It’s unclear how many cases of diphyllobothriasis there are today: The Centers for Disease Control and Prevention in Atlanta are aware that such infections happen, but according to the CDC’s Susan Montgomery, “there is no systematic reporting of cases in the U.S.”
While diphyllobothriasis is not life threatening, the tapeworm can grow up to 30 feet long and live for decades, sometimes causing anemia from B12 depletion. Most diphyllobothriasis cases are initially asymptomatic, so they often go undetected; people can live with a tapeworm for years without knowing they have one. Some make the unhappy discovery after passing a segment in their stool. Others learn of their cohort after seeking treatment for symptoms such as diarrhea, abdominal cramping, fatigue, and nausea. (Such was the case with Jeff Smith, aka The Frugal Gourmet, one of the first celebrity TV chefs, who confided to writer Alicia Arter during an interview in 1992 that he was being treated for an “enormous tapeworm” he said he got from eating salmon sushi made from salmon that had not been frozen.) If a physician relates these symptoms to the possibility of tapeworm infection, a diagnosis is made and the infection is treated with a prescription medication.
But if a physician has not had a tapeworm patient before, the diagnosis can be challenging. In 2006, a doctor described in The New York Times how his medical partner couldn’t figure out what was causing his normally energetic wife’s fatigue and, as blood tests showed, anemia. There were no gastrointestinal symptoms. The correct diagnosis came when a colleague, after learning that the patient tasted her fresh gefilte fish as she seasoned it before cooking, suggested she might have a tapeworm. (Diphyllobothriasis is sometimes referred to as “Jewish housewives’ disease” for that reason.) Bingo! Diphyllobothrium latum. After treatment, the patient discharged the parasite—a three-foot-long specimen.
How likely is it that the salmon you’re eating contains a tapeworm? According to Tammy Burton, a fish pathologist for the Alaska Department of Fish and Game, the levels of infestation fluctuate seasonally and from year to year, but in general Diphyllobothrium latum is “fairly common” in Alaska salmon. In its Bad Bug Book, the FDA warns that farmed salmon, if they spend time in fresh water, can also acquire the larvae. Farmed salmon from Chile, where juvenile salmon are grown in cages in freshwater lakes, have been implicated in diphyllobothriasis outbreaks in Brazil and elsewhere.
So a better question is: How likely is it that you will get a tapeworm if you eat raw, unfrozen salmon that contains a tapeworm larva? According to Phillip Klesius, research leader at the USDA Aquatic Animal Health Research Laboratory in Auburn, Alabama, “The consumption of one live larva can result in tapeworm infection.” So until you find out if that salmon has been frozen first, it would be best to hold your fork.