which is then consumed without adequate cooking, often after an
intervening period in which microbial growth occurs. This was the cause
of a major outbreak affecting an international airline in 1984. The
outbreak involved 631 passengers and 135 crew and was due to contam-
ination of an aspic glaze by a member of catering staff who returned to
work after illness but was still excretingSalmonellaEnteritidis PT4.
Direct person-to-person spread by the faecal–oral route is also possi-
ble but is usually restricted to institutional outbreaks such as occur in
hospitals, old people’s homes, and nurseries.
Food animals may acquire salmonella infection on the farm from wild
birds and rodents, but the principal sources are other animals, which may
be symptomless excreters, and contaminated feeding stuffs (Figure 7.8).
Measures that can be taken to minimize transmission between animals on
the farm include good animal husbandry, protection of feeds and water
from contamination, hygienic disposal of wastes, and maintenance of a
generally clean environment. Transfer ofSalmonellabetween animals is
particularly associated with situations where animals may be stressed and
crowded such as during transport, at markets, and when in lairage at the
slaughterhouse. It is best minimized by avoidance of overcrowded con-
ditions, ensuring a clean environment, and otherwise limiting the stress to
animals on such occasions.
An important factor in maintaining the cycle ofSalmonellainfection
in food animals has been the practice of using animal by-products as
animal feeds such as meat and bone meal. The heat process that these
materials undergo in their conversion to feeds should destroy any
salmonellas present. Nevertheless they are subject to post-process con-
tamination either in the plant or on the farm by contact with unprocessed
material or with bird and rodent faeces. The importance of animal feeds
Figure 7.8 TheSalmonellacycle of infection
(Reproduced with permission from WHO, 1983)
Chapter 7 245