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Tracking Salmonella Contamination in Cilantro Using Surveillance and Geographic Information System (GIS) Analysis
Darcy E. Hanes1, Laura Ewing1, Kim Dudley1, Joshua Clayton2, Sarah Peters2, Karen Jarvis1, Junia Jean-Gilles Beaubrun1
1U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition, Laurel, MD; 2Engility, College Park, MD.
ABSTRACT
Salmonella contamination of produce continues to cause foodborne illness in the US. Determining the source and environmental conditions that
contribute to Salmonella contamination could facilitate strategy development to significantly reduce this pathogen in produce. Cilantro, a
commodity with a history of Salmonella contamination, was used as a model of contamination in leafy greens. From July 2011 through October
2012, 567 cilantro samples with metadata including the date, state, and source, were obtained from the USDA Microbiological Data Program.
Samples were tested for Salmonella using the FDA Bacteriological Analytical Manual (BAM) method, and the serotype determined for all confirmed
isolates. Salmonella was recovered in 4% of all samples collected in 2011, and 2.2% of those collected in 2012. Positive isolates composed the
following distribution of serotypes: Newport 53%, Tennessee 30%, Montevideo 11% and St. Paul 6%. Salmonella was recovered from cilantro in 2
distinct seasonal periods, one from November through February (winter), and a second from late June through August (summer). Serotype
Newport was recovered during both periods, while St. Paul was only recovered during the summer and Tennessee and Montevideo were found
during the winter. To further investigate the seasonality and potential source(s) of the contamination, maps were generated to show the
geographic location of farms sampled during the study, and a Geographic Information System (GIS) was used to examine environmental
parameters that may contribute to Salmonella contamination of cilantro. Preliminary data suggests that cilantro harvested from the west coast is
more likely to be Salmonella positive than product harvested from the east and midwest regions of the US. Precipitation may play a role in this
disparity; in general areas of the west coast that raise cilantro are more arid than those of the east coast. Results also suggest that proximity to
beef and poultry farming, especially in areas that share a watershed with cilantro farms, may be risk factors for Salmonella contamination. As
emphasis moves from detection of contaminated product in the food supply to prevention on the farm as a means of reducing foodborne illness,
surveillance studies such as this will provide tactics for stopping contaminated produce from entering the market.
CILANTRO ANALYSIS
A total of 567 samples obtained from the US Department of Agriculture (USDA) Microbiological Data Program (MDP) during sampling periods
July-December 2011, and April-October 2012 were tested for Salmonella using a modified version of the FDA Bacteriological Analytic Manual
(BAM) method (http://www.fda.gov/Food/FoodScienceResearch/LaboratoryMethods/ucm070149.htm). Aliquots (100g) of cilantro were
weighed into sterile whirl-pak bags and 500 ml of Modified Buffered Peptone Water (mBPW ) were added to each bag. The samples were
manually mixed for 2 min. and then incubated overnight at 37°C. The mBPW enrichment broth cultures were streaked onto HE and XLT-4 agar
plates, and subcultured into TTB and RV. The TT and RV cultures were incubated and processed as described in the BAM. Colonies demonstrating
typical Salmonella morphology on selective agar plates were subcultures onto SBA plates and confirmed using the Vitek® 2 Compact. The
serotype was determined using the Premitest® and a PCR serotyping method developed by Dr. Beaubrun (Food Microbiology , 31(2):199).
RESULTS
Salmonella was recovered in 4% of the samples received in 2011, and 2.2 % of samples received in 2012. Figure 1A shows the serotype
distribution among the positive samples. As shown in Figure 1B, Salmonella was recovered from cilantro in 2 distinct seasonal periods, one from
November through February (winter), and a second from late June through August (summer). Serotype Newport was recovered during both
periods, while St. Paul was only recovered during the summer and Tennessee and Montevideo were found during the winter.
Figure 1A.
Figure 1B.
POSITIVE FARM
Farm I in California was positive for Salmonella in 2011. As shown in Figure 3A, multiple cattle farms have proximal locations within just a few miles of
Farm I, but there are no nearby poultry farms or poultry slaughtering facilities. GIS analysis also indicated that Farm I is located within a FEMA 1 Percent
Flood Zone (Figure 2B), and watershed analysis determined that it is possible for runoff to carry Salmonella from the neighboring cattle farms (Figure
2C).
Figure 3A. Figure 3B. Figure 3C.
