techniques towards greater use of big-bale ensilage. In big-bale ensilage,
the silage is made in large plastic bags rather than in a single large clamp.
Rupture of the bag or inadequate sealing at its neck can allow mould
growth to occur on the lactic acid present increasing the silage pH to a
value at whichL. monocytogenescan flourish.
Human listeriosis is described in more detail in Section 7.9.3 below.
The recent widespread concern it has caused is largely attributable to the
realization that food is a major source of the infection (a possibility first
suggested as long ago as 1927), the psychrotrophic character of the
organism, and the high mortality rate of the illness. Reported incidence
of human listeriosis increased in several countries during the 1980s, but
remains generally low when compared to other foodborne infections such
as salmonellosis. For example, in England and Wales reported cases of
listeriosis peaked in the late 1980s at around 300 per year while reports of
Salmonellaand Campylobacterinfections numbered nearly 27 500 and
29 000 respectively. Reported cases of listeriosis dropped in 1990 and 1991
to 118 and 131 respectively; a decrease attributed to the effect of Depart-
ment of Health advice to the immuno compromised and pregnant to
avoid soft cheeses and to reheat certain chilled foods adequately and to
withdrawal of contaminated pate ́from a single manufacturer (Figure 7.6).
Reported incidence of human listeriois in England and Wales remained at
around 100 cases p.a. until 2001 when numbers increased to more than
200 cases p.a. in 2003 and 2004. Unlike the earlier peak in 1988, this was
marked by an increase in non-pregnancy associated cases. In the United
States, the Center for Disease Control (CDC) has estimated an annual
incidence of around 1700 cases resulting in 450 adult deaths and 100
foetal and postnatal deaths.
Figure 7.6 Human listeriosis in England and Wales
Chapter 7 225