Life-threatening infections spread by a life-saving medical device used in open heart surgery may be more widespread than first thought.

The Centers for Disease Control and Prevention on Thursday issued an alert  warning heart-surgery patients to seek medical care if they are experiencing symptoms associted with infections, such as night sweats, muscle aches, weight loss, fatigue or unexplained fever.

At least 28 cases of a bacterial infection, including four deaths, have been linked to a commonly used heater-cooler device used during bypass surgery to control a patient's blood temperature, most of them in Pennsylvania.

Thousands of patients in the U.S. have been notified regarding the contaminated heater-cooler devices, but the number who were exposed to the M. chimaera bacteria might be much larger, according to the CDC.

Three patients who underwent heart surgery at Penn Presbyterian Medical Center were identified as having been infected, along with three at Penn State Milton S. Hershey Medical Center. In October, 2015, WellSpan York Hospital reported that eight of its patients had contracted similar infections in recent years, and that four of them died. Hospital officials have since learned of four additional cases. Five cases were reported in Iowa and two more in western Michigan.

The devices, Stockert 3T machines, are used in about 250,000 bypass operations in the U.S. each year.  The units have been widely used for decades, but researchers did not definitively connect them with infections until last year.

The CDC said the contamination had been to the 3T factory in Germany where the machines were built. Tests conducted by the manufacturer in August 2014 found M. chimaera on the production line.

Researchers have found that the fan on the heater cooler machine is able to blow bacteria from inside the machine into the operating room, said Michael Bell. the deputy director of the CDC's Division of Healthcare Quality Promotion.

"The problem with that is if those bacteria land on a heart valve that is about to be implanted or a surgical wound, there's a possibiltiy of it causing an infection," Bell said. "It's important to see your clinician if you are having any symptoms related to an infection."

In Seattle last month, physicians reported that the devices may pose yet another kind of infectious threat. At the University of Washington Medical Center, an investigation found that heater-coolers were contaminated with bacteria that cause Legionnaires' disease.

Overall, the risk of infection is thought to be very low. In hospitals where at least one case was found, the risk of additional infections was thought to be between 1 in 100 and 1 in 1,000, according to the CDC. Patients who had prosthetic implants are believed to be at higher risk. 

The bacteria, refered to as NTM, are slow-growing and an infection may take more than eight weeks to develop before it is identified,. Diagnoses may several years after surgery. The bacteria is common in soil and water and rarely makes healthy people sick. The infection cannot be spread from an infected patient to another person.

Heater-coolers have been in widespread use for decades to heat or cool the blood of patients on a heart-lung bypass machine, needed for procedures such as open-heart surgery. Temperature is modulated by circulating water that does not come into contact with the patient's blood, so the device was not thought to pose a risk of infection.

Infectious-disease experts now say otherwise, because small amounts of water can become aerosolized and escape through a vent in the device.

The Associated Press contributed to this report.