Agriculture Reference
In-Depth Information
dependent upon many factors related to the geography and ecology within and sur-
rounding the watershed, including the density of animals, hydrology, elevation/runoff,
meteorological conditions (e.g., rainfall and temperature), pathogen fi tness (Table 1.5),
water composition (salinity, nutrients), predation, and vegetation. Waterborne disease
outbreaks in the U.S. (1948-1994) and Canada (1975-2001) occur more frequently
following heavy rain events, indicating transport of pathogens from human, domestic
animal, livestock, or wildlife sources through runoff, and, ultimately, contamination
of drinking water supplies (Curriero and others 2001; Thomas and others 2006).
Although no defi nitive links between heavy rain events and human illness have been
reported, fl ood contamination of fi elds or irrigation water sources intended for growing
produce is a potential risk factor for illness (CDHS 2005).
Watershed hydrology may be crucial to understanding pathogen transport within
an environment. Hydrological processes are relevant to transport of pathogens in the
environment, including fecal disintegration and dispersion, resuscitation of pathogens
in arid environments, trapping of pathogens in wetlands, concentration of pathogens
on or in sediment particles, land - to - watershed - to - land movement, and exposure of
wildlife to pathogens (Ferguson and others 2003). Similarly, the soil and sediment
particles present in fl owing or static water bodies can interact and bind with microor-
ganisms by mechanisms that are not well defi ned, and likely vary depending upon
variations in soil, fecal and water composition, weather, and other factors (Gagliardi
and Karns 2000; Brookes and others 2004; Ferguson and others 2003). Transport
of pathogens in dust, on harvest equipment, in manure/compost and pesticide and
herbicide sprays diluted with surface water should be considered also.
Pathogens and microbial species as indicators of fecal contamination can be preva-
lent in environments near produce production (Tables 1.3 and 1.4). Sensitive and
accurate detection of specifi c pathogens in the environment to track the fate and
transport of pathogens to fi elds requires intensive sampling, successful isolation of
pathogens or fecal indicator microorganisms, and effi cient molecular genotyping
methods for microbial source tracking pathogens in relevant and complex environ-
ments (Field and Samadpour 2007; Meays and others 2004). A variety of different
source tracking methods have been developed to identify sources of fecal contamina-
tion, sometimes yielding mixed results and accuracy (Field and Samadpour 2007;
Stoeckel and others 2004). Microbial source tracking methods have evolved to include
modern genetic methods that involve fi ngerprinting isolates from the environment and
different animal hosts to create a database for comparing fi ngerprints of new strains
to those in the database and thus identify putative sources of fecal contamination (Field
and Samadpour 2007).
Pulsed fi eld gel electrophoresis (PFGE) remains a common method for fi ngerprint-
ing foodborne pathogens, mainly because of CDC's PulseNet database, which stores
PFGE profi les submitted by public health labs representing tens of thousands of spo-
radic and outbreak strains for comparison (Swaminathan and others 2001). However,
sequence-based typing methods, such as MultiLocus Variable number tandem repeat
Analysis (MLVA), MultiLocus Sequence Typing (MLST), and Single Nucleotide
Polymorphism (SNP) microarrays, are gaining in acceptance due to ease of use, speed,
and high-resolution data for comparisons.
MLVA is an effective method for genotyping
E. coli
O157:H7 (Hyytia - Trees and
others 2006) and is being evaluated also for
S.
Enteritidis. MLVA proved effective in