NEGATIVE FARMS
Farms B, C, J, and M in California were all negative for the presence of Salmonella. All 4 farms have cattle within a 2 mile radius
and farms J and M had poultry farms within a 1-2 mile radius. The watershed analysis maps places all farms within a flat area with
low runoff potential, and none of the farms are within a FEMA 1 Percent Flood Zone. Therefore, these farms would not be in the
path of runoff from any of the cattle or poultry farms, and these factors make it unlikely that flooding could cause Salmonella
contamination. The GIS maps for proximal locations to cattle and poultry farming (Figure 4A), FEMA 1 Percent Flood Zone (Figure
4B.), and watershed analysis (Figure 4C), are shown for Farm J.
Figure 4A. Figure 4B. Figure 4C.
ELEVATION
Elevation can also play a role in contamination due flooding and runoff. The area surrounding Farm O in CA is unique compared to
the other locations that tested positive for Salmonella. The nearest cattle farm is approximately four and a half miles away, and
there are no nearby poultry farms or poultry slaughtering facilities. The area surrounding Farm O is relatively flat, and unlike
California there are no flood zones and steep inclines to cause massive runoff. According to the Watershed Analysis map, no animal
farms could have directly affected Farm O during heavy rainfall, and Farm O is also not within a FEMA 1 Percent Flood Zone.
However, looking more closely at the elevation map (Figure 5A), a cattle farm has a proximal location upslope from these possible
irrigation ponds (outlined in red) that could have caused bacteria to runoff into these ponds causing contamination of nearby
water resources possibly used for irrigation purposes.
Farm F in Michigan was negative for Salmonella. Farm F is located in a flat area with a very low chance of runoff due to the area’s
high elevation compared to the surrounding locations (Figure 5B.) The nearest cattle farm has a proximal location over two miles
away and there are no nearby poultry farms or poultry slaughtering facilities. Farm F is not located in a FEMA 1 Percent Flood
Zone. All of these factors indicate a low probability of Salmonella contamination due to flooding.
Figure 5A. Figure 5B.
Salmonella DISTRIBUTION
To further investigate Salmonella distribution across the US, farm and distribution sites were mapped by the CFSAN Geographical Information Team
(GIT). Regions that had samples positive for Salmonella are indicated RED and those areas that were negative for the entire sampling period are shown
in GREEN. As shown in Figure 2A, preliminary data suggests that cilantro harvested from the west coast is more likely to be contaminated than samples
from the east and midwest regions of the country. There is some research suggesting that precipitation may play a role in pathogen contamination of
produce, so a precipitation map was generated. As shown in Figure 2B, the regions of the country with no contamination have higher annual
precipitation rates than those regions with positive samples.
Figure 2A. Figure 2B.
RISK FACTORS
One hypothesis on how cilantro becomes contaminated with Salmonella is proximity to beef and poultry farming and potential contamination through
flooding and run off. Representative Salmonella positive and negative farms were assessed for proximity to cattle and poultry farms using the National
Geospatial-Intelligence Agency (NGA) For Official Use Only (FOUO) data. The NGA data also provided the locations of the 1 Percent Flood Zones as
calculated by the Federal Emergency Management Agency (FEMA). The watershed analysis layers are a product of the Geographic Information Team
(GIT). In order to produce this data, a Digital Elevation Model (DEM) is needed. The DEM is added to a customized model that distinguishes the highest
and lowest elevation points; thus, is able to recognize the steepest paths in the region which help us understand water flow during heavy rainfall events.
The imagery used is Digital Globe imagery collected by Esri (source:
http://help.arcgis.com/en/communitymaps/pdf/WorldImageryMap_Contributors.pdf) to more closely inspect the areas surrounding the testing sites.
The actual locations for the cilantro, cattle, and poultry farms were used to conduct the risk assessments, but proximal (hypothetical) locations are
shown on the maps to protect the identities of the farms.
SUMMARY
Determining the sources and environmental conditions that contribute to Salmonella contamination are important factors for
designing strategies to significantly reduce this pathogen in produce. Although proximity to beef and poultry farms alone does not
seem to be a risk factor, sharing watersheds, runoff, and flooding from beef and poultry farms appear to contribute to risk for
contamination of cilantro with Salmonella. As emphasis moves from detection of contaminated products in the food supply to
prevention on the farm as a means of reducing foodborne illnesses, analyses such as this will provide new strategies for preventing
contaminated produce from entering the food supply.
ACKNOWLEDGEMENT
The authors thank Kathleen Cheeseman from the CFSAN Office of Analytics and Outreach for her support in providing access to the GIS tools and
analysis by the Geographic Information Team (GIT). CONTACT INFORMATION
Darcy E. Hanes
Darcy.hanes@fda.hhs.gov
240-402-3428