Biology Reference
In-Depth Information
VIRUS
The incidence of food-borne outbreaks of viral gastroenteritis is still
unknown. Viral disease transmission to humans via consumption of
seafood has been known since the 1950's (Roos, 1956), and human enteric
viruses appear to be the major cause of shellfi sh-associated disease.
Presently there are more than 100 known enteric viruses, which are excreted
in human feces and fi nd their way into domestic sewage. However, only
a few have shown to cause seafood-associated illness according to Kilgen
and Cole (1991). These are: Hepatitis-type A (HAV), Noro (Norwalk virus,
small, round structured), Snow Mountain Agent, Calicivirus, Astrovirus
Non-A and Non-B.
Viruses are inert outside the living host-cell, but they survive. This
means that they do not replicate in water or seafood irrespective of time,
temperature or other physical conditions. Their presence on seafood is
purely as a result of contamination either via infected food handlers or
via polluted water. Shellfi sh, which are fi lter-feeders, tend to attract virus
from the water in which they are growing. Large amounts of water pass
through active shellfi sh (up to 1,500 L/day/oyster) according to Gerba
and Goyal (1978), which means that the concentration of virus in the
shellfi sh is much higher than in the surrounding water.
Epidemiology and Risk Assessment
The infective dose is probably much smaller than that of bacteria for
causing foodborne disease (Cliver, 1988). The minimum infection dose of
some enteric viruses for man is close to the minimum dose detectable in
laboratory assay systems using cell cultures (Ward and Akin, 1983).
Animal/human bodies are sources of enteric viruses. The viruses are
found in large quantities in the feces of infected persons a few days to
several weeks after ingestion/infection depending on the virus. Direct or
indirect fecal contamination is the most common source of contamination
of food.
Bivalve molluscs dominate the list of food vehicles in outbreaks of
viral diseases. However, another important vehicle involves ready to eat
food prepared by infected food handlers. The available data show that
almost any food that comes into contact with human hands and does
not subsequently receive a substantial heat treatment may transmit these
viruses.
With only few exceptions, all reported cases of seafood-associated viral
infections have been from consumption of raw or improperly cooked
molluscan shellfi sh (Kilgen and Cole, 1991). However, there is clear
evidence that HAV has been transmitted by unsanitary practices during
processing, distribution or food handling (Ahmed, 1991). These seafood-
