Food Protection Trends - September/October 2022 - 366

summer 2012 (14). Excluding stable flies, which had an E. coli
O157:H7 carriage rate of 1.1 flies per 1,000 flies sampled, all
had similar carriage rates of 22.3 to 29.0 per 1,000 (n = 6,228).
Flies collected at 0, 60, 120, and 180 m from the feedlot were
not significantly different in their carriage rates.
Wildlife
Wildlife also can serve as human pathogen vectors between
animal agriculture and specialty crop production areas. Aside
from birds, wildlife come in contact with animal agriculture
more often in pasture settings, but even in facilities and
feedlots, other wildlife such as rodents have frequent contact
with production animals. Numerous surveys have been used
to track pathogen prevalence in wildlife with habitat and
migratory pathways in proximity to produce-growing regions
(31, 54, 69, 74, 77, 84, 124). Indistinguishable pathogen
strains are frequently present among wildlife and domestic
animals in the same geographical area, suggesting transmission
among species and/or contact with a common vector(s)
or reservoir(s) of contamination in the environment. Deer
that share habitat with cattle are often thought to be more
susceptible to pathogenic E. coli colonization. Díaz-Sánchez et
al. (38) found a positive association between STEC prevalence
in red deer feces and the presence of livestock. In other
studies, STEC-positive deer have been found in proximity
to dairy and cattle operations (47, 117, 124). Kilonzo et al.
(75) screened fecal samples from wild rodents trapped on 13
agricultural farms (9 produce farms, 3 cow-calf operations, and
1 beef cattle feedlot) in Monterey and San Benito counties,
California to investigate the prevalence and risk factors for
shedding of several foodborne pathogens. Cryptosporidium
spp. (26.0% prevalence), Giardia spp. (24.2%), S. enterica
serovars (2.9%), and E. coli O157:H7 (0.2%) were detected
in rodent fecal samples. These researchers also discovered that
pathogen presence was higher in rodent communities with a
higher number of deer mice and lower diversity than in more
diverse rodent communities. During studies of the ranch
associated with a major outbreak of E. coli O157:H7 infection
linked to baby spinach in 2006, a high percentage of feral pigs
captured on the ranch carried the outbreak strain, which was
also present in a high percentage of cattle fecal samples (29,
69). The same outbreak strain was isolated 5 years later from a
bird (Dark-eyed Junco) trapped ca. 10 miles (16 km) from the
outbreak ranch (30) (unpublished data).
With extensive sampling, Cooley et al. (29, 31, 32) used
multiple locus variable number of tandem repeats analysis
(MLVA) to identify additional environmental E. coli O157:H7
strains with identical 11-loci genotypes isolated many months
apart from the same sites and from different sites (water, cattle,
and wildlife), indicating the stability of at least some microbial
populations. Multiple outbreak-associated E. coli O157:H7
strains had the same 11-loci genotype as strains isolated during
these studies. For example, a strain associated with an outbreak
caused by contaminated leafy greens grown on the island of
366 Food Protection Trends September/October
Kauai was indistinguishable by pulsed-field gel electrophoresis
and highly related by MLVA to a strain isolated many months
before from cows in Monterey County, California. One
hypothesis for this distant relationship between strains is
the common transporting of cows from Kauai on container
ships for fattening in California, which again suggests genetic
stability of some strains of E. coli O157:H7 (unpublished
data). These findings illustrate that pathogens are transported
by comingled animals and emphasize how animal populations
and interactions can affect pathogen transmission pathways.
A better understanding of how the ratios of hosts (livestock)
to wildlife vectors (e.g., rodents, feral pigs, birds, coyotes,
and insects) and vector-host, host-host, and vector-vector
interactions affect pathogen transfer will play a critical role in
estimating the risk of transmission in any given environment.
A study of 21 New York produce farms was conducted to
investigate the association between field-level management
practices and field samples positive for Salmonella and
L. monocytogenes. The researchers reported that wildlife
observation within 3 days of sample collection increased
the likelihood of a L. monocytogenes-positive field (133).
Researchers at the USDA Meat Animal Research Center in
Nebraska sampled water from a stream flowing through a cattle
pasture and determined that waterfowl density significantly
affected the stream E. coli levels after a storm event in the fall
(56). In a year-long survey of birds in an agricultural region
of California, Navarro-Gonzalez et al. (97) reported finding
similar strains of STEC in wild geese and free-range cattle that
comingled and/or were found in the same geographical area.
Carlson et al. (21) researched how bird-livestock interactions
affected the spread of ciprofloxacin-resistant E. coli in cattle
feedlots across the United States and found European Starlings
positive for ciprofloxacin-resistant E. coli strains in every
feedlot tested. The total number of European Starlings was
positively associated with increased cattle fecal shedding of the
same antibiotic-resistant E. coli strains. These studies provide
evidence of the role wildlife may play in human pathogen
transmission pathways between animal agriculture and
specialty crops.
RISK FACTORS: HOW ENVIRONMENTAL
CONDITIONS INFLUENCE PATHOGEN
DISPERSION, TRANSMISSION, AND
DEPOSITION ON CROPS
Even when a human pathogen is attached to a dust particle
or an aerosolized droplet or droplet nucleus, its dispersion
to and deposition on crops is also greatly influenced by
inherent determinants of buoyancy and various environmental
conditions such as landscape topographical features,
gravitational settling, regional weather, and the effect of
crop traits on boundary layer surface dynamics. The roles
of some of these conditions are obvious, for example, the
susceptibility of a row crops located directly downslope from a
cattle feedlot. Other conditions are less obvious, for example,
Food Protection Trends - September/October 2022

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