FRIDAY, March 15, 2024 (HealthDay News) -- A salmon-and-mushroom sushi roll proved deadly for two restaurant patrons in Montana last year, and experts are sounding the alarm on the dangers of undercooked morel mushrooms.

A total of 51 patrons at an unnamed restaurant in Bozeman got very ill last April, including the two fatalities, and researchers soon traced the illnesses to undercooked or uncooked morels.

These mushrooms are generally thought to be edible, but the Montana report shows that cooking morels is essential for safety.

The batch of mushrooms in question were imported from China and distributed to numerous U.S. restaurants. Significantly, "no gastrointestinal illnesses were reported among patrons who ate morels at facilities where they were cooked before serving," noted a team led by Heather Demorest. She's with the Gallatin City-County Health Department in Bozeman.

The outbreak was first noticed on April 17 of last year, when multiple diners at the restaurant in Bozeman became violently ill with nausea, vomiting and diarrhea within an hour of eating salmon-and-morel sushi rolls.

"One restaurant patron was hospitalized and later died; the second died hours after being discharged from a hospital emergency department," Demorest's team reported.

Their deaths, and the subsequent investigation, led to local public health warnings regarding the morels, and a total of 49 other cases of similar (but nonfatal) illnesses linked to dining at the restaurant were reported.

Two other people required hospitalization but survived.

There seemed to be a "dose-response" relationship to the illnesses: The more morels someone had eaten at the restaurant, the sicker they became.

Uncontrolled vomiting and diarrhea can quickly dehydrate a person, and "the two patients who died had chronic underlying medical conditions that might have affected their ability to tolerate massive fluid loss," the team said.

The report also found that even a little cooking of the morels seemed to lessen the severity of illness.

When served to patrons on April 8, the morels were partially cooked (marinated for 75 minutes), but on April 17 they were "uncooked and cold-marinated" before serving, the researchers said.

Both fatalities occurred when uncooked morels had been served and eaten.

The report also noted numerous safety violations at the Bozeman restaurant -- "temperature control issues, improper [cooking] time control and sanitization procedures, and improper storage of personal items."

According to Demorest's team, to ensure safe eating, morels should be refrigerated at a temperature of 40 degrees Fahrenheit or colder, in a "breathable" package (such as a paper bag).

"Morels should be cooked thoroughly before consumption because cooking is likely to reduce toxin levels present in the mushrooms," the report's authors said.

After addressing its health code violations, the Bozeman restaurant reopened last May, although it "elected to stop serving morels."

The findings were reported March 14 in Morbidity and Mortality Weekly Report, a journal of the U.S. Centers for Disease Control and Prevention.

More information

Find out more about the proper collection and cooking of mushrooms at the National Poison Control Center.

SOURCE: Morbidity and Mortality Weekly Report, March 14, 2024