# Presentation: 208102

## Pathogen Study - Design Considerations

- Jeffrey T. Gray
- USDA, ARS,ARRU
-  Athens, GA

## Study Design Considerations

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

## Most frequent Salmonella isolates

- 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

## 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

## Organism Characteristics

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

## 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

## 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.

## 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?

## Measuring Effects

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

## 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?

## 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

## 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* *

## 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

## 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

## 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

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