Study links watersheds to increase in Legionnaires’ disease


NEW HAVEN — A study by the Yale School of Public Health links certain Connecticut watersheds, including the Naugatuck River, to increased instances of Legionnaires’ disease.

The study’s lead author, Kelsie Cassell, said she was approached by state epidemiologist Matt Cartter when he noticed an increasing trend in the disease statewide.

The Legionella bacterium can be found in freshwater lakes and streams but is spread through the air in the form of mist, according to the Centers for Disease Control and Prevention. It often spreads through showers and faucets, air conditioning cooling towers, hot tubs, fountains, hot water heaters and large plumbing systems.

The disease, which causes a severe form of pneumonia, mostly affects older adults and people with weakened immune systems.

The rate of the disease in Connecticut increased from about one case per 100,000 people in 2005 to closer to two cases per 100,000 people in 2015.

Because the symptoms of fever, cough, chills and muscle aches are similar to other diseases and less than 5 percent of those exposed to the bacteria develop the disease, the disease is likely underreported, Cassell said. She said the disease is treated with antibiotics and may never be tested. The disease is fatal in about 10 percent of cases but can involve long-term complications and hospitalization for those who survive.

Cassell said she started by mapping reported cases of Legionnaires over the past 17 years by ZIP code and soon noticed that certain ZIP codes saw higher instances of the disease.

The disease usually crops up at the end of summer through early October, Cassell said, but she also found that elevated rainfall and greater stream flow were associated with an increased incidence of the disease. She said rising stream temperatures could prolong the period during which people are at risk of contracting the disease.

“Increases in rainfall and humidity netted more cases of the disease about two weeks later, which aligns with the disease’s two- to 10-day incubation period,” Cassell said.

While there have been some big outbreaks, like the one recently connected to cooling towers at Disneyland, a 2015 outbreak in the Bronx and the 1976 American Legion convention in Philadelphia, which gave the disease its name, most cases are sporadic.

The public health department follows up on every case in the state to determine where it may have come from and initiates an outbreak investigation when they see that two people who caught the bacteria had similar activities during the same time, Cassell said.

Cassell’s study found that people living within 5 kilometers of the Naugatuck River had a 1.32 fold increase in the disease and those within 10 kilometers of the Quinebaug River were associated with a 4.73 fold increase compared to the rest of the state.

Just because there’s a correlation between the certain watersheds and the disease, the rivers may not have anything to do with the spread of bacteria, Cassell said.

She said she is working to sample river systems for the presence of bacteria and working on similar studies in nearby states like Rhode Island and Massachusetts to see if the trend is consistent.

There could be other explanations, like the fact that towns and population centers naturally settle along rivers, Cassell said. She said certain power plants or sewage treatment plants which use river water as part of the cooling process may create aerosols that spread the disease. Bacteria can also grow in well water if it isn’t chlorinated, Cassell said.

Heavy rainfalls and subsequent river turbulence could contribute to an increase in aerosolization of the bacteria or contamination of residential water, Cassell said.

“Our findings demonstrate that the natural environment could have a greater role in influencing the risk of disease than previously thought. This contrasts with the common view that building water systems and cooling towers are the main source of exposure for many cases,” Cassell said. “Rivers and watersheds could be proxies for areas of heightened risk due to poorly treated drinking water or well-water use.”


  1. Twice in a year Waterbuty company’s dumped huge amounts of horrible chemicals into the river. Where’s the fines? Dead fish and wildlife for who know how long from this poison.