In 2012, a Japanese man with a fondness for chilled salmon came down with what physicians described as a “watery” bout of gastrointestinal distress. This was uncomfortable enough, but his illness took a turn for the shocking. The 40-year-old discovered that a meter-long “tape-shaped object” had “emerged from his anus,” according to a 2012 article in BMJ Case Reports. At that point the man decided to check himself into a hospital.
The cause of the man’s woes was the parasitic Japanese broad tapeworm, also known by its Latin name Diphyllobothrium nihonkaiense. (A treatment of oral anti-worm medication quickly cured the man of his invertebrate resident. It was unclear whether his chilled salmon habit remained intact.) The organism primarily lives within fish, but it can infect humans as well as wolves and bears.
Infectious disease experts have identified the parasite in some 2,000 reported illnesses in Japan and other parts of northeastern Asia. The first such infection occurred in North America in 2008. And although the worms had been found previously in wild Asian fish, according to a new study in the Centers for Disease Control and Prevention’s journal Emerging Infectious Diseases, the tapeworm was recently identified in North American waters off the coast of Alaska.
The researchers behind the study, tapeworm experts from the Czech Academy of Sciences and biologists at the Alaska Department of Fish and Game, concluded that “salmon from the American and Asian Pacific coasts and elsewhere pose potential dangers for persons who eat these fish raw.” They hunted for the tapeworm larva via microscope, and confirmed it was the Japanese species using a recently developed molecular technique.
The fish species in the study involved several types of wild Alaskan salmon: chinook, coho, pink and sockeye salmon, as well as rainbow trout. They found a Japanese broad tapeworm burrowed in the muscles of a Pacific pink salmon, near the fish’s spine. (The researchers also pointed to four types of Pacific salmon that were the principal offenders in past human infections: chum, masu, pink and sockeye salmon.)
Raw foods, broadly speaking, carry more risks than their cooked counterparts. But before any sushi lover’s warning bells start chiming too loudly, the vast majority of raw salmon on the market does not come with the risk of a new type of parasitic infection.
“If it was anything that was of concern, increased risk or anything like that from a management standpoint, we would have said something,” Jayde Ferguson, a Department of Fish and Game ecologist and study author, said to Alaska Dispatch News. The fish represented in the study were wild, and, as such, “they’re going to have parasites, they’re out in nature.”
The incidence of acquiring a tapeworm in the United States, though possibly underestimated, is quite low. (Most Japanese broad tapeworm infections go unnoticed, according to the Centers for Disease Control and Prevention, as the parasites usually cause few symptoms.) There are only about 1,000 new cases of tapeworm from eating beef or pork, for instance, in the United States each year.
The researchers wrote that the goal of the study was not to alarm consumers but to “alert parasitologists and medical doctors about the potential danger of human infection with this long tapeworm resulting from consumption of infected salmon imported (on ice) from the Pacific coast of North America and elsewhere.”
What the study should show, Ferguson said, is that infectious disease experts are improving their ability to distinguish the Japanese broad tapeworm from other sorts of tapeworm parasites.”This worm has always been here, and we’re just getting better at identifying it.”
William Schaffner, who teaches preventive medicine at Vanderbilt University Medical Center in Nashville and was not involved in the research, told CNN that importation and changing dietary habits may transfer old diseases to fresh places. “We do things that we haven’t done before,” Schaffner said. “Now, we have these fresh caught fish that can be transported anywhere and eaten raw. … I am sure we will be on the lookout for this kind of tapeworm going forward.” The Japanese broad tapeworm began appearing in new locations in the middle of the last decade; one Japanese visitor acquired the parasite in New Zealand in 2009, marking the country’s first case.
Heating fish to an internal temperature of 145 degrees is considered safe by the U.S. Agriculture Department. Proper freezing of salmon will kill parasites, too — to 4 degrees below zero for a duration of at least seven days, per CDC guidelines, or frozen solid at 31 below for 15 hours.
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