Food Protection Trends - September/October 2022 - 368

water sources for drinking had a 4.2 times higher risk of testing
positive for fecal E. coli O157:H7 than did cattle on ranches
without surface water access (143). Cooley et. al. (29, 31, 32)
described transport of foodborne pathogens as likely fluid and
bidirectional from water runoff, wildlife, manure, and water
transport or flooding to wildlife, farms, and ranches.
Weather
Researchers have investigated various climate- and weather-related
conditions that influence pathogen dispersion
and deposition. Surface water flows due to rainfall serve as a
natural conduit for pathogen dispersion. Rainfall is associated
with increased bacterial levels in agricultural water (2, 29, 31,
56, 114, 133, 138). USDA researchers at the Meat Animal
Research Center in Nebraska studied the effects of animal
and waterfowl presence and rainstorm events on E. coli levels
in a stream traversing a pasture. The presence of cattle in
pastures adjacent to the stream the day of a rainstorm and the
increasing accumulation of cattle (density) throughout the
growing season had a significant impact on E. coli levels in
the stream following summer storm events (56). In the fall, a
significant positive correlation was found between waterfowl
presence and E. coli levels in the stream following rainstorms.
Weller et al. (138) also found a positive correlation between
rainfall and L. monocytogenes level in agricultural water downstream
from dairy operations in upstate New York. Cooley et
al. (29) reported increased incidence of E. coli O157 isolates
in rivers when heavy rain caused increased flow rates, and
heavy rainfall in elevated watersheds resulted in some indistinguishable
strains being isolated from sites in the contiguous
watershed up to 32 m from a point source. Researchers
studying land use effects on E. coli levels in water sources also
found increased prevalence and higher levels when storms
and overland water flows occurred more frequently (114). In
their study of farm ponds on Virginia's eastern shore, Truitt
et al. (133) found a significant effect on the probability of
detecting Salmonella when precipitation occurred the day
before or the day of sampling.
Wind can have a variable effect on pathogen transmission
and deposition depending on the medium in which the
pathogen resides and the environment in which it is located.
The importance of wind direction is self-evident. When
specialty crops are grown downwind from animal operations,
the contamination risk is higher than it would be if the crops
were grown upwind. Wind can carry pathogen-contaminated
dust particles from an animal operation and deposit them
in the surrounding environment. Sanz et al. (116) analyzed
air samples taken in November and July at various distances
from a dairy farm in all directions and at three elevations for
the presence of E. coli. Both higher temperatures and wind
direction positively affected the number of E. coli isolates
captured in the air samples. A comparison of the genomic
DNA profiles of E. coli strains from animal housing facilities
and those isolated from the surrounding environment
368 Food Protection Trends September/October
suggested that the strains were related. In pastures where
animals are often not concentrated, bacteria are more stable
(i.e., not airborne), and wind may affect bacterial survival
differently. In a study of cattle on pasture at three California
ranches, wind speed was negatively associated with E. coli
O157:H7 occurrence in fecal pats. When wind speed was
higher, researchers were less likely to detect E. coli O157:H7
in fecal pats, most likely due in part to desiccation (11).
Wind speed also plays a crucial role in spreading contamination.
According to the Beaufort scale, a moderate breeze with
wind speeds of 5.5 to 7.9 m/s (12.3 to 17.7 mph) is associated
with dust movement (11). Higher wind speeds are required
to move material from the ground (3.0 to 5.4 m/s [6.7 to 12.1
mph]) than are required to move material from plants (0.5 to
2.0 m/s [1.1 to 4.5 mph]) (71). In their study of environmental
factors affecting E. coli O157:H7 contamination of in-field
lettuce in Salinas Valley, Moyne et al. (94) recorded wind
speeds of ca. 0 to nearly 8 m/s (17.9 mph) over a 24-h period
in six trials, with consistently higher wind speeds during the
late afternoon and early evening. Dry, windy conditions are
the most likely times when bacteria-laden dust is moved from
animal sources to crop production areas.
Wind also can stir up surface water and release bacteria
sequestered in the underlying sediment, where E. coli levels
can be 10 to 1,000 times higher than those in the overlying
water (10, 11). Falbo et al. (45) measured viable E. coli levels
in sediments from roadside ditches along agricultural and
forested land in New York State; the mean was 4,616 most
probable number (MPN)/100 ml, and the maximum was
>240,000 MPN/100 ml. The highest levels were detected
after manure was spread in fields adjacent to ditches.
Total suspended solids were 0.51 to 52.2 g/liter. Roadside
ditches capture stormwater runoff, which is a source of
environmental fecal contamination from such sources as
wildlife, pet, septic system, and livestock waste, and transport
contamination to watersheds. Crabill et al. (33) found
that sediment agitation by storm surges was responsible
for increased fecal coliform levels in water in Oak Creek,
AZ. In Salinas, CA, for every 1 m/s increase in wind speed,
Benjamin et al. (10) reported a 60.1% increase in E. coli levels
in irrigation water.
Although wind speed and direction play significant roles
in dispersion, they also affect deposition. Deposition of an
airborne particle onto a plant involves how many particles
are involved and the deposition velocity. Deposition velocity,
measured in distance per time (e.g., centimeters per second),
is a function of gravitational settling, aerodynamic resistance
or drag, and resistance from the surface on which it is being
deposited (66). In general, more particles in a particular size
class are deposited closer to their source, but this phenomenon
deviates when upward currents take particulates higher into
the atmosphere, facilitating transport and deposition at long
distances (71).
Food Protection Trends - September/October 2022

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