Sunday, 14 May 2017

Sushi and Raw Fish, Parasitic Nematodes, and Anisakiasis

Sushi and sashimi can be delicious, but diners should know that eating a meal of raw fish is potentially dangerous. Unless the fish has been adequately frozen before use, it may harbour a parasitic nematode named Anisakis simplex. The nematode attaches itself to the lining of the gastrointestinal tract (or the digestive tract) and often stays alive for some time. It may cause severe pain as well as other unpleasant and sometimes serious problems.


Nematodes are also known as roundworms. Anisakis is a roundworm with a complex life cycle that involves marine mammals, crustaceans, and fish or squid. The basic steps in the cycle are as follows.
  • The parasite reproduces inside marine mammals such as seals and whales. 
  • The eggs of the parasite are released into the ocean in the animals' feces. 
  • The eggs become larvae, which are eaten by crustaceans.
  • The crustaceans are eaten by fish and squid. The larvae migrate to the muscles of these animals.
  • The fish and squid are eaten by marine mammals. Here the larvae mature and the cycle begins again.
  • If the fish and squid are eaten by humans instead of a marine mammal, a nematode infection may result. 

The parasite can only penetrate the outer layer of the lining of the human digestive tract and eventually dies. This may sound like a good outcome, but the problem is that the presence of the worm triggers a strong response by the immune system. Inflammation occurs and a mass of cells is formed in the tract.

Infection, Symptoms and Treatment

Until quite recently, cases of anisakiasis were generally restricted to countries such as Japan, where raw fish is very popular. In the last few years cases have appeared in other parts of the world as the popularity of raw seafood dishes has spread. The disease can develop after eating fish and squid that is raw and improperly prepared. Sushi and sashimi can both be problematic. Sushi consists of raw fish, vinegared rice, and sometimes additional ingredients such as seaweed or other vegetables. Sashimi consists of thin slices of raw fish.

Possible symptoms of the infection include abdominal pain and swelling, nausea, vomiting, diarrhea (which may be bloody or contain mucus) and a slight fever. Some people experience an allergic reaction when they are infected by the parasite. This reaction may involve a rash and itching. Rarely, anaphylaxis may develop. This is a severe and body-wide allergic response that is life threatening and a medical emergency. As with any symptoms and condition, a doctor is needed in order to diagnose the problem.

The treatment for the disorder is often removal of the parasite by endoscopy or surgery. Endoscopy is a process in which a flexible tube with a tiny camera and a light are inserted into the digestive tract so that a doctor can examine the area. Devices can be inserted into the tube to remove items from the tract if necessary. These items include a nematode attached to the lining of the digestive tract.

How to Kill the Parasite

A professional sushi chef will probably know how to prepare sushi and sashimi safely, at least in North America, where I live. Some communities have a law mandating the correct treatment of the fish. People who prepare the fish at home or without training are more likely to cause a problem.

Adequate cooking will kill parasitic worms. According to the CDC (Centers for Disease Control and Prevention), under specific conditions freezing raw fish will also kill the parasites. The CDC says that one of the following procedures for treating the fish should be used. Note that none of them can be obtained in a home freezer.
  • Freeze at -4 degrees Fahrenheit (-20 degrees Celsius) or below for 7 days.
  • Freeze at -31 degrees Fahrenheit (-35 degrees Celsius) until solid and then store at the same temperature or below for 15 hours.
  • Freeze at -31 degrees Fahrenheit (-35 degrees Celsius) until solid and then store at 4 degrees Fahrenheit (-20 degrees Celsius) or below for 24 hours.
It's important that a raw fish lover checks that their preferred source of sushi or sashimi prepares the food properly. If this is done, it should be possible to enjoy the food and stay safe.

References and Further Information

Information about anisakiasis from the CDC

A case report from the British Medical Journal