Abstract
Scrapie is a transmissible spongiform encephalopathy in sheep and an example of a disease that may be controlled through breeding for disease resistance. Member states of the European Union have introduced strategies for breeding against scrapie based on the selection of genetically resistant breeding rams. An ambitious strategy adopted in The Netherlands consisted of selecting resistant rams for breeding throughout both breeding and production sectors. Mathematical modelling of the effect of a breeding program on the spreading capacity of scrapie in a national flock is needed for making assessments on how long a breeding strategy needs to be maintained to achieve disease control. Here we describe such a model applied to the Dutch situation, with the use of data on the genetic content of the Dutch sheep population as well as on scrapie occurrence in this population. We show that the time needed for obtaining scrapie control depends crucially on two parameters measuring sheep population structure: the between-flock heterogeneity in genotype frequencies, and the heterogeneity of mixing (contact rates) between sheep flocks. Estimating the first parameter from Dutch genetic survey data and assuming scenario values for the second one, enables model prediction of the time needed to achieve scrapie control in The Netherlands.
Highlights
Scrapie is a fatal infectious neurodegenerative disease for which susceptibility is associated with polymorphisms in the ovine prion protein (PrP) gene
The results suggest that for compliance of 75%, scrapie control is achieved in The Netherlands when the overall ARR frequency exceeds a minimum value in the range of 63 to 70 percent across scenarios assuming moderate heterogeneity of between-flock mixing
By a sensitivity analysis we have shown that the model prediction for the time needed for obtaining scrapie control is dependent in particular on the heterogeneity of between-flock mixing, which usually is difficult to estimate due to a paucity of data
Summary
Scrapie is a fatal infectious neurodegenerative disease for which susceptibility is associated with polymorphisms in the ovine prion protein (PrP) gene. Polymorphisms at codons 136 (A/ V), 154 (R/H) and 171 (Q/R/H) largely determine resistance to scrapie with the VRQ allele being most susceptible, and the ARR allele being resistant to classical scrapie [1,2,3]. Based on selective breeding for resistance, national eradication programs have been implemented in several countries in Europe, including Great Britain [4,5,6,7,8,9] and The Netherlands [10]. One of the most ambitious programs was implemented in the Netherlands, where selection of rams with the ARR/ARR genotype for breeding started in 1998 (voluntary basis) and was obligatory for all sheep farmers from October 2004 to June 2007 [11].
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