Technical Updates to the Harvard BSE Risk Assessment: FSIS Scenario Analyses

Uday Dessai, MS, Ph.D., M.P.H.
Director, Microbiology Division
Office of Public Health Science
Food Safety and Inspection Service
U.S. Department of Agriculture

Presentation Overview

• Modeling considerations in the Harvard BSE model
• Known cases of BSE in the United States
• FSIS Scenarios
  • SRM removal
  • Compliance

Modeling Considerations in the Harvard BSE Model

• The source of the infectivity (indigenous or external BSE infected cattle) used to initiate the model does not influence model results.
• Model is generally initiated with the hypothetical introduction of 10 BSE infected animals into the United States.
• The potential human exposure expressed as cattle oral ID50s is a cumulative measure over a period of 20 years.
• The model results are in a 3% range for numerical stability.
• To measure the effect of FSIS mitigations, alternative scenarios were compared to the base case. The results are expressed as % reduction in potential human exposure.
• Perfect compliance was assumed in modeling the mitigation scenarios unless stated otherwise.

Known Cases of BSE in the United States

• First BSE positive case (Washington State): A 6.5 year-old cow tested positive in December 2003. This cow was imported into the United States from Canada.
• Second BSE positive case (Texas): A 12 year-old cow born and raised in the United States, confirmed BSE positive in June 2005.
• Third BSE positive case (Alabama): A 10 year-old cow born and raised in the United States, tested positive for BSE in March 2006.

Known Cases of BSE in the United States: In the Context of Harvard BSE Model

• The revised 2005 Harvard BSE model can be initiated using different hypothetical numbers of BSE infected cattle (1, 10, 500...) to measure the effect of mitigations of interest.
• The revised 2005 Harvard BSE base case scenario predicted ~3.5 new cases of BSE when 10 BSE infected cattle were introduced in the United States.
• The revised 2005 Harvard BSE model does not take into account the current number of BSE cases detected in the United States.

FSIS Scenarios

• Background/need for FSIS scenarios:
  • The revised 2005 Harvard BSE model showed that a ban on use of SRMs has the biggest impact on the spread of BSE among cattle and human exposure.
  • The International Review Committee recommended removal of SRMs from cattle = 12 months.
  • The 2005 Harvard BSE scenarios considered SRM removal from cattle = 30 months (except for the International Review Committee scenario).
  • Potential human exposure to BSE in the United States as a result of less than perfect compliance in SRM removal had not been modeled.
    
    (Distal ileum and tonsil (both SRMs) are removed from all cattle irrespective of age.)
• FSIS modeled the following scenarios that could further strengthen the SRM removal mitigations adopted by FSIS post January 2004.
  • SRM removal from younger cattle
    • SRM removal from cattle > 24 months and cattle >12 months of age
    • SRM removal from cattle > 24 months and cattle >12 months of age, combined with a ban on non-Ambulatory cattle
  • Less than perfect (100%) compliance in SRM removal
    • SRM removal compliance from 100% to 95%

FSIS Scenario: Removal of SRMs from Younger Cattle

FSIS Scenario: Less than Perfect Compliance in Removal of SRMs

The model output indicates that for every 1% drop in SRM removal compliance (from 100% to .. 95%), there is a corresponding 1% increase in potential human exposure to BSE.

All the other SRM removal scenarios modeled by Harvard and FSIS assumed perfect compliance.

Concluding Remarks

1. Removal of SRMs from cattle under the age of 30 months did not provide additional benefit to further reduce potential human exposure to BSE.
2. The model output shows that for every 1% drop in compliance for removal of SRMs, there was a 1% increase in potential human exposure to BSE.

Although the hypothetical introduction of 500 BSE infected cattle was for computational convergence and to reduce model run time (~30 days to ~3 days), the results (% change) for FSIS mitigation scenarios are equally relevant for any number of animals (from 1-500).

Thank you.