The Delmarva Peninsula, consisting of Delaware and parts of the eastern shores of Maryland and Virginia, is a concentrated produce growing area for the three states, where tomatoes, cucumbers, snap beans, potatoes, cantaloupe, watermelon, strawberries, and other crops are grown. Of the three states, Virginia is the largest tomato producer and a major source of fresh tomatoes consumed in the U.S. The Delmarva Peninsula is also home to major U.S. poultry producers, and some of the poultry houses and processing plants are in proximity to produce fields. From 2002 to 2007 there were 11 salmonellosis outbreaks in the U.S. associated with tomatoes. Four of those outbreaks were linked to tomatoes from the Delmarva Peninsula. What was unclear was how the tomatoes were becoming contaminated. FDA teamed with university researchers to better understand the possible sources and routes of contamination on the peninsula.
Researchers from FDA and the Virginia Tech Eastern Shore Agricultural Research and Extension Center (AREC) collected more than 3,000 produce and environmental samples, tested each for the presence of Salmonella, and conducted whole genome sequencing on the Salmonella isolates recovered from each sample that tested positive. The samples included, but were not limited to: tomatoes (fruits, blossoms, and leaves), native vegetation, insects, agricultural water (including surface water and sediment), wildlife scat, and agricultural soil. Most of the samples were collected from growing areas owned by AREC, which have similar environmental characteristics to those of the surrounding commercial growing fields. In parallel, Salmonella’s ability to internalize in maturing tomato fruits was studied in a simulated growing environment using elevated levels of inoculum.
Outbreak-related strains of Salmonella were consistently found in surface water and sediment. A handful of other samples, including goose droppings, a tomato leaf, and a few tomatoes collected from road-side stands also tested positive for Salmonella, although none of the strains isolated from the road-side stand tomatoes were associated with illnesses. Conversely, hundreds of insects, native vegetation, soil, well water, seagull droppings, and numerous other agricultural commodities grown at AREC were sampled and all were found to be negative for Salmonella. These findings suggest that surface waters and associated sediments may be persistent reservoirs of Salmonella in the Eastern Shore farm environment.
The inoculation studies in a simulated growing environment revealed that when tomato blossoms were contaminated with Salmonella, 70% of the tomato fruits that developed were contaminated with Salmonella internally, externally, or both internally and externally. Researchers also found that when the root system of the tomato plants were exposed to water contaminated with Salmonella within 3 days of transplant, 5% of the tomatoes that developed tested positive for Salmonella. Internalization of Salmonella via the route system appears at least to be a function of the serovar and the plant’s growth state, with the plant being more susceptible to internalization within 3 days of transplant.
Collectively, these data suggest that agricultural application of surface water may be an important vehicle for contaminating produce under the conditions studied, and that further research is needed in real-life production systems.
- Colonization and Internalization of Salmonella enterica in Tomato Plants. Zheng J, Allard S, Reynolds S, Millner P, Arce G, Blodgett RJ, Brown EW; Applied and Environmental Microbiology, 2013, 79(8):2494-2502.
- Ecological prevalence, genetic diversity, and epidemiological aspects of Salmonella isolated from tomato agricultural regions of the Virginia Eastern Shore. Bell RL, Zheng J, Burrows E, Allard S, Wang CY, Keys CE, Melka DC, Strain E, Luo Y, Allard MW, Rideout S, Brown EW; Frontiers in Microbiology, 2015, 6(415).
- Agricultural Practices Influence Salmonella Contamination and Survival in Pre-harvest Tomato Production. Gu G, Strawn LK, Oryang DO, Zheng J, Reed EA, Ottesen AR, Bell RL, Chen Y, Duret S, Ingram DT, Reiter MS, Pfuntner R, Brown EW, Rideout SL; Frontiers in Microbiology, 2018, 9(2451).
- Diversity and Dynamics of Salmonella enterica in Water Sources, Poultry Litters, and Field Soils Amended With Poultry Litter in a Major Agricultural Area of Virginia. Gu G, Strawn LK, Zheng J, Reed EA, Rideout SL; Frontiers in Microbiology, 2019, 10(2868).
Study Related Announcements
- Virginia Tech Eastern Shore Agricultural Research and Extension Center (AREC)
- Delmarva Produce Safety Task Force (April 2015): The Delmarva Produce Safety Task Force, consisting of representatives from Delaware’s, Maryland’s, and Virginia’s principal state agriculture agencies, the land-grant universities of each state, their state extension services, FDA, and CDC, was formed to ensure continued close collaboration on the issue of produce safety. The Task Force coordinates efforts, monitors progress, and discusses future actions to prevent Delmarva produce from becoming a source of foodborne illness. Major areas that the Task Force addresses include: current research and research needs; epidemiology and surveillance; good agricultural practices; and communications.
- Delmarva Action Plan (June 2015): Delaware, Maryland, and Virginia, each developed an action plan, specific to its own state, for reducing foodborne illnesses from produce. Cumulatively these plans form the Delmarva Action Plan, which represents an adaptive management approach, with the emphasis and strategies changing as the situation warrants.