The potential role of riparian corridors in overland dispersal of bacteria among vegetable farms

Background/Question/Methods

Understanding dispersal pathways of foodborne pathogens is essential to forecasting disease risk from farm-to-fork. Indeed, intrusion into produce farms by wildlife species features prominently as a risk factor for foodborne illness outbreaks in food safety assessment guidelines. However, to predict when wildlife intrusions constitute a significant foodborne illness risk on farms, we must understand how wildlife movements contribute to foodborne pathogen dispersal. These studies can also enhance our basic understanding of how landscapes shape bacterial populations by constraining dispersal pathways. We sought to test whether Escherichia coli disperses overland among produce fields via riparian corridors at faster rates and longer distances than among sites that were isolated from riparian corridors. Escherichia coli were collected in nineteen produce fields and adjacent forests located along two waterways in New York State during autumn of 2012. The land surrounding the Flint Creek was 69% agricultural and 12% forest land cover whereas the Hoosick River sampling area was 27% agricultural and 38% forest. Models predicting the movement cost among sample sites were developed using GRASS GIS. Over 2800 confirmed E. coli were isolated from 571 samples, including 278 soil samples, 157 fecal samples, 116 surface drag swab samples, 20 water samples.

Results/Conclusions

Escherichia coli isolates were obtained from 73% of drag swab samples, 53% of soil samples, 76% of fecal samples, and 100% of water samples. While the proportion of fecal samples yielding E. coli was somewhat similar at 74% in Flint Creek and 78% in the Hoosick River area, there was a pronounced difference in the proportion of positive soil samples between the two riparian areas: 35% of soils in the Flint Creek area and 72% of soils in the Hoosick River area, respectively. Soil samples obtained from riparian forest soils were more likely to yield E. coli than those from produce field soils, with prevalence values of 70% and 48%, respectively. These preliminary results seem to suggest that forest habitats harbor E. coli in extra-host environments more frequently than produce fields and that it remains possible that riparian forests actually do act as dispersal corridors or, at least, sources for E. coli dispersal to produce fields. Future work will examine the multi-locus sequence types of E. coli isolates to estimate dispersal rates and distances among produce field sites.  Goodness-of-fit tests will also be conducted between dispersal data among sites and competing GIS models predicting various pathways of E. coli dispersal.