ASA Competition: Section on Epidemiology
Session Slot: 8:30-10:20 Wednesday
Estimated Audience Size: xx-xxx
AudioVisual Request: xxx
Session Title:
Estimating the Magnitude of Transmissible Spongiform
Encephalopathy Epidemics in the United Kingdom: Bovine
Spongiform Encephalopathy (BSE) and the Probable Link to the
New Variant of Creutzfeldt-Jakob Disease (vCJD) in Humans
Theme Session: Yes
Applied Session: Yes
Estimation techniques for the magnitude of the BSE
epidemic in Great Britain will be presented. The Age-at-onset approach
estimates an upper bound to the epidemic with few parametric
assumptions. The backcalculation approach simultaneously estimates
necessary parameters and the size of the epidemic. The third
presentation will address the problem of estimating the magnitude of
the vCJD epidemic in humans given that the incubation distribution is
long, variable and unknown.
Session Organizer: MaWhinney, Samantha University of Colorado Health Science Center
Address: Department of Biometrics, Campus Box B119 University of Colorado Health Science Center 4200 East Ninth Ave Denver CO 80262
Phone: 303 315 7509
Fax: 303 315 3183
Email: sam@greeneggs.uchsc.edu
Session Timing: 110 minutes total (Sorry about format):
Opening Remarks by Chair - 5 or 0 minutes First Speaker - 30 minutes (or 25) Second Speaker - 30 minutes Third Speaker - 30 minutes Discussant - 10 minutes (or none) Floor Discussion - 10 minutes (or 5 or 15)
1. Estimating an Upper Bound on the BSE Epidemic in British Cattle
Samantha, MaWhinney, University of Colorado Health Science Center
Address: Department of Biometrics, Campus Box B119 University of Colorado Health Science Center 4200 East Ninth Ave Denver CO 80262
Phone: 303 315 7509
Fax: 303 315 3183
Email: sam@greeneggs.uchsc.edu
Abstract: The Bovine Spongiform Encephalopathy (BSE) epidemic in British cattle has generated public health concerns and has significantly impacted the British agricultural industry. Backcalculation methods have been used to estimate infection incidence requiring distributional forms for incubation period and age at infection, allowing for maternal and horizontal transmission. Assuming all infections occur early in life, an upper bound is obtained for the number of infections without separately parametrising these distributions. This method relies on a generalised linear model estimated using an EM algorithm to account for the truncated data. As the epidemic matures, the average age at infection decreases and stabilises (a function of exposure and the feed ban imposed in July 1988) and the upper bound approaches the actual number of infections. The primary analysis was restricted to the birth cohorts of cattle from 1984 to 1993. The bound for the total number of infections was estimated to be 1,061,400, which is consistent with earlier backcalculation results for all cohorts of 903,000 (840,000-1,250,000). The method is shown to yield robust estimates for the magnitude of the epidemic, assuming all infections occur early in life, without requiring the estimation of epidemiological parameters.
2. Statistical Analysis of the British BSE Epidemic: Past, Present and Future
Donnelly, Christl, University of Oxford
Address: Wellcome Trust Centre for the Epidemiology of Infectious Disease Department of Zoology University of Oxford South Parks Road Oxford OX1 3PS
Phone: 011 44 1865 281 244
Fax: 011 44 1865 281 241
Email: christl.donnelly@zoology.oxford.ac.uk
Abstract: Backcalculation models describing the key processes determining the pattern of the Bovine Spongiform Encephalopathy (BSE) epidemic in British cattle are derived that allow for infection from feed as well as maternal and direct horizontal transmission. Heterogeneous susceptibility classes are also incorporated into the analysis. Maximum likelihood methods are used to estimate parameters and to obtain confidence intervals from available experimental and epidemiological data. A comprehensive sensitivity analysis of model parameters and distributional assumptions is presented. Additional validation is provided by fitting the model to independent data collected in Northern Ireland. Model estimates and predictions based on BSE case data for Great Britain and Northern Ireland, together with their implications, are reviewed, and future research priorities discussed.
3. Analysis of New Variant CJD Cases in Great Britain: What Can We Say Now? What Will We Be Able to Say Later?
Ferguson, Neil, Affiliation University of Oxford
Address: Wellcome Trust Centre for the Epidemiology of Infectious Disease Department of Zoology University of Oxford South Parks Road Oxford OX1 3PS
Phone: 011 44 1865 281 228
Fax: 011 44 1865 281 241
Email: neil.ferguson@zoology.oxford.ac.uk
Abstract: By September 30, 1997 22 cases of new variant Creutzfeldt-Jakob disease (vCJD) had been confirmed in Great Britain. In March 1996, the Secretary of State for Health announced that 'There remains no scientific proof that BSE can be transmitted to man by beef, but the Committee have concluded that the most likely explanation at present is that these (10 vCJD) cases are linked to exposure to BSE before the introduction of the specified bovine offal ban in 1989.' Since this time, experimental studies have yielded strong scientific evidence linking BSE and vCJD. Backcalculation methods can be used to estimate future case numbers based on these initial cases; however, the lack of knowledge about the incubation period distribution results in high levels of uncertainty. The performance of backcalculation performed at various points throughout the time scale of the epidemic is evaluated under a variety of assumptions consistent with the currently available data.
List of speakers who are nonmembers: Neil Ferguson, Christl Donnelly