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People undergoing heart surgery may be getting infected with a deadly strain of bacteria, spread by machines used to cool blood.

The design of blood-cooling machines is flawed, Daniel Diekema at the University of Iowa told the European Congress of Clinical Microbiology and Infectious Diseases in Vienna, Austria, last week. “This was an infection risk that was hiding in plain sight for decades,” Diekema said.

The risk arises during open-heart surgery when inserting a device, such as a valve or blood-vessel graft. This process requires a machine to cool and later warm up the blood. During the operation, machines contaminated by the bacteria can blow them out into the operating room, where they can land on the devices to be implanted.


It was thought that the microbe, called Mycobacterium chimaera and common in soil and water, was present in only a certain brand of blood-cooling machine, due to factory contamination. But doctors are now reporting that other machines seem to be affected too, and there is no known way of decontaminating them.

The problem is causing alarm among doctors worldwide, because M. chimaera infection is difficult to treat. There are 110 known cases of this happening in heart patients so far, and half of those infected have died.

Indestructible biofilm

The problem is that once M. chimaera gets into an implant, it forms an indestructible “biofilm” which antibiotics can’t penetrate, said Diekema. “It results in continuous reseeding [of the bacteria] in the bloodstream.”

People who show signs of infection are usually treated first with antibiotics, but Diekema said the only solution was additional surgery to replace the implant.

According to the US Centers for Disease Control and Prevention, even if a hospital has had one case of infection in this way, the observed risk to other patients is very low – between 1 in 1000 and 1 in 100.

However, there may have been many more cases than the 110 identified so far. M. chimaera is difficult to grow in the lab, so tests of samples from machines or patients may wrongly come back negative. It can also take months or years before an infected person shows symptoms, such as weight loss, tiredness and night sweats.

Patients warned

Hospitals around the world have started notifying patients who are at risk to tell them to watch out for symptoms, mainly if the type of machine first identified as contaminated was used in their surgery. In the UK, Public Health England has sent letters to doctors warning them that any patient who has had valve replacement or valve repair surgery could be at risk – although they have not specified any particular brand of machine.

A spokesperson for Public Health England says the Medicines Healthcare Regulatory Agency is working with manufacturers of blood-cooling machines to engineer “solutions which could be safely and universally implanted”.

One solution may be to use long tubing, so the machine can be placed outside the operating room, Diekema said. Alternatively, venting the machine’s exhaust outside the room might work.