US salmon may carry Japanese tapeworm, scientists say
If you eat raw or undercooked fish, you risk developing an infection from parasites.
One of the most gruesome is tapeworm, a species of digestive tract-invading parasites that includes Diphyllobothrium nihonkaiense, or the Japanese broad tapeworm.
Though this worm was commonly believed to infect only fish in Asia, a study published Wednesday in the Centers for Disease Control and Prevention’s monthly journal Emerging Infectious Diseases says wild salmon caught in Alaska had also been infected by this parasite.
Based on those results, researchers warn that salmon caught anywhere along the Pacific coast of North America may be infected.
Meet the tapeworm
The most common fish tapeworm is Diphyllobothrium latum. In 1986, scientists identified another member of this family, the Japanese broad tapeworm, and believed it had been responsible for about 2,000 infections reported to that point, making it the second most common cause of tapeworm infection.
However, continuing to study the tapeworms using new molecular methods, researchers funded by the Czech Science Foundation discovered they’d been wrong.
Almost all of the previous cases of tapeworm infections occurring in Japan, South Korea and the Pacific coast of Russia had actually been caused by Japanese tapeworms rather than D. latum. In fact, Japanese tapeworm larvae, known as plerocercoids, could be found in salmon caught off the coasts of eastern Russia and Japan.
Could Japanese tapeworms also be infecting salmon caught in the United States?
In July 2013, a team of scientists examined 64 wild Alaskan salmon. After filleting the musculature into narrow slices, the scientists observed these and the internal organs of each fish under a magnifying glass.
They discovered larvae, between 8 and 15 millimeters long, that continually elongated and contracted (as worms are known to do). With gene sequencing, they were identified as Japanese tapeworms.
Based on the study results, four species of Pacific salmon are known to carry Japanese tapeworm infections: chum salmon, masu salmon, pink salmon and sockeye salmon. Because these salmon are exported on ice — unfrozen — and then appear in restaurants around the world, infections caused by the Japanese tapeworm may occur anywhere, from China to Europe, from New Zealand to Ohio.
Few have symptoms
Compared with an infection resulting from D. latum, “we think this Japanese version would not be any different, although very little is actually known about this variant of tapeworms,” said Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University School of Medicine who was not involved in the new study.
Because the Japanese version is from the same family of tapeworms, illness and symptoms should be largely the same, he said.
But D. latum and related species (including the Japanese tapeworm) can grow up to 30 feet long, according to the CDC.
“Actually, most of the people who are infected don’t have symptoms,” Schaffner said. Some feel a little bit of abdominal discomfort, some have nausea or loose stools, and some even lose a little weight.
Most often, tapeworm leads to only minor symptoms, but in exceptional cases the infection can turn into a serious medical problem, according to Roman Kuchta, lead author of the study and a research scientist at the Academy of Sciences of the Czech Republic.
“Massive infections may result in intestinal obstruction” and painful inflammation of the bile ducts, said Kuchta. “The infections can have a substantial emotional impact on patients and their families, because segments are evacuated over a long period of time. More severe cases may require specialized consultations and complementary analyses, which are costly.”
Along with emotional impact and expenses, there’s also the initial shock — terror, really — of discovery.
“The reason you know you have tapeworms is you look in your stool and you find bits of tapeworm floating in the water — and that usually panics you enormously,” Schaffner said. After all, tapeworm infection is very unusual in the United States.
After discovering that you’re infected, you can collect a sample from your toilet bowl and send it to a lab for testing, and then, with your doctor’s help, the tapeworm can be identified and “treated very effectively,” Schaffner said.
According to Dr. Patrick Okolo, chief of gastroenterology at Lenox Hill Hospital in New York, the medications used are not typical antibiotics but specialized drugs targeting specific types of parasites.
Though the study suggests that infections by Japanese tapeworm may be much more common in the US than anticipated, there’s still “no evidence at all about how common it is,” Schaffner said.
“Is this a teeny-tiny proportion of the population, or is this something the average family doctor better learn about?” he asked.
Other options: cooking or freezing
Those who prefer the safe side can stick with adequately frozen or cooked fish, according to the CDC.
“Cooking for 145 Fahrenheit for four or five minutes will destroy the tapeworm,” said Okolo, who was also not involved in the study. “Freezing fish under certain conditions will also destroy the worm and its larvae.”
Schaffner admits the new study has given him “a little bit of pause — because I like salmon sushi.”
He said talk of “emerging infections” or new infections comes about, in part, because new scientific methods are able to identify them.
This view is supported by Jayde Ferguson, a co-author of the new study and a scientist at Alaska Department of Fish and Game.
“The tapeworm itself is probably not new — it’s just that more skilled parasitologist started looking for it. Identifying these parasites is challenging,” said Ferguson. “This was simply a more detailed evaluation of the Diphyllobothrium that has occurred here for over a millennium.”
Still, there’s another important reason “old” infections from one part of the globe emerge as “new” infections on another part of the globe, said Schaffner.
“Because 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.”