Questions:

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Questions What is the probability (risk) of
detecting BSE cases (late pre-clinical) with a
targeted surveillance scheme? Can the difference
in prevalence in target groups be explained by
different age structures?
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Important INPUT Variable considerations BSE
Prevalence - Variable by country -
Uncertain in all countries? - Varies with
time - Variable by target group? Age specific
incidence - Variable by country? - Vary with
time Risk groups - Variable by country -
Uncertain in most countries Sample
size(s) - All? Some? Target groups? Population
structure - Varies by country
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• Some outputs needed/wanted
• Number of BSE cases slaughtered
• Number of BSE cases in each target group
• Distribution of BSE cases within age groups
• Probability of detection a BSE case with a
  given sample size and prevalence.
• Number of BSE cases in population?

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Model steps Simulate the prevalence in target
groups Simulate number of cases in target
populations Simulate number of cases in sample
groups by age cohort
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Routine slaughter 3/7138F Fallen stock 16/7178
Emergency slaughter 6/3578 Combined 25/17,894
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Age specific incidence rates - UK
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• The Surveillance Model
• Population data used
• Danish female fallen stock population gt2yrs -
  Age Distribution
• Estimated number of emergency slaughtered - FS
  age distribution
• Number routinely slaughtered - Estimated number
  and age distribution
• Feed cohort - Cohorts all 4 yrs or greater
• Prevalence data used
• Simulated prevalence distribution (beta) based
  on routine slaughter cases in Switzerland ,
  1999, and Denmark 2001
• Simulated prevalence distribution for risk
  group, 2000-2001
• BSE case age distribution
• Derived from age specific incidence rates in
  59,000 UK homegrown cases 1989-99

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• The Simulations
• Distribution of the number of BSE cases in
  routine slaughter group generated
• Cases per age cohort allocated based on BSE case
  age distribution
• Cases in fallen stock, emergency slaughter and
  feed cohort (Risk group)
• allocated based on the percentage of the total
  population.
• Risk group cases based on testing 2000-2001

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Part II Estimation of probability a BSE infected
animal will develop clinical BSE Depends
on Probability of surviving to a given
age Probability of BSE clinical signs at a given
age
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Monte Carlo Simulation Cattle gt 2yrs age
distribution Age at clinical onset
distribution Sampling from both distributions If
cow age gt BSE age you have a case 1 If cow age lt
BSE age no case0. Probability of
iterations1
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Age specific incidence rates -UK
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Effects of Population Age Structure and Incidence
Rates on Probability of BSE cases