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MONDAY, May 19, 2014 (HealthDay News) -- Windshield washer fluid may contain bacteria that cause the deadly form of pneumonia known as Legionnaires' disease, according to a new study.

Researchers found Legionella bacteria in the windshield washer fluid of 75 percent of school buses they tested in one central Arizona school district.

The investigators also discovered that Legionella bacteria can grow in windshield washer fluid and maintain stable populations in the fluid for up to 14 months, according to the study presented May 18 at the American Society for Microbiology's annual meeting.

"Washer fluid spray can release potentially dangerous numbers of these bacteria into the air. These results suggest that automobiles may serve as a source of transmission for Legionella infections," study author Otto Schwake, a doctoral student at Arizona State University, said in a society news release.

Legionella bacteria, which are naturally occurring and typically found in water, are transmitted to people through mist or vapor from water containing the bacteria. For example, air conditioners and hot tubs can be sources of infection. The bacteria do not spread from person to person.

Most people exposed to Legionella bacteria don't become ill, but some develop Legionnaires' disease. The bacteria can also cause a milder illness similar to the flu.

Schwake and his colleagues decided to conduct their study after previous research suggested a link between cars and increased risk for Legionnaires' disease. One of those studies concluded that nearly 20 percent of Legionnaires' cases in Great Britain were associated with windshield washer fluid.

"This study is the first to detect high levels of Legionella in automobiles or aerosolized by washer fluid spray," Schwake said. "While potential transmission of a deadly respiratory disease from a source as common as automobile windshield washing systems is significant, the study also points to the fact people can be exposed to pathogens [germs] -- particularly those occurring naturally in the environment -- in previously unknown and unusual ways."

According to the U.S. Labor Department, Legionnaires' disease got its name from the first outbreak in which the germ was found to be the cause. This outbreak occurred in 1976 at a Philadelphia hotel where the Pennsylvania American Legion was holding a convention. More than 200 Legionnaires and visitors developed pneumonia, and some died.

Research presented at meetings should be considered preliminary until published in a peer-reviewed journal.

More information

The U.S. National Library of Medicine has more about Legionnaires' disease.