Photo/IllutrationAnisakis (Provided by the Tokyo Metropolitan Institute of Public Health)

Parasitic anisakis worms were the number one cause of food poisoning in 2018 for the first time, with the number of patients from the disease known as anisakiasis doubling from the previous year.

Reports of food poisoning owing to anisakis nematodes in internal organs of fish have rapidly increased in recent years, especially from eating raw skipjack tuna, according to data compiled by the health ministry and released March 13.

In 2018, 468 cases among 478 patients were reported, double the 2017 figure of 230 cases among 242 patients.

As a result, cases of anisakis poisoning exceeded food poisoning by the campylobacter bacteria in chickens, with 319 cases among 1,995 patients.

Until now, infection was caused mainly by eating raw mackerel, squid or saury. However, in 2018, infection from raw skipjack tuna increased to 100 cases among 103 patients, 10-fold the previous year's figure.

“Last year, there may have been a lot of food in which anisakis parasitized in seas where skipjack tuna was fished owing to changes in the seawater temperature,” a ministry official said.

A ministry research team is currently investigating the matter.

The ministry listed four measures to avoid food poisoning by anisakis: cook fish at 70 degrees Celsius or higher; place in a freezer at minus 20 degrees Celsius for at least 24 hours; visually check for and remove anisakis larvae; buy fish fresh and remove the internal organs as soon as possible.

Larvae of anisakis, 2 or 3 centimeters in length, parasitize the internal organs of fish and easily spread to muscles of the abdomen when a fish loses its freshness. If such fish is eaten raw, symptoms such as a severe stomachache and vomiting may appear within a few hours.

Anisakis larvae do not die if immersed in vinegar and salt.