The VA is warning thousands of veterans across the country that they may have been exposed to a surgical heater-cooler infection risk.

Letters went out in December 2016, notifying veterans who underwent open-heart surgery at VA medical centers about a risk of infection that has been linked to a heater-cooler device used in certain types of open heart surgery.

The device in question is the LivaNova PLC Stöckert 3T cardiac heater-cooler. The Stöckert 3T system uses temperature-controlled water to keep the patient’s blood at a certain temperature during surgery.

The same device was previously branded as the Sorin 3T heater-cooler, before its manufacturer Sorin Group Deutschland was acquired by Italian company LivaNova PLC.

The Stöckert 3T heater-cooler is in widespread use in operating rooms around the country. More than half the health care facilities in the U.S. that perform open-heart surgery use this particular device. It’s used on an estimated 250,000 patients per year in the U.S. who undergo open-heart surgery.

The heater-cooler system is designed so that the water it uses does not come into contact with the patient during surgery. However, testing and inspections have revealed that mycobacteria can sometimes get inside the device and colonize.

These mycobacteria may be ejected into the air of the operating room through the device’s exhaust vent. They may then be able to migrate to the open surgical site on the patient’s body.

It’s possible these devices have been contaminated since before they left their manufacturer’s facility. In September 2014, testing at the Sorin manufacturing facility revealed the presence of mycobacteria.

CDC Issues Warning About Heater-Cooler Infection Risk

The same heater-cooler infection risk was addressed in October 2016 in an announcement from the Centers for Disease Control.

According to the CDC, these heater-cooler devices may create a risk that patients could be infected with nontuberculous mycobacterium, or NTM. As many as half a million former open heart surgery patients nationwide could now be exposed to this heater-cooler infection risk, the CDC says.

Since the CDC issued this warning, the VA and other health care providers have been sending out their own warnings to individual patients.

The FDA also addressed the issue in a communication issued October 2016. The agency issued instructions to health care providers on what they could do to prevent the spread of NTM infection through the use of these devices.

NTM infection grows so slowly that it can take months or even years to develop. For that reason, warnings are going out to patients whose surgeries occurred as far back as 2012.

Infection symptoms may not appear until long after the infection has begun to develop. Symptoms can include fatigue, muscle aches, weight loss, night sweats and fever that is not otherwise explainable.

The risk of infection is reportedly very low. However, several deaths have already been reported in conjunction with these NTM infections.

So far, two confirmed cases of NTM infections have been reported in Minnesota, according to that state’s Department of Health. About 10,000 patients in that state were exposed to potential heater-cooler infection risk between 2012 and 2016, the department says.