The dangers of eating raw seafood

PUBLISHED : Sunday, 03 April, 2011, 12:00am
UPDATED : Sunday, 03 April, 2011, 12:00am


Tacye asks: Common sense tells us that we should avoid uncooked animal products. But the Japanese enjoy longevity even though they eat a lot of raw fish. Is raw fish actually good for our health?

Wynnie says: As with most animal-based foods, there are health risks associated with eating raw fish. But these can be minimised if proper hygiene and strict regulations are adhered to.

Food poisoning from sushi and sashimi is fairly common, and accounted for 3 per cent of all food poisoning cases reported to Hong Kong's Department of Health in 2000.

Fresh and frozen raw seafood can contain bacteria such as salmonella and listeria, and tapeworm, roundworm or flatworm parasites. It's impossible to see, smell or taste any of this contamination.

The risk of roundworm infection is higher from freshwater fish, such as mackerel and salmon. Parasitical infection is more commonly caused by fish tapeworm: these can grow to just under a couple of metres long and may cause stomach cramps, diarrhoea, nausea, and tiredness.

However, most people don't have any symptoms of parasites, so they may only realise they are infected when they pass bits of the worm during a bowel movement.

To prevent food poisoning from raw seafood and to kill parasite larvae, raw fish must be frozen to at least minus 20 degrees Celsius for 24 hours.

A type of food poisoning not related to parasites but common from eating raw fish is scromboid poisoning. This occurs when the fish get too warm after they're caught. As a result, a chemical substance called histamine builds up in the flesh. Some types of fish such as tuna, mackerel, skipjack and bonito are particularly susceptible. Fish affected with the Scromboid toxin may taste metallic or peppery.

Symptoms of food poisoning start within an hour after eating. They resemble an allergic reaction - tingling and burning feelings around the mouth, facial flushing, sweating, nausea, vomiting, headache, dizziness and a rash.

Eating raw shellfish such as mussels, oysters and clams is more 'dangerous' than raw fish. Naturally occurring microbes like red tide can contaminate shellfish, which build up these toxins in their flesh, so the chances of getting hepatitis (liver inflammation) is high especially for people with liver disease.

Watch out also for marine prawns that are commonly used for making prawn sashimi. Hong Kong's Department of Health warns that these prawn shells are easily contaminated by pathogens. If you love raw shellfish and can't give it up, make sure it's alive when you buy it or only go to reputable restaurants.

Tacye's diary

Food: bread, rice, noodles, chicken and beef during weekdays, seafood during weekends, vegetables, fruit

Drinks: water, green tea, fruit juice

Exercise: 3 hours ballet, 2 hours Chinese drumming, 2 hours playing ball and skipping rope every week

Wynnie Chan is a British-trained nutritionist. If you've got a question for her or would like to be featured in this column, e-mail