Food Protection Trends - May/June 2024 - 232

to contaminate tree fruit than are overhead sprinkler systems
for irrigating row crops such as strawberries and lettuce (35,
80, 110, 111, 144). Table 1 summarizes how agricultural
water is used during cultivation and growing operations in
produce production and describes the associated risk of crop
contamination related to those operations.
PATHOGEN PREVALENCE IN AGRICULTURAL
WATER
Water can be a carrier of many human pathogens, including
bacteria such as Shiga toxin-producing Escherichia coli
(STEC), Salmonella, Listeria monocytogenes, Shigella, and
Campylobacter (1, 24, 37, 41, 53, 56, 57, 69, 72, 103, 109,
143); parasites such as Cyclospora cayetanensis, Cryptosporidium
parvum, and Giardia (27, 30, 31, 58, 79, 85, 120, 143);
and human enteric viruses such as hepatitis A and E and
norovirus (39, 102, 118, 122). When present in the environment,
human pathogens can be introduced into an agricultural
water system anywhere to and from the water source
through its distribution and use.
Human pathogen prevalence in surface and ground
agricultural water sources is attributed primarily to the
proximity of these sources to pathogen sources in the
surrounding environment and varies considerably among
U.S. produce-growing regions, both in the types and levels
of pathogens present (3, 18, 24, 29, 110). As would be
expected based on the risk of contamination, surface water in
produce-growing areas has been studied far more extensively
than has groundwater. In addition, pathogen presence
and levels in surface water may vary seasonally and are
undoubtedly related to and affected by other conditions and
human activities in the surrounding environment. Surveys
of agricultural water in locations around the United States
and Canada also provide evidence that the distribution and
prevalence of human pathogens (strains and species) differ
among produce-growing regions (82, 112).
Table 2 summarizes studies published between 2013 and
2023 that address watersheds in which produce-growing
operations occur, and Table 3 summarizes studies published
in the same time frame that address agricultural water supplying
produce farms. Studies of agricultural water used for
irrigation on produce-growing ranches and farms (Table 3)
typically indicate that human pathogens are less prevalent
in irrigation water sources than they are in the surrounding
regional watersheds (Table 2) (10). Of the studies listed
in Tables 2 and 3, water in surrounding watersheds had an
overall pathogen prevalence of 33.3% and irrigation water
used on produce farms had an overall pathogen prevalence
of 17.9%. Of the three major human pathogens (Salmonella,
L. monocytogenes, and pathogenic E. coli) surveyed in
agricultural water studies, Salmonella and L. monocytogenes
had the highest prevalences in agricultural watersheds and in
agricultural water supplies for produce farms and pathogenic
E. coli had the lowest.
Seasonality and temperature
Studies in which human pathogens such as L. monocytogenes,
Salmonella, and STEC have been surveyed in agricultural
water have frequently included evaluation of whether
fluctuations in occurrence, levels, or strain diversity are
seasonal. Seasonality encompasses weather conditions, wildlife,
domestic animals, and cultivation activities that occur
seasonally. For example, L. monocytogenes has been reported
as consistently more prevalent in water when temperatures
are cooler, most often during winter but also in early spring
and/or late fall (1, 24, 32, 82, 114). This pattern has been fairly
consistent for surface waters surveyed across the country
from New York, Maryland, and Virginia to California. Rivers
(n = 147) and reclaimed water plants (n = 41) on Maryland's
eastern shore and surface water (n = 120) on produce farms
in Virginia had significantly higher L. monocytogenes prevalence
in winter months than in summer (1, 82). However, in
Ohio, Ferguson et al. (34) reported higher L. monocytogenes
prevalence in irrigation water from surface water sources (n =
69) in the summer than in fall and spring. These authors suggested
that the difference was due to domestic animals having
more frequent direct contact with surface water in summer
months. On the west coast, Falardeau et al. (32) reported
significantly higher L. monocytogenes prevalence in irrigation
ditch water (n = 223) from agricultural areas in southern
British Columbia in fall (22.9%) and winter (16.3%) than
in spring (7.8%) and summer (3.7%). A survey of five
watersheds (n = 1,405) within the California central coast
region revealed two watersheds with higher L. monocytogenes
prevalence in winter and spring months than in summer and
fall months (24). However, in three watersheds within the
region, L. monocytogenes prevalence was not correlated with
season (24).
Seasonal trends in Salmonella prevalence throughout the
United States and Canada were less consistent than trends for
L. monocytogenes. In several studies of Salmonella prevalence,
higher recovery rates were found in one season compared
with others (18, 40, 48, 65), whereas in other studies greater
serotype diversity was found during a particular season
(48, 117) or no seasonal association was found (46, 79,
114, 117). Haley et al. (48) and Li et al. (65) both reported
seasonal differences in Salmonella prevalence in their surveys
of agricultural water in southern Georgia's Little River (n =
72) and Suwannee River (n = 170) watersheds, respectively.
Salmonella prevalence was significantly higher in July, August,
and September (summer) than in other months. In northern
Georgia's Upper Oconee watershed, Cho et al. (18) also
reported higher Salmonella prevalence in summer months
over a 3-yr surface water survey (n = 688) from 2015 through
2017. In a survey of shallow wells on tomato farms along
Virginia's eastern shore (n = 196), Salmonella was detected
more often in weekly samplings during November and December
than in other months throughout the 70-wk sampling
period (47). In their 5-yr survey of surface water (n = 2,979)
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