County health officials said this week they have confirmed three cases of a bacterial infection that has in the past been traced to hospital use of a specific type of heater-cooler during open heart surgery.

All of the Los Angeles County patients identified with the Mycobacterium chimaera—or M. chimaera—bacteria had undergone “open-chest cardiac surgery,” according to the county Department of Public Health. The agency noted that M. chimaera is a slow-growing bacteria, and the infection may not become apparent until months or years after surgery.

It was not immediately clear when or where the three patients in the county had undergone surgery, when they were diagnosed with the bacteria or their current conditions.

County health officials said they were also investigating a possible fourth case.

According to the health department, studies have linked the bacterial infection with the use of the LivaNova 3T heater-cooler device during surgery. As of October 2016, more than 70 cases have been reported worldwide.

Health officials noted that more than 250,000 heart-bypass surgeries are performed using heater-cooler devices, and about 60 percent of them involve the specific brand associated with the infections—meaning the risk of developing the infection “is very low, including at hospitals where a previous infection has been identified.”

The health department is working with the hospitals that have confirmed cases “to increase surveillance and implement control measures.”

“Public health has also been communicating with all hospitals in Los Angeles County that have used these devices to assess compliance with (Centers for Disease Control) recommendations,” according to the agency.