Study highlights role of climate in rising US incidence of legionellosis
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Study highlights role of climate in rising US incidence of legionellosis
Study highlights role of climate in rising US incidence of legionellosis
Chris Dall, MAToday at 2:39 p.m.
Legionella
Public Health
In two neighborhoods in Manhattan this summer, 94 people were sickened, and 11 died, in an outbreak of Legionnaires’ disease. It was the second summer in a row that New York City had experienced a cluster of cases of the severe type of pneumonia, which can cause lung failure and death.
Both outbreaks were linked to cooling towers in the city’s large apartment buildings and office towers that had been contaminated with Legionella bacteria. The warm water in cooling towers can provide a perfect environment for the bacteria, which proliferate if the towers aren’t properly maintained. People can get sick if they inhale the evaporated water the towers emit into the atmosphere.
Hot tubs and fountains, even showerheads and sinks, can also be sources of infection. Outbreaks linked to contaminated cooling towers and other water sources tend to occur in the hotter summer months.
“There’s a seasonality to Legionella,” Natalie Exum, PhD, a professor in the department of environmental health and engineering at Johns Hopkins Bloomberg School of Public Health, told CIDRAP News. “July, August, September is really when we’re getting in the height of Legionella season.”
While summertime Legionnaires’ disease outbreaks make the headlines, most cases are sporadic, with an unknown source of infection. Overall, the incidence of the disease has been rising in the United States since 2000, according to the Centers for Disease Control and Prevention (CDC). Several studies have suggested climate-driven environmental changes, such as rising temperature and increased rainfall, are playing a role by enabling proliferation of Legionella bacteria in soil and water.
A new study in PLOS Global Public Health highlights the role that warming temperatures and increased rainfall are playing in rising incidence of Legionella infection in the United States.
Increased temps, precipitation linked to rising incidence
The study, by Xiang-Yang Han, MD, PhD, a clinical microbiologist, pathologist, and medical director of laboratory medicine at The University of Texas MD Anderson Cancer Center, examined the ecological evolution and main causes of the rising incidence of legionellosis (infection with Legionella) in the United States from 1999 to 2021.Han found that age-adjusted legionellosis rates rose more than sixfold over the study period, from 0.40 per 100,000 people in 1999 to 2.37/100,000 in 2021, with a peak of 2.69/100,000 in 2018.
Using publicly available data on annual and monthly infection rates, temperature, and precipitation, Han found that, compared with the first 100 years of annual temperature and precipitation data (1895-1994), temperature and precipitation levels rose by an average of 7.6% and 4.7%, respectively, from 1995 to 2021.
Regression analysis indicated strong correlations between annual legionellosis incidence rates and cumulative temperature anomalies (ie, the change in temperature compared with the historical baselines) and rising annual precipitation amounts, which combine to promote the growth of Legionella bacteria in the soil.
“It is the global warming in the US over the last 40 years that has driven the rise of incidence,” Han told CIDRAP news. “After 40 years of warming temperatures, the load of the organism in natural water and soil has increased.”
The data also indicated the emergence in 2018 and 2019 of a seasonal peak in legionellosis incidence lasting from early June through October.
Potential role of road exposure
Han’s analysis also found correlations between increased motor vehicle mileage and a rising incidence of legionellosis over the 23-year study period. The combination of increased vehicle miles and more rainfall, he believes, made for more “road exposure” to Legionella bacteria.Han has previously written about this concept. He theorizes that increased heat and more rainfall result in more Legionella bacteria accumulating in puddles on wet roads; the bacteria then get aerosolized by car wheels and can enter vehicles through open windows and car ventilation systems. He suggests that studies in Japan, the Netherlands, and Greece, which have isolated Legionella bacteria in road puddles and car cabin air filters, have established the loop of road exposure.
Han believes that data from 2020 and 2021 bolster this theory. When the COVID-19 pandemic hit and stay-at-home measures were recommended, far fewer Americans were driving to work; Han found that motor vehicle miles fell from 3.3 trillion in 2019 to 2.9 trillion in 2020—a level not seen since 2003. The drop in motor vehicle miles paralleled a drop in legionellosis incidence, particularly during the peak incidence months.
Han noted that weekly cases, particularly in June and July of 2020, were only at 40% of the 2018-2019 level. Cases picked up a bit in the fall of 2020, but they were still much lower than those years, he said. In 2021, when motor vehicle miles rebounded, so did legionellosis incidence.
“We have extensive data here, particularly during COVID: We don’t drive, and the incidence drops,” Han said. He believes road exposure could be a primary source of isolated legionellosis cases.
Exum, who was not involved in the study, said that while road exposure is a plausible route of Legionella exposure, she’s doubtful that it’s a primary source of infection. But she said the literature is clear that climate-induced changes are playing a role in the rising incidence of Legionnaires’.
“There’s increased rain, increased heat, increased cases,” she said. “These are clearly related to climate.”