Agriculture Reference
In-Depth Information
environmental studies involving tracking
E. coli
O157:H7 strains in produce produc-
tion environments, watersheds, and cattle feedlots (Cooley and others 2007; Murphy
and others 2008). An intriguing fi nding in the 2006 investigation of the
E. coli
O157:H7 multistate outbreak linked to bagged baby spinach was the isolation of
multiple strains of
E. coli
O157:H7 from the feces of multiple feral swine trapped in
the vicinity of the suspected spinach fi eld; some of these isolates, and isolates from
cow fecal, river. and dirt samples also collected within a mile of the fi eld, were indis-
tinguishable from the clinical outbreak strains (Jay and others 2007; Cooley and others
2007). Similarly, evidence of transport of
E. coli
O157:H7 strains between dairy farms
by wild birds has been reported (Wetzel and LeJeune 2006).
How Do Pathogens Get onto Preharvest Produce and Survive?
Hypotheses from Recent Outbreaks
The transient incidence of pathogens in livestock, wildlife (Table 1.3), and watersheds
(Table 1.4), the environmental fi tness characteristics of foodborne pathogens (Table
1.5), and recurring outbreaks of foodborne illness associated with ready-to-eat produce
(Table 1.1) are consistent with the fi ndings of low-level, but signifi cant, incidence of
generic
E. coli
on fresh produce obtained from distribution centers and retail markets
(Table 1.2). Although some of this
E. coli
could be present at harvest, postharvest
contamination also could occur in a variety of ways, such as rodents, contaminated
bins or transport vehicles, commingling of food at retail markets or restaurants, or ill
workers. Postharvest cross-contamination could exacerbate what might have been
a limited contamination event initially.
Preharvest contamination of produce occurs by obvious processes, but perhaps also
by unknown, or less well understood, processes. Although no defi nitive conclusions
have been offered about the sources of preharvest contamination of leafy vegetables
and tomatoes associated with recent outbreaks (Table 1.1), reasonable hypotheses
involve transport of pathogen in animal fecal waste by 1) watershed to fl ooded fi elds
(CDHS 2005), 2) feral swine intrusion (Jay and others 2007), 3) irrigation by pipes
used previously to remove dairy holding pond waste (CalFERT 2008), and 4) amphib-
ian or other wild animals emerging from contaminated surface water to intrude into
fi elds (MMWR 2005a).
Water is a central factor in hypotheses of contamination, so studies of the dispersion
and dissemination of microbes in water and the use of microbes as tracers of water
movement are relevant to understanding dissemination of enteric pathogens in water.
Heavy rainfall is associated with rapid dispersal of pathogens from fecal matter on the
ground into surface and groundwater (Ferguson and others 2003). Pathogen incidence
and survival in feces, water, soil, and other matrices (Table 1.3, 1.4, 1.5) are relevant
for modeling environmental contamination of preharvest produce, identifying sources,
and controlling contamination, but details are lacking about how different species of
bacteria, including pathogens, disperse and survive in water and other sites in the
production environment and how this might relate to preharvest contamination.
Bacteria, yeasts, and bacteriophage have been used as tracers by dosing a large
number of laboratory-grown cells (approximately 10
14
cells) into a river and monitor-
ing movement (Wimpenny and others 1972). The bacterial strain traced,
S. marcescens
