Food Protection Trends - September/October 2022 - 363

Because animals are well-known primary reservoirs or
transient hosts for certain human pathogens, animal production,
harvesting, and processing operations have for decades
dealt with zoonotic pathogen infection and contamination
in their food products. Much research has been conducted
to investigate pathogen prevalence, loads, transmission, and
dispersion in animal feeding and harvesting operations and
methods to control and minimize subsequent contamination
in by-products and end products. One meta-analysis of 46
studies published between 1980 and 2012 in North America
revealed an Escherichia coli O157 prevalence of 7.35% among
110,641 cattle (64). Results of multiple studies have indicated
that environmental contamination within cattle operations
greatly increases the spread of E. coli O157:H7 to uncolonized
animals (4, 48). Although several mechanisms and modes of
transference are highly plausible, what is less well known is
how specifically these zoonotic pathogens get from animal
operations to specific produce fields, on a case-by-case basis.
However, as more research is published and contamination
events and outbreaks are studied more thoroughly, the
complexity of these events is becoming more evident. Rarely
has the scientific evidence pointed to a simple transmission
from pathogen source to preharvest crops. Despite these
challenges, the empirical method has not failed but has
provided volumes of evidence that when pieced together
is methodically forming a clearer picture of how zoonotic
pathogens pose a threat to the safety of preharvest crops.
Research exploring how these individual risk factors contribute
to and affect pathogen transmission from animals to in-field
crops is revealing how multiple risk factors work in concert
as a conduit for pathogen transmission. Therein lies the path
to a better understanding of the contamination problem-
capturing data from scientific research and real-world
contamination events to use with analytical methods such as
mathematical models.
This review includes an exploration of the published
research for evidence of how human pathogens move from
zoonotic origins in animal agriculture to in-field fruit and
vegetable crops. Although pathogens from an animal operation
may be confined to their immediate environment and never
disperse at consequential levels beyond the pen or feedlot,
numerous studies have documented pathogen dispersion
beyond the confines of the animal operation (13, 31, 45, 66,
67, 79, 112). The objective of this review was to explore and
summarize recent research findings that shed light on human
pathogen transmission pathways from animals to crops and the
various factors that play a role in pathogen transfer, dispersion,
deposition, survival, and persistence.
BACKGROUND: WHAT IS KNOWN ABOUT HOW
HUMAN PATHOGENS MOVE FROM THEIR
DOMESTIC ANIMAL HOSTS TO CROPS
After leaving the gastrointestinal tract of its animal host,
a zoonotic pathogen begins a journey in the surrounding
environment traveling primarily with the aid of various
types of biological and nonbiological vectors. To study
movement through the environment, researchers have used
various genetic, chemical, and phage-based methods to help
them identify pathogens in the environment and track their
movements.
* Genetic tracking methods use host or pathogen subtypespecific
gene sequences as markers or tags to follow
pathogens from sources (i.e., concentrated animal
operations and cow-calf operations) to surrounding areas
within a region and sometimes even over long distances
via dissemination pathways (e.g., watersheds).
* Chemical source tracking methods search for chemicals
that are unique to a specific source and remain stable in
the environment (e.g., metabolites of drugs given only to
animals).
* Bacteriophages (phages) are viruses that infect specific
bacterial hosts. Because phages are typically present in an
environment where their bacterial host is also present, they
could be used as a surrogate of sorts to investigate pathogen
presence. Because a phage destroys its host after replicating
within it, a negative correlation often exists between phage
presence and bacterial host presence and, in some cases,
phage presence without the presence of culturable bacterial
hosts, and vice versa (79, 81, 100, 111).
Much of this work has been pioneered and developed to
identify and track human pathogens in drinking water or other
water sources (e.g., irrigation water, watersheds, lakes, and
canals) but is also applicable to tracking pathogens in media
other than water (52, 60). To assist pathogen transmission
investigations, tracking methods are coupled with pathogen
databases, such as that of the National Center for Biotechnology
Information (https://www.ncbi.nlm.nih.gov/), that contain
genetic and phenotypic data and descriptive information such
as the location where and vehicle in which pathogens have been
found (109). Additional databases that have been useful for
source tracking of pathogens are PulseNet, PulseNet-MLVA, and
more recently GenomeTrakr (49, 62, 132).
Water pathways
Contaminated water can spread pathogens from animal
sources directly to crops (i.e., through irrigation and other
applications such as dilution of pesticides or nutrients) and
has been studied possibly more than any other transmission
pathway. Numerous surveys of watersheds supplying
agricultural water to or in the vicinity of major specialty crop
production areas have been published (2, 10, 11, 29, 31, 32, 51,
70, 81, 128, 133). In one such survey, scientists from the U.S.
Department of Agriculture (USDA) and the U.S. Food and
Drug Administration (FDA) collected surface water samples at
public access points in watersheds within California's Central
Coast agricultural region over a 2-year period and tested these
samples for Shiga toxin-producing E. coli (STEC), Salmonella
enterica, and Listeria monocytogenes (32). Of 1,386 samples
September/October Food Protection Trends 363
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Food Protection Trends - September/October 2022

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