Foodborne viral gastroenteritis is characterized by an incubation
period of 15–50 h followed by diarrhoea and vomiting which persists
for 24–48 h. The infectious dose is not known. Studies in model systems
have suggested that doses as low as one cell culture infectious unit can
produce infection but in polio vaccination an oral dose of 100 000
infectious units is given to ensure a success rate of at least 90%.
The onset of symptoms such as projectile vomiting may be very
sudden and unexpected and this can contribute to the further spread
of illness (see below).
8.5.4 Sources of Food Contamination
The importance of viral gastroenteritis is clear from the huge under
reporting revealed by the Infectious Intestinal Disease Study in England.
The degree to which this is foodborne is uncertain since a considerable
amount of human to human transmission must also occur. Estimates of
the proportion of viral gastroenteritis which is foodborne, made in the
UK, Australia and the USA, vary between 10% and 40%.
Enteric viruses may be introduced into foods either as primary con-
tamination, at source where the food is produced, or as secondary
contamination during handling, preparation and serving. It is possible
that salad vegetables fertilized with human excrement or irrigated with
sewage polluted water could be contaminated with viruses while in the
field. Salads and fruits such as raspberries have been implicated in
outbreaks, though in some cases this could also have been the result of
Figure 8.22 Small round-structured viruses (SRSVs), magnification200 000 (Photo:
H. Appleton)
304 Non-bacterial Agents of Foodborne Illness