the developed world have reduced exposure to the virus so that, when it
does occur, it tends to be later in life when the illness is more severe.
The incubation period varies between two and six weeks. During this
period the virus multiplies in the cells of the gut epithelium before it is
carried by the blood to the liver. In the later part of the incubation period
the virus is shed in the faeces. Early symptoms are anorexia, fever,
malaise, nausea and vomiting, followed after a few days by symptoms of
liver damage such as the passage of dark urine and jaundice.
Like other enteric viruses, hepatitis A is transmitted by the faecal–oral
route. Primarily it is spread by person-to-person contact but food-and
waterborne outbreaks do occur. Milk, fruits such as strawberries and
raspberries, salad vegetables such as lettuce, and shellfish are common
food vehicles. With the exception of those caused by shellfish, common
source outbreaks are usually due to contamination by an infected food
handler. The long incubation period of the illness often makes identifi-
cation of the source extremely difficult. For the same reason, it is difficult
to say with any accuracy what proportion of hepatitis A cases are
transmitted by food, although it has been estimated that about 3% of
cases in the United States are food or waterborne.
The agent of enterically transmitted non-A, non-B hepatitis has now
been designated hepatitis E virus and molecular biology studies indicate
it is a calici-like particle with an unusual RNA structure. It too is
transmitted by the faecal–oral route and produces illness after an incu-
bation period of 40 days.
8.5.3 Gastroenteritis Viruses
A number of different viruses have been implicated in gastroenteritis by
their presence in large numbers (up to 10^8 –10^10 g^1 ) in diarrhoeal stools.
In most cases it has not proved possible to culture the virus thus
preventing their full characterization. As a result, classification has been
based largely on morphology and geographical origin.
Although other, better characterized, viruses such as rotavirus, calici-
virus and astrovirus are also known to cause diarrhoea, it is these less
well-defined agents that are responsible for most outbreaks of foodborne
gastroenteritis where a virus is identified. In the United States they were
originally known as Norwalk-like agents after the virus which caused an
outbreak of gastroenteritis in schoolchildren in Norwalk, Ohio in 1968.
In the UK, they were described as small round structured viruses
(SRSVs) based on the fact that, when viewed in the electron microscope,
they are particles about 25–30 nm in diameter possessing an amorphous
structure lacking geometrical symmetry (Figure 8.22). They are now
classified as a distinct genus, Norovirus (NoV), within the family
Caliciviridae.
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