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  1. Pathogen Study - Design Considerations Jeffrey T. Gray USDA, ARS,ARRU Athens, GA

  2. Study Design Considerations • Organism Characteristics • Host-Range / Clinical Status • Study Design • Challenge studies • Natural inoculations • Measuring Effects • Confounding Factors

  3. Swine choleraesuis-k C1 typhimurium B derby B choleraesuis C1 typhimurium-c B anatum E1 heidelburg B brandenburg B worthington G2 agona B Human typhimurium B enteritidis D1 heidelburg B newport C2 hadar C2 agona B infantis C1 thompson C1 braenderup C1 javianna D1 Most frequent Salmonella isolates

  4. Organism Characteristics • Salmonella • Broad Host Range “ubiquitous” • Clinical status questionable • Campylobacter • Lack of clinical signs in food animals • E. coli O157:H7 • Narrow host range • No clinical signs in food animals

  5. Organism Characteristics • Invasiveness • Salmonella • “Shedding” • Carrier State • Stress / Diet / Environment • Strain differences among Salmonella isolates • Infectious Dose • Shedding vs. Infectious dose

  6. Study Design • Challenge Studies • Challenge Dose • Dose response curve Salmonella in swine1 • 103cfu- no shedding, no deep tissue infection • 106cfu- 101 shedding peak, 8 weeks carriage • 109cfu- 103 shedding peak, long term carriage • Controlled Natural Infections2 • Seeder Animals 1. Gray et al AJVR Vol. 57, no 3, 1996 2. Gray et al. Appl. Environ. Microbiol. 62: 1996

  7. Study Design • Controlled Natural Infections • 103 shedding peak of seeder animals • All naïve animals infected • 103 shedding peak in ‘naturally’ exposed animals • Long term carrier state • Swine infected with infected, desiccated feces1 • Dose = 105cfu • All naïve animals infected / No shedding • 103/g deep tissue infections 1. Gray et al J. Food Prot. V 64 no 7, 2001.

  8. Measuring Effects • Pathogen Detection Methods • E. coli O157:H7 • 1998-99’ Hancock et al. feedlot prevalence 5%-8% • 2000-2001 Hancock et al. /Smith et al. feedlot prevalence 23-25% • Differences based on detection methods • Are detection methods sensitive enough to detect treatment effects?

  9. Measuring Effects • Temporal Measurements • Salmonella high dose challenge shedding curve - cfu/g of feces • At what point do we intervene?

  10. Measuring Effects • Temporal Measurements • Intervention points • Prior to infection – alterations in flora • During peak shedding - ? Clinical signs • Late in production cycle – Induction of carrier state?

  11. Measuring Effects • Measurement of effect • What measurements are used • Method sensitivity • Feces, Tissue • What time point • For how long • “No” Salmonella • Not currently feasible • Salmonella in many animals – clinical? • Detectable in a few animals at low levels • Likely the norm • Food safety risk

  12. Confounding Factors • Seasonal Changes • E. coli O157:H7 • Seasonal prevalence 3% - 5% • Salmonella • Species, serotype differences • Peak season prevalence 8-12% • Diet • Composition of feedstuffs can effect prevalence and transmission of Foodborne zoonotic pathogens

  13. Confounding Factors • ‘Stress’ • Manner in which stress is measured • Effect on pathogen shedding • Transportation effects • Lairage exposure • Recent swine data indicate lairage may be important exposure point • Deep tissue (lymph node) infections • Salmonella serotype differences

  14. Confounding Factors • Environment • Treatment effects may be environment specific • Mechanism of action of treatment • Temporal relationship of shedding curve • Effect on flora or directly on pathogen • Long term survival and infectivity of Salmonella • Years in desiccated state

  15. Conclusions • Organism Characteristics • Studies must adequetely account for specific characteristics of a foodborne zoonotic pathogen • Study Design • Model a realistic infection cycle in target host species • Measuring Effects • Must be robust enough to show differences under realistic circumstances • Confounding Factors • Study must reasonably account for factors that exist in production systems and pathogen strain differences