Agriculture Reference
In-Depth Information
Cattle shedding high levels of E. coli O157:H7 in their feces have been identifi ed
in some surveys. The majority of cows positive for E. coli O157:H7 in a herd have
<
100 CFU/g of feces, and this usually is detectable only by preenrichment and immu-
nomagnetic selection methods. However, high-level shedders (“super shedders”) have
been identifi ed that shed between 1,000 and 1,000,000 CFU/g of feces (Low and
others 2005; Chase-Topping and others 2007). Similarly, mice shedding
10 8 CFU
viable Salmonella cells per gram of feces have been identifi ed in laboratory studies,
and high-shedding status appeared linked directly to the health of the intestinal micro-
fl ora and level of infl ammation in the colon (Lawley and others 2008).
Indeed, models of prevalence, heterogenous shedding, and human infectious dose
data are consistent with the “80/20 rule” suggesting that 80% of the transmission of
an infectious agent results from the 20% of the most infectious members of the popu-
lation (Matthews and others 2006). Therefore, colonized animals shedding large doses
of a pathogenic strain (or strains) relative to the majority of a herd, or any population,
in a region are relevant epidemiologically because the strains they shed are likely to
be predominant in the environment. If predominant strains are virulent members of
the species also, they are candidates for outbreaks of foodborne illness or other forms
of infectious disease (Matthews and others 2006).
Other factors important epidemiologically are the survival of a virulent pathogen
in complex environments and its fi tness in water, in soil, and on fi eld crops. It is
noteworthy then that E. coli O157:H7 strains linked to four outbreaks associated with
bagged leafy vegetables in 2005 and 2006 (including the baby spinach outbreak,
2006) appear to be part of a phylogenetically distinct group (“clade 8”) that includes
virulent strains associated with outbreaks from patients who had been hospitalized
with hemolytic uremic syndrome and strains associated with increased frequency of
hospitalization (Manning and others 2008).
Increased virulence correlates also with a lower infectious dose required for illness.
The estimates of the dose of E. coli O157:H7, for example, capable of causing illness
in a population exposed to contaminated food ranges from 4 to
>
40 CFU/g of food
(Strachan and others 2001; Teunis and others 2004). Thus, a more virulent strain
capable of causing illness at an even lower infectious dose emphasizes the risks asso-
ciated with any pathogen contamination of environments near produce production.
<
Incidence of Potential Pathogens in Municipal and
Agricultural Watersheds
Pathogens shed onto soil on the range, in feedlots, or in other habitats are dispersed
and disseminated further by runoff into watersheds. Table 1.4 summarizes the results
of some selected recent studies of the incidence and fi tness of E. coli O157:H7 and
S. enterica in municipal or agricultural watersheds because they have been the bacte-
rial pathogens linked most frequently with recent outbreaks associated with preharvest
contamination of fresh produce (Table 1.1 ).
The incidence of E. coli O157:H7 in watersheds has been reported to be low gener-
ally (
2%) compared to Salmonella , refl ecting probably the general concentration of
the pathogens in the water samples. Strains of E. coli , potentially pathogenic based
on the presence of known virulence genes ( tir and stx ), were isolated frequently in
one U.S. study, indicating that specifi c urban watersheds can be contaminated heavily
<
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