Food Protection Trends - September/October 2022 - 367

a row crop ranch located 2 miles (3.2 km) downwind from a
swine production facility. The distance that a crop production
facility should be from an animal agriculture facility to prevent
contamination is an ongoing and somewhat controversial
discussion among fresh produce industry stakeholders. In one
study conducted in Colorado and Texas, E. coli counts on infield
spinach plants were significantly increased by proximity
(within 10 miles) to dairy, beef, and poultry farms (105). In
another study, a relationship was found between pathogenic
E. coli presence in agriculture water and how much of the
waterway was bordered by animal operations (44). These
studies have provided evidence of how environmental factors
may influence bacterial dispersion, and they provide valuable
information that can be used to assess the contamination risk
from microbial hazards. However, considering the complexity
and perhaps uniqueness of each agricultural region, the results
do not provide equal value for risk assessment.
Animal operation features
Considering the numerous studies reporting various
levels of human pathogens in animal agriculture, controlling
pathogen concentrations at the animal operation level would
undoubtedly have a beneficial effect on transfer of pathogens
to food crops. An extensive review of research on human
pathogen prevalence in animals and risk factors associated with
high levels of pathogens such as STEC in grazing and feedlot
cattle is beyond the scope of this review, and published reviews
of those studies are readily available (20, 42, 92). Nevertheless,
certain risk factors are related to features and conditions of an
animal operation and of the animals themselves.
Distance between specialty crop production and animal
operations is an obvious risk factor for human pathogen
transmission. Berry et al. (13), Park et al. (105), Sanz et al.
(116), and Theofel et al. (130) described the proximity to
animal operations as an influence on the microbial profiles of
nearby specialty crops and on human pathogen contamination
of crops. Connections between crops and animals, such
as waterways or landscape topographical features creating
wind tunnels, also influence the risk of pathogen transfer
from animal operations. In a study in British Columbia of
connections between pathogens in surface water used for
irrigation and the length of upstream borders with animal
operations (44), the researchers described the significant
correlation between pathogenic E. coli levels in irrigation water
and the length of a waterway (2 to 3 km) bordering properties
containing cow or poultry operations.
Animal density is another well-established risk factor in
pathogen transmission; areas with multiple animal operations
or individual high-density animal operations pose a higher
risk of pathogen transfer (35, 50, 70, 80, 130, 143). However,
independent of density, high levels of human pathogens have
been measured at some animal facilities and not at other
similar operations. Thus, some characteristics of the operation
and/or the resident animals affect the dispersion and the
concentration of human pathogens. Researchers have explored
conditions other than density, such as management practices,
related to animal operations to see how they might contribute
to pathogen transmission to preharvest in-field specialty crops.
Contamination can spread to the hides of additional animals in
the feedlot via pathogen-containing dust particles (93). Many
animal operations spray water to control dust. At the USDA
research facility in Nebraska, Berry et al. (13) noted that cattle
pen dryness and animal activity affected the amount of dust
that was dispersed to nearby test fields of leafy greens and the
detection of E. coli O157:H7 on test plants. In experiments on
which uninfected cattle were exposed to cattle contaminated
with E. coli O157:H7, pathogen transmission rates were
significantly increased by contamination in the surrounding
environment (48).
In numerous studies, pathogens have been found in water
troughs, feedstuffs, feedbunks, incoming water, and silage
(59, 76, 98, 99, 113, 118, 134, 135). However, water troughs
may affect human pathogen levels in cattle even when the
water is not contaminated. Beauvais et al. (8) found a positive
relationship between water levels in automatic refilling troughs
in a Texas feedlot and the prevalence of E. coli O157:H7 fecal
shedding in the feedlot cattle. When water levels were low, E.
coli O157:H7 shedding by cattle increased, and when water
levels were high, E. coli O157:H7 shedding decreased. The
study authors proposed some explanations for this finding but
were admittedly unsure of the cause of the association.
The presence of supershedders in a cattle herd has also been
associated with increased pathogen prevalence within cattle
operations. Cattle shedding E. coli O157:H7 in their feces
at ≥104
CFU/g are labeled supershedders and transmit the
pathogen to other cattle (horizontal transmission) in feedlots
(3, 4, 25, 126). Matthews et al. (88) presented data that suggest
that the spread of E. coli O157:H7 in cattle herds could be
effectively controlled by prevention strategies targeted at the
top 5% of cattle that shed high levels (104
to 105 CFU/g).
Findings from other studies support this strategy of controlling
the supershedders in a cattle herd to prevent widespread
infection throughout the herd (27, 89, 102). Supershedder
strains become the predominant strains in the environment,
and when they also are highly virulent they can cause illnesses.
In studies of herds in pasture, cattle access and density was
significantly associated with increased E. coli levels in water
sources and feces (11, 56, 138, 140, 141, 143). At the USDA
research facility in Nebraska, Hansen et al. (56) found higher
E. coli levels in a stream flowing through the pasture when
more cattle were grazing close to the water during the summer
months. Benjamin et al. (11) also reported increased odds
of detecting E. coli O157 in feces in larger herds. Wilkes et
al. (140, 141) found higher levels of ruminant Bacteriodales
markers and E. coli O157:H7 prevalence in water samples from
streams where cattle had unrestricted access compared with
streams where cattle access was restricted. Contamination
flows both ways; cattle on California ranches that used surface
September/October Food Protection Trends 367
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