A year after a recall of Snoqualmie ice cream linked to listeria food poisoning, a third victim has been sickened by bacteria that lingered inside a machine used to make milkshakes for hospital patients.

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A year after a giant recall of Snoqualmie ice cream tied to listeria, a third illness has been blamed on the bug after it apparently lingered in a machine used to make milkshakes for hospital patients.

A woman in her 40s being treated at the University of Washington Medical Center was diagnosed in November with listeria. When experts did tests, to their surprise, they found the bacteria matched the genetic fingerprint of the germ that sickened two other UWMC patients in 2014.

The common factor? All three drank milkshakes made with ice cream from the same UWMC machine, said Dr. Jeff Duchin, health officer with Public Health — Seattle & King County.

“We’re assuming it’s linked to Snoqualmie ice cream from last year that persisted in the machine,” Duchin said. “The most likely explanation is it persisted in some nook or cranny somewhere where it escaped the cleaning process.”

The 2014 illnesses were traced to an ice-cream mix used by Snoqualmie Gourmet Ice Cream and sparked a voluntary recall of a year’s worth of ice cream, gelato and other products. The plant underwent extensive cleaning before reopening.

However, UWMC isn’t using Snoqualmie mix in their ice cream now. Instead, they use another commercial brand.

The latest patient developed symptoms of an infection and, on Nov. 17, a blood culture was positive for listeria. Because listeria is often spread through food, hospital officials reviewed her diet and realized she consumed several milkshakes a day during her stay.

When genetic tests matched the bug to the Snoqualmie outbreak, investigators realized that the victims last year had consumed milkshakes, too.

No listeria was found in the new commercial ice-cream mix. But the bacteria were detected on several sites inside an ice-cream machine, including the dispenser nozzle, Duchin said.

That means the listeria from the Snoqualmie mix likely remained inside the machine for more than a year, he said. UWMC cleaned and sanitized the equipment twice weekly, which is less often than recommended by manufacturers, health officials noted in a blog post.

The ice-cream machines were used to make milkshakes for patients admitted to UWMC, not in the general cafeteria and other areas of the hospital, Duchin said. UWMC served hundreds of milkshakes to patients in the past year and conducted more than 22,000 blood cultures. Only the patient in November tested positive for listeria, said Tina Mankowski, a UWMC spokeswoman. She has since recovered.

Listeria is a common bug, and about 3 percent to 4 percent of healthy people harbor it in their guts, Duchin noted. It usually doesn’t make people sick, but those with compromised immune systems are particularly vulnerable.

Pregnant women, the elderly and others are urged to avoid foods such as lunchmeat and soft cheeses because of the risk of listeria. Soft-serve ice cream isn’t usually included on the list in the U.S., though officials in Australia and elsewhere warn pregnant women against consuming the treat.

The unusual outbreak at UWMC may underscore the need for more research into a link between listeria and the ice-cream machines, Duchin said.

“I think for people who are really concerned, they may want to consider that these soft-serve ice creams might pose some risk,” he said. “Maybe we’re seeing a signal here.”