A deadly mushroom outbreak has been reported in Montana after 51 people were sickened with gastrointestinal illnesses and two were killed.
Scientists from the U.S. Centers for Disease Control and Prevention (CDC) launched an investigation after diners at Dave’s Sushi in Bozeman reported diarrhea, nausea, vomiting and abdominal pain shortly after their meals.
The culprit: undercooked morels.
Although mushrooms are not considered toxic, it is the toxins in undercooked, edible wild mushrooms that have led to illnesses.
Undercooked morels have been linked to an outbreak of serious intestinal upset among Montana restaurant patrons last year.
CDC investigators initially struggled to identify what was wrong with the mushrooms that had made people sick.
Calvatia gigantea, commonly known as giant puffball, is a colossal mushroom found in grasslands. Young puffballs have soft, clean white skin and firm flesh when cut.
“Vomiting and diarrhea were reported to be profuse and hospitalized patients had clinical signs of dehydration,” CDC officials wrote in their findings, released as part of the Morbidity and Mortality Weekly Report.
In the three weeks the restaurant served a special salmon and morel sushi roll, 51 people became ill after eating there — three were hospitalized and two died.
Once people began reporting their illnesses in March and April of last year, the CDC began investigating the source of the apparent outbreak.
They discovered that 51 people had eaten at Dave’s Sushi restaurant between March 27 and April 17.
Morels grown in China and imported fresh – not dried – were part of a special sushi roll at the restaurant.
In interviews with people who ate at restaurants during this period, a matched case-control study found that diners who ate morels got sick, while those who didn’t did not get sick.
“Consumption of morels, generally considered edible, was strongly associated with gastrointestinal illness,” The report bed. “A dose-response relationship was identified and consumption of raw morels was more strongly associated with illness than consumption of at least partially cooked morels.”
They also found that the more people ate, the sicker they seemed to get.
Investigators tried to determine what the problem might have been. They collected samples of mushrooms from the restaurant and tested them for the presence of pesticides, heavy metals, bacterial toxins and pathogens.
Morels are a popular mushroom to harvest in the wild, but experts warn that they need to be cooked well.
Although it seemed clear that morels were the cause of the disease, all tests came back negative.
Additionally, six other restaurants that received shipments of morels from the same mushroom seller reported no illnesses among their customers.
What CDC investigators found, however, was that some people ate the mushrooms more cooked, while others ate them more raw.
Other restaurants in the area served morels from the same supplier, but none of their customers got sick.
Depending on the day, the morels were prepared differently for the sushi special.
“On April 8, morels were served partially cooked: hot boiled sauce was poured over the raw morels, after which they were marinated for 75 minutes,” the report reads. “On April 17, the morels were raw and cold marinated before being served.”
Investigators found that people who ate the least cooked mushrooms on April 17 were about nine times more likely to get sick than those who ate partially cooked mushrooms on April 8.
Clearly, the cooking had some effect.
Morels contain toxins that can make people sick if not cooked properly.
There is some debate among scientists about what these toxins are, but many believe they are called hemolysins.
This class of toxins destroys red blood cells, leading to illness and even death.
But hemolysins can be destroyed by cooking mushrooms carefully, experts say.
“Morels should be cooked thoroughly before eating, as cooking is likely to reduce the levels of toxins found in the mushrooms,” the CDC scientists wrote in their report.