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Predicting an Epidemic. A Quantitative Assessment of TSE Sampling Data to Predict Outbreak Magnitude Aspen Shackleford HONR299J. Variant Creutzfeldt-Jakob Disease (vCJD). Linked to BSE Unknown number of individuals who may be infected Iatrogenic contamination.

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predicting an epidemic

Predicting an Epidemic

A Quantitative Assessment of TSE Sampling Data to Predict Outbreak Magnitude

Aspen Shackleford

HONR299J

variant creutzfeldt jakob disease vcjd
Variant Creutzfeldt-Jakob Disease (vCJD)
  • Linked to BSE
  • Unknown number of individuals who may be infected
  • Iatrogenic contamination

The National CJD Research & Surveillance Unit. (2014). Variant Creutzfeldt-Jakob Disease current data (March 2014) [Data file]. Retrieved from http://www.cjd.ed.ac.uk/documents/worldfigs.pdf

how are outbreak predictions made
How are outbreak predictions made?
  • Reporting
  • Statistical Models
dealler kent 1995
Dealler & Kent, 1995
  • Age of onset of clinical BSE symptoms was sampled
  • Results
    • Decrease in peak age of onset
    • Led to further investigation of under-reporting

Dealler, S.F. & Kent, J.T. (1995). BSE: an update on the statistical evidence. British Food Journal, 97 (8), 3-18. http://docserver.ingentaconnect.com

under reporting
Under-Reporting
  • MAFF Requirements
    • 2 visits by a veterinarian
    • Slaughter and send in tissue sample
  • Case denial
    • Dealler and Kent Suspicion
    • Paul Brown’s predictions
  • Farmer Initiative
paul brown s prediction
Paul Brown’s Prediction

Brown, Paul. (2004). Mad Cow Disease in cattle and human beings: Bovine spongiform encephalopathy provides a case study in how to manage risks while still learning facts. American Scientist, 92 (4), 334-341. http://www.jstor.org/stable/27858422

under reporting1
Under-Reporting
  • MAFF Requirements
    • 2 visits by a veterinarian
    • Slaughter and send in tissue sample
  • Case denial
    • Dealler and Kent Suspicion
    • Paul Brown’s predictions
  • Farmer Initiative
dealler and kent statistical model 1995
Dealler and Kent Statistical Model (1995)
  • Based upon data collection year (i) and bovine age (j)
  • Follows a Poisson Distribution
  • Predicts the expected number of deaths at age j in year i from 1984 to 2001
  • Reporting effect and parameters compensated for under-reporting
poisson distribution
Poisson Distribution
  • Requirements
  • Used to determine the frequency of an abnormal event
  • Graphical representation
    • r-curve closer to zero signifies a rare event

The Warring States Project. (2007, August 24). Statistics: The Poisson distribution. Retrieved from http://www.umass.edu/wsp/resources/poisson/index.html

dealler and kent statistical model 19951
Dealler and Kent Statistical Model (1995)
  • Based upon data collection year and bovine age
  • Follows a Poisson Distribution
  • Predicts the expected number of deaths at age j in year i from 1984 to 2001
  • Reporting effect and parameters compensated for under-reporting
results
Results
  • Model predictions from 1984 through 2001 (solid line)
  • Actual reports from 1984 to 1993 (dotted line)
  • Under-reporting
  • Peak in 1994
  • Overlap

Dealler, S.F. & Kent, J.T. (1995). BSE: an update on the statistical evidence. British Food Journal, 97 (8), 3-18. http://docserver.ingentaconnect.com

hagenaars et al 2006
Hagenaars et al., 2006
  • Scrapie
    • Within-flock model
      • Sheep age
      • Genotypes: ARR (0.45), ARQ (0.5), VRQ (0.05)
    • Between-flock model
  • Model determines duration and magnitude of an outbreak
slide13

N - flock-size

  • n - geometric mean of the size distribution
  • c1 , c2 – constants

Hagenaars, T.J, Donnelly, C.A., & Ferguson, N.M. (2006). Epidemiological analysis of data for scrapie in Great Britain. Epidemiology and Infection, 134 (2), 359-367. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870388/

within flock results
Within Flock Results
  • Large case rate
  • Case rate less than 5 per year
  • Average number of cases: 2.8 per year

Hagenaars, T.J, Donnelly, C.A., & Ferguson, N.M. (2006). Epidemiological analysis of data for scrapie in Great Britain. Epidemiology and Infection, 134 (2), 359-367. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870388/

slide15

γ – the rate of recovery of affected farms

  • λ - the rate per capita that a farm becomes affected
  • t - time (in years)
  • at>0 - the number of sheep flocks that have experienced at least one BSE case since the starting year

Hagenaars, T.J, Donnelly, C.A., & Ferguson, N.M. (2006). Epidemiological analysis of data for scrapie in Great Britain. Epidemiology and Infection, 134 (2), 359-367. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870388/

between flock results
Between Flock Results
  • Low basic reproduction rate
  • High basic reproduction rate
  • Breeding for resistance
    • ARR/ARR genotype
  • Changes in management

Hagenaars, T.J, Donnelly, C.A., & Ferguson, N.M. (2006). Epidemiological analysis of data for scrapie in Great Britain. Epidemiology and Infection, 134 (2), 359-367. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870388/

conclusion
Conclusion
  • Statistical models are widely used
  • Statistical models offer information that uses parameters and constraints that model real life
  • The predictions of statistical models can be used to make decisions about how to best prevent an outbreak that threatens human and animal populations
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