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Food Safety Focus (177th Issue, April 2021) – Incident in Focus

Food Poisoning Caused by Consumption of Marinated Raw Crabs and Cockles

Reported by Mr. Arthur YAU, Scientific Officer,
Risk Communication Section, Centre for Food Safety

In early March 2021, the Centre for Food Safety (CFS) investigated two epidemically linked food poisoning cases referred by the Centre for Health Protection (CHP). Two clusters, with five persons affected, suffered from gastrointestinal symptoms such as abdominal pain, diarrhoea and vomiting after their dinner at a restaurant in Mong Kok. Four victims sought medical attention but did not require hospitalisation.

The epidemiological assessment of the CHP suggested that the suspected causes were norovirus and Vibrio parahaemolyticus in suspected food items marinated raw crabs and marinated raw cockles. The crabs and cockles served were purchased from a wet market and were not intended for raw consumption. The cockles were only briefly blanched for less than a minute, and the crabs underwent no heat treatment after dressing. The items were then marinated with soya sauce, garlic and chilli, and refrigerated overnight before serving. After on-site investigation, the CFS had immediately instructed the premises to stop selling the uncooked seafood dishes. No further outbreaks concerning raw seafood were later on reported at this premises.

Figure 1: Marinated raw mud crabs (left) and marinated raw cockles (right)

Hazards from Consumption of Raw Aquatic Products

Raw seafood is a kind of high-risk foods. There are various types of hazards, including microbiological hazards and parasites, from eating seafood raw. Marinating, which adds flavours to foods by soaking them in a seasoning liquid, can neither draw enough moisture from foods to prevent bacterial growth nor kill any parasites present. Basically, marinating cannot replace thorough cooking.

Bacteria and Viruses

Raw aquatic products may harbour harmful microorganisms, including both bacteria (e.g. Vibrio parahaemolyticus (VP)) and viruses (e.g. norovirus). These microorganisms may be naturally present or introduced through environmental contamination. VP prevails in estuaries and coastal areas. Therefore, some seafood, such as crabs and bivalves, is inevitably contaminated with VP. Norovirus is a prevalent foodborne virus commonly found in contaminated seawater, where shellfishes collected may uptake the virus.


Parasites are more likely to be present in wild-caught aquatic animals and certain farm-raised ones that are not exclusively raised in a parasite-free environment. These aquatic products may acquire different parasites. The parasites of concern include flukes (liver flukes and lung flukes), roundworms (e.g. Angiostrongylus cantonensis) and tapeworms (e.g. Diphyllobothrium spp.). Some parasites may cause no or mild symptoms, while some may result in severe consequences later. For instance, some can cause bile duct or intestinal blockage, some can cause symptoms similar to tuberculosis or bronchitis, while others can affect the nervous system.

Preventive Measures

Avoid Consuming Raw or Undercooked Marinated Seafood

Raw fish and crabs may contain harmful bacteria and parasites, while shellfishes are more commonly contaminated with harmful bacteria and viruses. While some practise light cooking of shellfishes, such as cockles, to retain their delicate texture, it cannot kill foodborne pathogens, including antimicrobial-resistant pathogens.

Consumption of raw or undercooked foods is inherently more risky than cooked ones. Susceptible populations, such as pregnant women, infants, young children, the elderly and people with weakened immunity, are at higher risk of being infected or having complications if they consume raw or undercooked foods. They should avoid them and consume cooked foods instead.

Key Points to Note

  1. Marinated raw seafood products are high-risk foods.
  2. Cooking seafood thoroughly is the only way to prevent foodborne diseases caused by harmful microorganisms and parasites.
  3. Seasonings, wine, herbs and spices cannot effectively kill bacteria and parasites.

Advice to the Trade

Advice to the Public