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Hepatitis A virus in shellfish

Introduction

 

1. Hepatitis A is an infectious disease affecting the liver caused by hepatitis A virus (HAV).
   
2. Food borne transmission through faecal-oral route is the most important mode of transmission for hepatitis A.
   
3. Hepatitis A may be asymptomatic, or produces acute symptoms depending on the age of the infected persons. Adults exhibit more severe symptoms while infants usually show no signs.
   
4. The incubation period of the disease ranges from two to six weeks. Symptoms can include fatigue, poor appetite, nausea, vomiting, diarrhoea, abdominal discomfort, jaundice and tea-coloured urine.
   
5. Because of its unique habitat and special feeding mechanism, shellfish especially bivalve shellfish is an important vehicle responsible for transmission of hepatitis A.
   
 

Risk Assessment and Public Health Significance

 

1. Shellfish is a broad term for all aquatic animals that have a shell of some kind. It generally divided into four groups namely crustaceans (examples: crabs, crayfish, lobster, and shrimp), gastropods (examples: abalone, limpet, snail, and whelk), bivalves (examples: oysters, clams, mussels, and scallops), and cephalopods (examples: octopus, squid, and cuttlefish).
   
2. Shellfish living in water contaminated by sewage can carry the HAV. Because bivalve shellfish are filter feeders, they absorb food particles and nutrients by filtering out the seawater. Hence they tend to concentrate the virus present in the polluted water. Man may contract hepatitis A after consumption of contaminated shellfish that has not been thoroughly cooked. Consumption of bivalve shellfish, like cockles, oysters and clams, is strongly associated with hepatitis A outbreaks.
   
3. Attention to environmental, food and personal hygiene can prevent hepatitis A. When handled properly, shellfish is as safe to eat as any other source of food.
   
 
Control Measures and Surveillance Findings

 

1. Under the Public Health and Municipal Services Ordinance, Cap.132, fresh or frozen shellfish is classified as a restricted food item, which is subject to licensing control.
   
2. At the import level, we strongly encourage importers to obtain health certificates issued by health authorities of the countries of origin certifying that each imported consignment of shellfish is fit for human consumption.
   
3. When a consignment of shellfish arrives at entry points, it may be subject to inspection or sampling by us. If the importer concerned is unable to present a health certificate for inspection, we may take samples from the consignment for examination before its release.
   
4. We take regular samples of shellfish both from the retail outlets and at the entry points for testing of HAV. From 1999 to June 2000, 809 samples of shellfish (including 734 samples of bivalve shellfish) were taken for HAV testing. 92 samples (11.4%) were found unsatisfactory. All these unsatisfactory samples were bivalves. The surveillance findings reflected that bivalve is more vulnerable to HAV contamination than other groups of shellfish.
   
 

Advice to the Public

 
1. Buy only those shellfish which are fresh, with intact shell and free from abnormal odour.
   
2. Do not buy shellfish from illegal hawkers and unlicensed food premises.
   
3. Scrub and rinse the shellfish in clean water.
   
4. All shellfish should be cooked at boiling temperature for not less than five minutes before eating.
   
5. Mud oysters should not be eaten raw.
   
6. If possible, remove the shells before cooking as they impede heat penetration.
   
7. Remove the viscera of the shellfish before cooking.
   
8. When having hotpot, use separate chopsticks and utensils for handling raw and cooked food to avoid cross contamination.
   
 
Advice to the Trade
1. Do not obtain the supply of shellfish from unreliable sources.
   
2. Importers should obtain health certificates from the relevant health authorities certifying the safety of each consignment of shellfish.
   
3. Observe good manufacturing, preparation and hygiene practices.
   
4. Observe hygienic practice during food preparation and storage.
   

 

 

Risk Assessment Section
30 December 2000

 

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