Food Protection Trends - March/April 2023 - 129

eratures) (26, 163, 190). L. monocytogenes is a facultative
anaerobe that occurs naturally in the environment in soil and
on plants and in the intestines of humans, other mammalian
species, and birds (32, 100, 104, 107, 147, 163, 165).
Although not directly informative to the prevalence of
L. monocytogenes in specialty crop production soils, Liao et al.
(100) detected Listeria spp. in 31% of 1,004 soil samples in
a survey of U.S. topsoil taken from natural environments that
are minimally disturbed by human contact.
A systematic review of studies investigating Listeria spp.
and L. monocytogenes prevalence and persistence in the
produce supply chain revealed the greatest prevalence of
Listeria spp. in outdoor production and natural environments
(163). Investigations of the produce production environments
in New York State reported more frequent isolation
of L. monocytogenes and Listeria spp. than of Salmonella and
STEC (155). In New York State, Listeria isolation in the
produce production environment was highly associated
with soil moisture and proximity to water and pastures (32).
Gorski et al. (64) tested environmental samples from San
Benito and Monterey Counties, California (a major produce
growing region) for Salmonella prevalence and found that
2.6% (16 of 617) of soil and sediment samples were positive
for this pathogen. In another study, 5% of 2,460 samples of
manure-amended soil taken from 19 organic farms in California,
Maine, Maryland, and Minnesota tested positive for L.
monocytogenes (130). In their study of five farms in New York
State, Strawn et al. (155) reported finding STEC at approximately
equal frequency in soil, water, feces, and drag swabs,
whereas L. monocytogenes and Salmonella were detected more
frequently in water samples. A study of L. monocytogenes
distribution within a single farm in New York State revealed
that the pathogen persisted on or was repeatedly introduced
to the farm over the course of the growing season (68).
Risk factors for pathogen transfer to and persistence on
harvesting equipment in the production environment
Although studies surveying the microbial condition
of food-contact surfaces in the outdoor production
environment have not been published, studies that included
surveillance and monitoring of food-contact surfaces in
produce packinghouses have revealed pathogen survival
on inanimate surfaces in indoor environments (53, 94,
116, 137). When considering the potential for crosscontamination
of product from contaminated equipment
surfaces, the microbial hazards in the field may have overlapping
similarities to those encountered in controlled
environment agriculture and indoor manufacturing and
processing environments. However, equipment may be
differentially exposed to microbial hazards in indoor and
outdoor environments. Microbial hazards in the outdoor
environment differ from those in indoor environments
primarily in exposure frequency and a reduced ability to
control exposure or provide protection. In addition to more
frequent contact with and less control over contamination
sources in the outdoor setting, surfaces of equipment used
outdoors also have increased exposure to environmental
conditions such as radiation and desiccation that may
adversely affect human pathogen survival and persistence
(95, 149, 163, 181).
Environmental conditions
When equipment comes into contact with pathogen-contaminated
soil, plants, water, insect vectors, bioaerosols, etc.,
and is not properly cleaned and sanitized prior to subsequent
use, pathogens may persist or grow and can be transferred to
harvested crops (21, 93, 144, 162). Depending on the environmental
conditions such as temperature, the availability of
moisture, and presence of organic matter, human pathogens
can survive on field equipment surfaces for varying lengths of
time (111, 181, 182). Among potential sources of contamination
for harvesting equipment, animal feces, soil, and plant
debris appear to be the primary ones (12, 77, 126, 155). Contaminated
water (1, 13, 38, 50, 66, 164, 177) or detergents
(33, 142, 187) used in cleaning activities also pose a contamination
risk for equipment, as do cleaning tools, which
may be contaminated (148). When harvesting equipment
is stored outdoors between uses, additional opportunities
exist for contamination due to climatic factors such as wind
blowing dust onto the equipment or due to contact with various
pathogen carriers (e.g., rodents and other pests). Wind
may transfer contaminants to equipment in the production
environment, and an understanding of normal wind direction
and speed can allow harvesters to determine whether unusual
circumstances warrant additional preventive measures. When
equipment is stored in proximity to animal feeding or holding
operations, even small-scale operations, flies may be attracted
to the harvesting equipment, especially when crop residues
and water remain between uses.
Although some human pathogens such as L. monocytogenes
are ubiquitous in the environment, others are normally not
prevalent but are introduced to cropland soil by manure
amendments, animal feces, or flooding (14, 71, 81, 82,
122, 127, 185). Numerous field studies have revealed that
pathogens may persist in soil for extended periods of time
(81, 82, 122, 185). Depending on field conditions and soil
type, soils may encrust equipment in varying degrees during
harvesting, and contaminated soil can introduce pathogens
onto harvested product.
Risk factors for isolation of Listeria spp., Salmonella, and
pathogenic E. coli in soil include soil type, soil properties,
soil amendments, temperature, wind speed, animal presence,
proximity to water, precipitation, and irrigation (10, 68,
78, 83, 85, 101, 102, 105, 113, 119, 123, 124, 126, 127,
129, 133, 177, 178). Environmental conditions may also
affect pathogen survival. Wind and high temperatures may
desiccate pathogens on food contact surfaces, affecting their
ability to survive (94, 181). Other environmental conditions
March/April Food Protection Trends 129
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