A second Pennsylvania hospital has reported a cluster of unusual infections in patients who underwent open-heart surgery, prompting the state Department of Health to require immediate replacement of "heater-cooler" devices that have been linked to similar illnesses elsewhere.

Penn State Hershey Medical Center said Tuesday that in the last four years, three open-heart surgery patients had become infected with nontuberculous mycobacteria - a common bug found in tap water and soil that rarely causes illness.

The hospital said that there was a very low chance that additional cases would turn up, but that it was sending information about the risks to 2,300 additional patients who had undergone open-heart surgery during that time.

Last month, WellSpan York Hospital reported that eight of its patients had contracted similar infections in recent years, and that four of them died.

Those infections were attributed to bacteria in water that circulates through heater-coolers - devices that are used to modulate a patient's body temperature when they are on a bypass machine during heart surgery.

The units have been widely used for decades, but researchers did not definitively connect them with infections until this year - in part because the bacteria in question are slow-growing, so patients may not experience symptoms until months later.

The York cases first came to light because three of their patients were later treated at Penn State Hershey, where physicians noticed that all three had the same kind of infection and had been at the same hospital.

Penn State notified York, which then uncovered additional cases in an investigation. Penn State also undertook a review of its own heart surgery patients and found the three cases announced Tuesday, said Carol Freer, that hospital's chief medical officer.

She said the dogged detective work by her colleagues will help others in the field.

"I think we've done health care a favor by doing this," Freer said. "It's putting the pieces together."

Penn State said tests of its six heater-coolers did not find any evidence of the bacteria, but state health department officials said that upon learning of the three infections on Nov. 5, they required the hospital to replace its devices just in case.

The water circulating in the units does not come in direct contact with the patient, but infectious-disease experts think infections may arise when bacteria are "aerosolized" through the device's exhaust vent.

Two of the three Penn State patients died, but the hospital said that they already were gravely ill before exposure to the bacteria, and that the deaths had not been linked to the infections. At York, however, officials said the infections were "likely a contributing factor" in four patient deaths there.

Still unclear is how many other hospitals may find cases. Last month, the U.S. Food and Drug Administration said it had received 32 reports worldwide of contamination or infection associated with heater-cooler devices, most of them in Europe. The agency said it knew of four deaths and 11 serious injuries associated with the heater-coolers.

Biomedical engineer Lawrence Muscarella, an infection-control consultant based in Montgomeryville, said other cases were likely to emerge. Among other reasons, the instructions for cleaning are so complex as to invite the cutting of corners, he said.

No cases have been reported in New Jersey, but state health departments in that state and in Pennsylvania have urged hospitals to be on the lookout.

Among those following through are Our Lady of Lourdes Medical Center in Camden and Main Line Health, which are among their respective states' highest-volume health systems for open-heart surgeries. Officials at both said they had looked back at several years' worth of patients and found no evidence of a problem.

The units at Penn State Hershey were made by Sorin Group, an Italian device maker that recently merged into a larger company called LivaNova.

Freer said the hospital replaced its units with some made by Sorin and some from another manufacturer, in case of a recall by either.

Earlier this year, Sorin issued enhanced maintenance instructions for its devices - box-shaped units that measure less than three feet tall. The company told customers that in addition to periodically cleaning the units' tanks with bleach, technicians also should fill them with filtered water and hydrogen peroxide.