Flies as Possible Vectors for Transfer of Shiga-toxigenic and Salmonella
Gorman, S., V. Nettles, D. Paulsen, R. T. Trout Fryxell, A. L. Wszelaki, J. R. Buchanan, and F. Critzer.  2017.  Internation Assocation for Food Protection Annual Conference, Flies as Possible Vectors for Transfer of Shiga-toxigenic. P1-42. International Association for Food Protection, 2017 Annual Conference, Tampa Convention Center, Tampa, FL, July 9-12, 2017.

Introduction: Cattle and wildlife are known reservoirs for Shiga-toxigenic Escherichia coli (STEC) and Salmonella. Cattle operations may be in close proximity to farms growing fresh produce, allowing insects to transfer these pathogens to the crops. The role of insect vectors in produce contamination is not fully understood, and data characterizing the prevalence of foodborne pathogens in farm fly populations is lacking. Purpose: Investigate the prevalence of STEC and Salmonella associated with various genera of flies sampled/collected on a farm with both produce and beef cattle. Methods: Flies were collected using ten separate bait traps spread throughout the farm ranging in distance from cattle to plots used for growing and harvesting fresh produce. Flies were identified and grouped by trap location, collection date, and genus/species. Pooled samples were preenriched, followed by selective enrichment of Salmonella and STEC, respectively. Selective enrichment for Salmonella was plated onto XLT4 and colonies were confirmed using immunological latex agglutination. Selective enrichment for STEC was plated onto Chromagar STEC and presumptive-positive colonies were confirmed using quantitative real-time PCR for stx1, stx2, and eae genes. Results: Over eight weeks, 152 pooled samples were collected representing 26 genera from five taxonomic families. Seventeen samples were positive for the presence of Salmonella (11 Muscidae, one Sarcophagidae, two Calliphoridae, one Tachinidae). Four samples were positive for the presence of STEC (three Muscidae, one Sarcophagidae). There was no significant correlation between trap location, capture date, and pathogen presence among samples. Significance: Our findings suggest that certain genera of flies may be more likely to carry and transmit foodborne pathogens from fecal material to produce on-farm. Further research is needed to determine the impact of cattle density, produce type and farm size as well as interventions that may alter fly patterns to reduce the risk of produce contamination.