Health officials are asking hospitals to warn heart surgery patients about a potentially life-threatening infection linked to heating and cooling devices used during some operations.

It’s not the first time the devices have been flagged for infection-related concerns, but the latest warning from the U.S. Centers for Disease Control and Prevention suggests the risk is more widespread than originally thought.

“Although thousands of patients in the United States have been notified regarding potential exposure to contaminated heater-cooler devices, the number who were exposed might be much larger,” the new CDC report warned.

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The Stöckert 3T heater-cooler devices – made by German company LivaNova PLC, formerly Sorin Group Deutschland GmbH – are commonly used during open-heart surgery to keep a patient’s circulating blood and organs at a specific temperature during the procedure.

Schematic representation of heater–cooler circuits tested for transmission of Mycobacterium chimaera during cardiac surgery. Blue arrows indicate cold water flow; red arrows indicate hot water flow and patient blood flow. FDA

There are close to 2,000 of the devices in the United States, according to the U.S. Food and Drug Administration. The CDC said they might have been contaminated during manufacturing.

The bacteria, Mycobacterium chimaera, is a species called nontuberculous mycobacterium, also referred to as NTM. It’s common in soil and water and the odds of it making someone sick are small. Patients who have had invasive procedures and have weakened immune systems are most at risk.

More than 250,000 heart bypass procedures – about 60 percent of the total performed in the U.S. every year – rely on the heater-cooler devices.

Overall, the odds of someone getting the infection are still low, officials said. “In hospitals where at least one infection has been identified, the risk of infection was between about 1 in 100 and 1 in 1,000 patients,” the CDC report said.

It can take months, or even years, for an infection to show up in a patient because NTM is a very slow-growing bacteria.

Patients who had open-heart surgery in the United States before 2014 should be aware of the signs and symptoms of infection, including night sweats, muscle aches, weight loss, fatigue, pus around a surgical incision, or unexplained fever, the CDC said.

At least 28 cases have been identified at hospitals in Iowa, Michigan and Pennsylvania. Cases have also been reported in Europe.

There is no test to determine whether a person has been exposed to the bacteria, but a culture can be performed and after eight weeks – the timeframe needed for the bacteria to grow – a diagnosis can be made or ruled out.

The CDC said doctors should be aware of the risk and suspect NTM infections among patients who have signs of infection and a history of open-chest cardiac surgery.

“While these infections can be severe, and some patients in this investigation have died, it is unclear whether the infection was a direct cause of death. Available information suggests that patients who had valves or prosthetic products implanted are at higher risk of these infections,” the CDC said.

CDC also released a Health Alert Network advisory to help hospitals and health care providers identify and inform patients who might have been put at risk.

“It’s important for clinicians and their patients to be aware of this risk so that patients can be evaluated and treated quickly,” said Dr. Michael Bell, deputy director of the CDC’s Division of Healthcare Quality Promotion.

“Hospitals should check to see which type of heater-coolers are in use, ensure that they’re maintained according to the latest manufacturer instructions, and alert affected patients and the clinicians who care for them.”

Concerned patients and health care providers can contact CDC Info at 800-232-4636 or via its website.