Assessment of the genetic risk and impact of lateral transmission in a valine-associated scrapie outbreak in sheep.

Abstract

OBJECTIVE: To characterize an outbreak of valine-associated scrapie, assess the relative risk of scrapie infection in relation to allele frequency at codon 136, and investigate lateral transmission of infection in a sheep flock within the United States. ANIMALS: 1,006 sheep. PROCEDURE: To determine genotypes, blood or semen samples were assessed via commercial testing; in 190 slaughtered sheep, scrapie status was determined via immunohistochemical evaluation of tissues. Scrapie-positive sheep born to scrapie-negative dams and sheep infected after 1 year of age were identified to assess lateral transmission. RESULTS: Genotypes were determined for codon 171 (164 sheep) or codons 136 and 171 (842 sheep). Forty-four of 160 slaughtered sheep of known genotype were scrapie positive. In these sheep, the presence of at least 1 valine allele at codon 136 (V136) was highly correlated with scrapie-positive status. Lateral transmission was the probable source of infection for 4 scrapie-positive sheep born to scrapie-negative dams and for 11 sheep in which scrapie was diagnosed at > 50 months of age. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that the outbreak of scrapie was associated with a relatively high frequency of V136 in the flock, introduction of a valine-dependent scrapie strain, and the occurrence of lateral transmission. Genotyping of sheep may assist management decisions following diagnosis of scrapie in a sheep with at least 1 V136. It may be prudent to remove sheep of the diploid genotype AVQR (at codons 136 and 171) from infected flocks.