing over the past 20 years (Cassell, Davis, & Berkelman). In this article, I have provided a list of possible contributors to the increase in Legionnaires’ Disease cases within the US.

Increase of At-Risk Demographics

The upward trend of the aging population of the United States may also be a contributor. One of the risk factors for Legionnaire’s Disease is being over the age of 50. The percentage of the population that is over the age of 50 in the United States has increased 67% since 1980 and is projected to double the 1980’s report by 2050 (Statista).

The presence of an underlying condition (i.e Obesity, COPD, Type 2 Diabetes, etc) is another risk factor for contracting Legionnaires’ Disease, which has also increased among Americans. In 2018, Boersma, Black, and Ward found that 51.8% of their 25,417-sample set had at least 1 chronic condition, with 27.2% having 2 or more. This is a 1.7% increase from 2012 and a 5.4% increase from 2001 (Boersma, Black, & Ward).

Climate Change

Several studies have shown that climate change may be a contributor to the recent increase in Legionella outbreaks around the globe. Warm, stagnant water is a hospitable environment to Legionella bacteria. Current trends in climate change have shown that an increase in global precipitation and temperature is conducive for this type of environment (Mazzotta, Salaris, Pascale, Girolamini, & Cristino; Walker).

One study of interest was done by Xiang Y. Han of the University of Texas. In his study, Han discusses the relationship between surplus precipitation, rising temperatures, and traffic-generated aerosols with the elevated density of Legionella bacteria in the environment. He postulates that a combination of these environmental changes is contributing to the upward trend of legionellosis cases within the United States (Han).

Increased Virulence and Co-Evolution Mechanisms

Legionella and Covid, Legionella Lungs

One study by Personnic, Striednig, and Hilbi, found that a subpopulation of sessile, nongrowing Legionella pneumophila bacteria in biofilm still maintained metabolic activity, expression of virulence genes, and antibiotic tolerance. This collection of traits classifies this subpopulation of Legionella pneumophila as microbiological persisters, where growth is arrested or slowed until the stress has lessened in their environment (Personnic, Striednig, & Hilbi).

Culture is considered the gold standard by the CDC and is the primary methodology used in Legionella outbreaks for confirmation and isolation. Culture allows for serotype grouping where most PCR methodologies available do not. There is a potential shortcoming to the culture methodology, however. According to a study conducted by Dietersdorker, et al, Viable But Not Culturable (VBNC) Legionella bacteria is capable of infecting human macrophages and amoebae; the primary vector in Legionnaires’ Disease infection. Their study also showed evidence of infection even after a year of starvation (Dietersdorfer, et al.). Most environmental PCR tests for Legionella are severely limited, with at the time of this post, there is only one known PCR test available commercially that tests viable-only Legionella with serotyping, EST’s vPCR.   


Even with COVID-19 taking the center stage of infection prevention in the United States, pathogens like Legionella are ever-present and require preventative measures just as much, if not more so. Co-infections with COVID-19 and other opportunistic pathogens like Legionella have been observed, with increased lethality to the individual. In a Post-COVID-19 world, it is important to acknowledge the danger that Legionella bacteria can pose to the public, and that appropriate prevention and testing be done to minimize risk.


ANSI/ASHRAE. (2018, June 28). Legionellosis: Risk Management for Building Water Systems. Retrieved from ASHRAE.org: https://www.ashrae.org/file%20library/technical%20resources/standards%20and%20guidelines/standards%20addenda/188_2015_h_20180628.pdf

Boersma, P., Black, L. I., & Ward, B. W. (2020, September 17). Prevalence of Multiple Chronic Conditions Among US Adults. Preventing Chronic Disease, 17, 1. doi:200130

Cassell, K., Davis, J. L., & Berkelman, R. (2021, January 6). Legionnaires’ disease in the time of COVID-19. Pneumonia, 13. doi: https://doi.org/10.1186/s41479-020-00080-5

CDC. (n.d.). History, Burden, and Trends. Retrieved from CDC: https://www.cdc.gov/legionella/about/history.html

Dietersdorfer, E., Kirschner, A., Schrammel, B., Ohradanova-Repic, A., Stockinger, H., Sommer, R., . . . Cervero-Arago, S. (2018, September 15). Starved viable but non-culturable (VBNC) Legionella strains can infect and replicate in amoebae and human macrophages. Water Research, 141, 428-438. doi:https://doi.org/10.1016/j.watres.2018.01.058

Duncan, E. (2021, July 23). Chicago Department of Public Health Tracking Increase in Legionnaires’ Disease Cases in July. Retrieved from Chicago.gov: https://www.chicago.gov/city/en/depts/cdph/provdrs/health_protection_and_response/news/2021/july/chicago-department-of-public-health-tracking-increase-in-legionn.html

Google. (n.d.). Google Trends: Understanding the data. Retrieved from Google: https://newsinitiative.withgoogle.com/training/lesson/4876819719258112?image=trends&tool=Google%20Trends

Han, X. Y. (2021). Effects of climate changes and road exposure on the rapidly rising legionellosis incidence rates in the United States. PloS one, 4. doi:10.1371/journal.pone.0250364

Mazzotta, M., Salaris, S., Pascale, M. R., Girolamini, L., & Cristino, S. (2021). Occurrence of Legionella spp. in Man-Made Water Sources: Isolates Distribution and Phylogenetic Characterization in the Emilia-Romagna Region. Waterborne Pathogens: Epidemiology, Surveillance, Control and Risk, 552. doi:https://doi.org/10.3390/pathogens10050552

Michigan reports big increase in Legionnaires’ disease across 25 counties. (2021, 7 19). Retrieved from Detroit News: https://www.detroitnews.com/story/news/local/michigan/2021/07/19/michigan-increase-legionnaires-disease-early-july-flooding-heat/8017540002/

Personnic, N., Striednig, B., & Hilbi, H. (2021). Quorum sensing controls persistence, resuscitation, and virulence of Legionella subpopulations in biofilms. ISME Journal(15), 196-210. doi:https://doi.org/10.1038/s41396-020-00774-0

Statista. (2020, September). Share of old age population (65 years and older) in the total U.S. population from 1950 to 2050. Retrieved from Statista.com: https://www.statista.com/statistics/457822/share-of-old-age-population-in-the-total-us-population/

Walker, J. (2018, August 29). The influence of climate change on waterborne disease and Legionella: a review. RSPH, 282-286. doi:https://doi.org/10.1177/1757913918791198

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