Study: Parasites In 10% Of Salmon Sushi

Parasites were found in 10 percent of the 279 samples of salmon sushi purchased at Seattle-area restaurants in a recent study, a government researcher reports.

The parasites all were dead - and thus harmless to humans. But researcher Ann Adams said the findings illustrate the value of this advice: If you eat sushi, make sure the fish was once frozen.

Because the fish in sushi is uncooked, a sushi eater can become infected with the potentially harmful Anisakis nematode if the live parasite is in the fish.

This thin, whitish worm, about an inch long, is not destroyed by human stomach juices but is killed by freezing, Adams said. The fact that only dead parasites were found in the sampled sushi indicates all the fish had been frozen, she said.

King County Cooperative Extension Agent Jan Grant recommends freezing fish at or below zero degrees for at least four days to kill parasites, if the fish is to be eaten raw.

Adams, a parasitologist with the Food and Drug Administration's Seafood Products Research Center in Bothell, conducted the FDA parasite study.

Fewer or no parasites were found in samples of sushi made with three other kinds of fish, Adams said. Those that were found were not alive. Sampled were:

-- Mackerel sushi. Parasites were detected in one out of 19 samples, with 135 samples tested.

-- Tuna sushi. No parasites were found in 201 samples. There have been reports in other states of anisakiasis (human infection with the parasite) involving tuna, but it's not known what kind of tuna dish was involved, Adams said.

-- Rockfish sushi. No parasites were detected in the 69 samples. However, because rockfish sushi is less common and the sampling was small, the results may not be representative, said Adams.

The samples were collected by ordering take-out sushi from 32 of the approximately 50 sushi-serving restaurants in the Seattle area, said Adams. She believes the results probably would have been about the same with sushi purchased anywhere in the country.

"There's a lot of misinformation out there" about parasites in sushi, she said. For instance, some people believe that a good sushi chef can see the worms and discard the fish.

"But even when we were looking for them," the worms went undetected until subjected to special laboratory procedures. One of them: using enzymes to "digest" the fish flesh, leaving only the worms.

Adams said scientists don't know how likely someone is to get anisakiasis from a one-time exposure to fish containing live parasites.

About 50 cases of anisakiasis are on record nationally, said Adams. But there actually could have been more cases because doctors aren't required to report them to the Centers for Disease Control. Also, the infection is sometimes misdiagnosed as ulcers, appendicitis or even stomach cancer. Although not known to be fatal, anisakiasis can cause serious discomfort.