Food Protection Trends - May/June 2024 - 233

collected from six watersheds in Monterey County in California,
Gorski et al. (41) reported higher levels of Salmonella
in the spring than in the fall. They also found differences in
seasonal recovery among Salmonella strains; Heidelberg and
Senftenberg were isolated more often in winter, Infantis and
Oranienburg were more common in spring, Enteritidis and
Anatum were recovered more frequently in fall and spring,
and Montevideo was more common in spring and summer.
Studies of surface waters in eastern Virginia (n = 147 and
196), central Florida (n = 202), New York (n = 146), southern
Ontario, Canada (n = 235), and lower mainland British
Columbia (n = 223) revealed no association between Salmonella
prevalence and season (1, 32, 46, 78, 113, 117).
Because pathogenic E. coli is found less frequently in agricultural
water than are Salmonella and L. monocytogenes, its
prevalence is more difficult to correlate with season. Several
studies in the produce-growing region of California's central
coast were conducted to evaluate a possible association
between pathogenic E. coli prevalence in water and season,
but the results were inconclusive. In their study of E. coli
O157 prevalence over three growing seasons in the region
from May 2008 through October 2010, Benjamin et al. (10)
found no significant association between season and E. coli
O157 presence in water (n = 256) or sediment on farms or at
public access areas within the watershed. In contrast, Cooley
et al. (24) conducted a study over a 2-yr period from October
2011 to November 2013 in the same region with five times
the sample size (n = 1,386) and reported significantly higher
prevalence of E. coli O157 in water in the winter and spring
than in the summer and fall. However, these authors did not
find any seasonal difference in non-O157 pathogenic E. coli
prevalence. In another central California study with a smaller
sample size (n = 244) conducted from April 2008 to October
2010, significantly higher prevalence of pathogenic E. coli was
found in January, February, and March when temperatures
are cooler and rainfall is more frequent than in warmer summer
months with typically fewer rainfall events (23).
Like studies evaluating the association between seasonality
and pathogen prevalence, studies evaluating water temperature
also frequently revealed a correlation with pathogen
prevalence or levels (48). Many of the most prominent human
pathogens causing foodborne illness (e.g., L. monocytogenes,
Salmonella, and STEC) survive longer in water at lower
temperatures due primarily to the slowing of their metabolic
processes (22, 92). However, as water temperatures become
warmer, many pathogens become more metabolically active
and resume reproduction (62).
Some researchers have tried to more specifically determine
whether pathogen prevalence is directly related to seasonal
environmental conditions (e.g., precipitation, water temperature,
and humidity) or to seasonal agricultural practices (e.g.,
runoff from soil amendment applications and cattle moved
to nearby pasture). Thomas et al. (117) studied the seasonality
of Salmonella prevalence in urban and rural streams in
Ontario, Canada and found an association between season
and Salmonella serotypes of human health significance in
rural (agricultural) streams but not in urban streams, which
suggests that agricultural activities are primarily responsible
for Salmonella prevalence in these environments. Weller et al.
(140) studied nine waterways providing water to commercial
farms in Arizona and New York and also suggested that
upstream activities related to farm production may have contributed
to the seasonal variation in microbial water quality.
Most likely seasonal weather effects (e.g., ambient temperatures
and rainfall and related runoff) in conjunction with seasonal
agricultural activities contribute to and influence pathogen
prevalence. Seasonal weather conditions such as rainfall
and runoff undoubtedly affect contamination of agricultural
water by introducing pathogens or increasing pathogen levels
and will be further discussed as contamination risk factors in
a future article in this series on agricultural water.
PATHOGEN PERSISTENCE IN AGRICULTURAL
WATER
Human pathogens can survive in water for extended
periods of time and survive longer in environments with
favorable conditions that reduce or eliminate competitors
and provide sufficient nutrients and protection from
predators (7, 8, 13, 25, 37, 61, 138). Pathogens may
also persist in agricultural water by entering a viable but
nonculturable state or being enclosed in vesicles within
free-living protozoa, and survival may be enhanced by
adherence to sediment or aquatic plants and by the presence
of agrochemicals, as discussed in more detail below (5, 61,
69, 74, 75). Pathogens may persist longer in stagnant water
or ephemeral waterbeds that contain water only when there
is substantial rainfall than in continually flowing waterbodies.
Chase et al. (17) studied an ephemeral river in Florida and
observed significantly greater fecal coliform and E. coli levels
under no-flow conditions than when the river flowed. Fecal
coliform levels in the water column and sediments were
negatively correlated with the time since the last rain event
(i.e., as time between rain events increased, microbial levels
decreased) (17). Pathogens are removed from water by
sedimentation, filtration, and absorption or from natural dieoff
due to UV exposure, temperature fluctuations, starvation,
or predation (61, 69, 146). Two recent reviews have taken
in-depth looks at the ability of human pathogens to persist
in water (8, 37). Bell et al. (8) reviewed the persistence of
STEC, Salmonella, Campylobacter, and L. monocytogenes
in surface water sources. Gartley et al. (37) reviewed the
published science on the prevalence and persistence of
L. monocytogenes in irrigation water and the factors that
contribute to L. monocytogenes contamination of various
types of water sources. Here we briefly discuss published
research findings on specific environmental conditions
that may enhance the persistence of human pathogens in
agricultural water sources after contamination occurs. In a
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