Pathway Analysis: Assessment of the Pathogen Transmission Capacities of Disease-Carrying Insects

This project develops an FDA science base for insect vectors of Salmonella, Listeria and E. coli that is comparable to similar science bases under development for livestock. Flies are a recognized contributing epidemiological factor in the spread of foodborne pathogens, especially E. coli, Listeria, Salmonella and Shigella. House flies are a recognized risk factor as vectors of E.coli O157:H7 in dairy farms and cattle farms and as vectors of Salmonella in broiler houses and dog kennels. The house fly is the most competent fly vector of Salmonella. Recent studies show that wild populations of house flies and blow flies harbor pathogens over entire breeding seasons. Studies conducted on litter beetles in broiler houses found that litter beetles infected with Salmonella transmit the infection to previously uninfected flocks under laboratory and field conditions. No research has been conducted to determine if flies have a similar vector competence for infecting layer house flocks. No studies have been conducted to assess the reservoir competence or other risk factors associated with insects in or near layer houses and fresh produce facilities. Recent findings that fruit flies can transmit E. coli O157:H7 to apples and that litter beetles can also transmit E. coli indicate that this project should be expanded to include fresh produce and to explore the vector competencies of vectors other than the house fly. Layer house phase: Although rural environments have many potential reservoirs of Salmonella, the physical realities of layer house construction and operation limit the likely vehicles of contamination of uninfected flocks to human workers, water, feed, rodents and insects. The layer house phase of the project is designed to determine if the wild flies are a risk factor as natural reservoirs of Salmonella in and around infected layer house flocks. Flies are aseptically collected during traceback inspections for salmonellosis outbreaks involving shell eggs and tested for Salmonella using the procedures that are used for the other traceback environmental samples. Analysis of samples from the first, and to date only, traceback inspection of the project isolated S. enteriditis and S. infantis from house flies in and around the infected layer house. Samples of water and feed were negative for Salmonella. The relative densities of fly populations are also measured during the traceback inspections using the WHO/CDC protocol for estimating fly densities. Fly colony phase: This phase is designed to determine the mechanical vector competence of house flies for Salmonella and Listeria. Individual fly adults are tested to determine the mean pathogen carrying capacity of a single fly under laboratory conditions. Vector competence is an important factor for comparing the relative risks presented by different species of flies. There are 12 species of flies, including the house fly, that are reasonably competent mechanical vectors of pathogens but there is evidence that vector competence may vary among fly species. Tests will be conducted to compare the vector competence of the other species with the house fly baseline. This phase is in abeyance until such time as the FSI program provides microbiological analytical support.

The vector competence studies determine which vectors should be included in risk assessment models for shell eggs and produce. The studies also provide estimates of vector population densities, expressed in the standard WHO/CDC epidemiological formula, for use in risk assessment modeling. The fly colony phase, when funded, will provide a science base for developing CPGs for unacceptable numbers of flies in layer houses or other food production facilities. The results of the fly colony tests will also be used to develop HACCP guidance for fruit juice and seafood. The entire project provides a needed addition to the FSI science base for the Center=s enforcement strategy for filth and extraneous materials. Suggested project expansion: In view of the newly-discovered capability of fruit flies to contaminate apples with E. coli O157:H7, it is recommended that the layer house phase be expanded to include FDA surveys and traceback inspections involving pathogen contamination of fresh produce. As is the case with layer house tracebacks, the produce study can be accomplished using existing survey and traceback resources. In view of the diversity of vectors of foodborne pathogens and the diversity of vector competencies, it is further recommended that the project be expanded to include other vector species in all phases of the project.