Fish Microbiology and Quality Assurance

Unit 6 - Biological hazards in foods

6.6. Foodborne viruses
The incidence of foodborne outbreaks of viral gastroenteritis is quite common. Viral disease transmission to human beings via consumption of seafood has been known since the 1950s and human enteric viruses are implicated as a major cause of shellfish- associated disease conditions. Presently, more than 100 known enteric viruses are excreted along with faeces by infected individuals and finally find their way in to domestic sewage.
The seafood associated viral infection causing illness are ;

• Hepatitis- typeA (HAV)
• Norwalk virus (small, round structured)
• Snow Mountain agent
• Calcivirus
• Astrovirus
• Non-A and Non-B viruses

Their presence pathogenic viruses in seafood are mainly due to the contamination through infected food handlers and sewage polluted water. Shellfish being filter feeders filter large volume of water (upto 1,500 litres/day/oyster) and concentrate virus from the water. The concentration of virus in the shellfish is much higher than in the surrounding water. The infection with a few number of virl particles can cause food-borne disease.

Infected persons discharge large quantities of virus in the feces and contamination of food occurs by direct or indirect contact with faecal matter. Among the foods the outbreaks of viral diseases is dominated by bivalve molluscs. Infected food handlers are involved in the transmission of the virus and any food that comes into contact with human hands and does not subsequently receive a substantial heat treatment get contaminated by viruses.

Survival of viruses in the environment and in food

• The survival of viruses in the environment and in food is dependent on factors such as temperature, salinity, solar radiation, and presence of organic solids. Enteric viruses survive much longer than coliform bacteria, and for several months in seawater at temperatures <10ºC. Thus, there is little or no correlation between presence of virus and coliforms which are the common indicator bacteria for faecal pollution.All enteric viruses are resistant to acid pH, proteolytic enzymes and bile salts in the gut.
• Hepatitis type A virus is one of the more heat stable viruses and has an inactivation time of 10 min. at 60ºC, thus are able to survive some commonly used culinary preparation methods (steaming, frying).
• Enteric viruses are resistant to some commonly used disinfectants (e.g. phenolics, ethanol, quaternary ammonium compounds), but sensitive to halogens (e.g. chlorine, iodine). Ozone is highly effective in clean water.

Foods involved in viral infection
Seafood-associated viral infections are mainly due to the consumption of raw or improperly cooked molluscan shellfish. HAV transmission has been attributed to unsanitary practices during processing, distribution or food handling. One of the largest outbreaks of food-borne illnesses involving 2, 90,000 cases was reported in China in 1988 due to the consumption of contaminated and inadequately cooked clams.

Prevention of foodborne viral disease

• Prevention of foodborne viral disease mainly relies on measures taken to prevent direct or indirect faecal contamination of food that will not receive a virucidal treatment before consumption.
• Consumption of bivalve shellfish harvested from pollution-free waters or depurated in clean seawater or by cooked.
• Preventing contamination by food handlers by adopting good personal hygiene and health education.
• Preventing food handlers suffering from intestinal infections from handling foods.