Agriculture Reference
In-Depth Information
serotypes are identified in any one year in the United
Kingdom. They are members of the Enterobacteriaceae,
are Gram-negative and can readily grow on a wide range
of media including foods. They are temperature sensitive
and readily destroyed by cooking.
The different serotypes are identified by means of
the somatic (O) and flagellar (H) antigens using the
“Kauffmann-White scheme”. Further sub-typing can be
carried out by phage typing and, increasingly, molecular
methods such as plasmid profiling.
Some salmonellae usually only affect a single animal
species including, for example,
S. typhi
in humans,
S.
abortus ovis
in sheep and
S. pullorum
in poultry. Most
salmonella species including those associated with food
poisoning can infect many species of animal, although
they may not cause illness in all of these. A good example
of this is
S. enteritidis
, now the most common salmonella
in humans, which is widespread, but usually causes no
illness in poultry.
Within the EU,
S. enteritidis
and
S. typhimurium
sero-
vars account for approximately 70% of confirmed cases
of human infection.
While not as resistant as the spore-forming organisms
such as the
Clostridium
spp.,
Salmonella
spp. can exist for
many months in the environment, especially if protected
from extremes of temperature and sunlight. This means
that recycling through the environment is an important
route for animal (and human) infection (Fig. 12.2).
This ability of salmonella to exist within different
self-contained compartments makes eradication difficult.
Control is a more practical option, although specific
salmonellae such as
S. pullorum
have to all intents and
purposes been eradicated in commercial poultry. There is
considerable ongoing effort being expended by the
poultry industry to reduce salmonellae causing human
illness, in particular
S. enteritidis
.
Source of human infection
The lack of a routine
discriminatory typing scheme has often hindered
investigation of the source of human infections. Unlike
salmonellosis, most cases of campylobacter infection are
not recognised as part of an outbreak, and detailed inves-
tigation is not carried out. The exceptions to this are out-
breaks associated with milk and water, when hundreds of
patients can be involved. The Advisory Committee on the
Microbiological Safety of Food investigated infections
with campylobacter and reported in 1993 that poultry
was the most common vehicle of infection. While it is
now considered that 60-80% of human infection can be
linked to poultry, it should be remembered that the types
of campylobacter affecting humans can also be isolated
from all species of animals (including pets) and a wide
range of foods including, milk, water and cooked meats.
Person-to-person spread is also important.
The low infective dose for campylobacter means that
cross-contamination is a particular risk since a relatively
small amount of contamination may be sufficient to
establish human infection. This cross-contamination
must be prevented throughout the whole food chain
with efforts being made to reduce the prevalence of the
bacteria on farm through good biosecurity as well as
through interventions during transport of the birds, in
the slaughterhouse and during storage and retail.
Salmonella
spp.
Within the EU,
Salmonella
are the second most common
food-borne pathogen with around 100,000 confirmed
cases annually. Given the constraints upon surveillance
discussed earlier, this figure has been estimated by the
EFSA Panel on Biological Hazards to represent a true
incidence of 6 million cases.
The salmonellae constitute a large group of over 2200
different serotypes, although only 100-200 different
Abattoir
Human food
Environment
Wildlife
Imported
food
Food animals
Man
Sewage
Imported
livestock
Immigrants
Imported
feed
Animal feed
Figure 12.2
Salmonella recycling in food animals.
