Agriculture Reference
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with fresh produce have occurred (Mandrell and Brandl 2004). In agreement perhaps,
is the absence of any isolation/detection of C. jejuni on
6,800 produce samples in
recent studies reported (Sagoo and others 2001; Thunberg and others 2002; Moore
and others 2002; Sagoo and others 2003), suggesting that C. jejuni may be of lesser
fi tness compared to E. coli O157 and Salmonella in environments relevant to fresh
produce production and preharvest contamination (Brandl and others 2004).
Nevertheless, high numbers of sporadic C. jejuni illnesses compared to E. coli O157
and Salmonella (MMWR 2005b, 2007b) suggest surveillance to identify food sources
associated with C. jejuni illness, including produce, should be continued.
The results summarized in Table 1.3 confi rm there are multiple livestock and
wildlife sources of pathogens and suggest modes of transport of pathogens for con-
tamination of fresh produce in fi elds or orchards. Livestock are located near produce
production in many locations, but not close enough usually to be considered a major
risk. However, resident wildlife species are potential sources of pathogens also, and
commingle with livestock on ranches, dairies, or feedlots, thus increasing exposure
of livestock and wildlife to pathogens. Wildlife colonized by pathogens will roam and
potentially disseminate them to produce or other locations in the vicinity of produce
(Jay and others 2007). This presents problems for controlling wildlife intrusion into
fi elds depending upon the size and roaming capability of the species. Small mammals
(e.g., squirrels, mice, raccoons), large mammals (feral swine, deer, elk), and birds
illustrate the diversity of population sizes, barriers (fencing height, depth, gage), and
habitat that are issues in considering interventions to control exposure of wildlife to
fi elds. Therefore, only obvious risk factors can be addressed until defi nitive data are
obtained about major sources of pathogen in an environment.
A few conclusions can be drawn from the selected livestock and wildlife inci-
dence data. First, they refl ect the dynamic fl uctuations in the incidence of enteric
pathogens that can occur and that relatively high incidence of certain pathogens may
occur at specifi c times. Second, there appears to be a general trend in higher inci-
dence of S. enterica strains in surveys of animal and environmental samples com-
pared to E. coli O157:H7, a trend consistent with the general amount of illness
reported for these pathogens in the U.S. and U.K. (MMWR 2005b, 2007b; CDR
2006). In contrast, the recurrent outbreaks of E. coli O157:H7, in the absence of any
known Salmonella outbreaks, associated with leafy vegetables grown in the same
region (Table 1.1) is inconsistent with this trend. Perhaps, a study of the incidence of
Salmonella in the environment of leafy vegetable production would provide clues to
explain this paradox.
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High-level Shedding of E. coli O157:H7 and Salmonella by
Some Animals
Measuring the prevalence of pathogens in animals and other environmental reservoirs
relevant to produce production are informative, but the concentration and total amount
of pathogen disseminated is perhaps more relevant to identifying potential risks in a
produce production region. However, quantifying pathogen in complex samples is
diffi cult due to the inability to survey livestock and wildlife populations comprehen-
sively and to obtain accurate values with environmental samples containing low
concentrations of pathogens in a complex microbial fl ora.
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