Age-dependent windows for cohort culling in BSE herds

With the discovery of a BSE case in a herd, practice in Denmark prior to 2005 has been to cull any herd in its entirety, to avoid the risk of further cases. However, a growing dissatisfaction with this practice has led to a desire to be able to cull a minor fraction of a BSE herd only, while still removing the majority of the risk. One proposed method has been cohort culling. All animals in a certain age window around the age of the infected animal are culled, assuming that the BSE case and all other potential cases were infected at roughly the same time. This paper presents a method to characterize the animals in a BSE infected herd most at risk for developing BSE from the exposure that resulted in the index case. The results suggest that a ±one-and-a-half year cohort cull would work well in Denmark for index BSE cases with an age of 6 years or less, but for older BSE cases the method is completely inadequate, making age-dependent windows a necessity. Formulas for calculating age-dependent windows are provided, and age-dependent windows for a herd size corresponding to a herd size equal to the 85% percentile of the Danish herds are provided in graphical form. A case study with a 9-year-old index case is presented.