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Advantages of Probability-Based Approaches to Animal Disease Surveillance - Dr. Aaron Scott, Center Director, USDA/APHIS/VS/CEAH, from the 2012 Annual Conference of the National Institute for Animal Agriculture, March 26 - 29, Denver, CO, USA. More presentations at: http://www.trufflemedia.com/agmedia/conference/2012-decreasing-resources-increasing-regulation-advance-animal-agriculture
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NIAAMarch 27, 2012
Aaron Scott, DVM PhD DACVPM (epi) Center Director, NSU
Probability based approaches to animal health surveillance
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National Surveillance Unit“Building Partnerships and Leading Change”
Safeguarding Animal Health
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Using risk factors and probability to get the most
information at the lowest cost
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Why?
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Federal/State budgets
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What is surveillance?
Find disease
if its not there…
Prove it!!!
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Surveillance is evidence!
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Reverend Thomas Bayes
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Evidence may be combination of sample testing and
other knowledge about risk factors
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“Risk” = likelihood * consequence
= animals more likely to get disease? = animals more likely to have disease? = animals more likely to be detected?
= more dangerous to people????
Risk-based versus probability
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Inference …
…is drawing a conclusion about the population from one or
more pieces of evidence
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Sampling and inference
Convenience
Random
Representative
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Random Selection
•Inference is based on random sampling•Listing of all subjects (sample frame)•Each has equal chance of selection•Is always representative
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Random Sample size
“n” = 3X when 1:X
3,000,000 samples to detect 1 in 1,000,000
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Representative – non-biased
A representative sample could be just anybody!
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Geographic representation
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Representative
If not random, must be other supporting evidence for
representativeness…
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Targeted (“risk based”) sampling
Concept: Every sample gives information
Corollary: Some are worth more than others
Targeted = intentional bias!
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Must clearly define sub populationsMust have knowledge of risk factors
Must know magnitude of risk factorsTargeting represents sub-populationsMust know relationship
Targeting and inference
Population
Clear risk factor
Clear risk factor
3X
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What is “n”?
40 H-risk * value 3 = 12010 L-risk * value 0.5 = 5
50 tests = 125 “virtual” samples
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What are the risk factors?
Intentionally biased sample
+ other information
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Sows and boars (value = 2.5x to 5x ) Exposure to feral pigs (10x ? 100x ?) Non-confinement (??)
Some samples from all populationsMarket swine (< 1x)
PRV Targeting
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Slaughter sampling
From 3 herds: 5,000, 30, 10
Need “n” of 30/farm for inference30 + 30 + 10 =70 samples
Without additional info,n = 5,040!!!
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Collect all with traceable IDSend to lab and sortOnly knowledge is State of originDiscard all but 5% from each State
Current PRV
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PIN tag pilot
Collaboration:• NPB, AASV, industry reps• 6 State Veterinarians• Veterinary Services
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High value (feral swine exposure)Limit to number neededCollect “pink tag”, scan ID, combine
zip with feral risk, target # needed Other tagsSpend the money on other diseases
PIN Pilot
31Safeguarding Animal Health
national.surveillance.unit@aphis.usda.gov
http://www.aphis.usda.gov/vs/ceah/ncahs/nsu/
Thanks!!
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