150 THE ROLE OF THE PRE-IMPLANTATION EMBRYO IN THE VERTICAL TRANSMISSION OF NATURAL SCRAPIE INFECTION IN SHEEP

Abstract

The objective of this study was to examine whether or not the pre-implantation embryo can act as a carrier of scrapie. The study was carried out on quarantined premises with sheep of highly susceptible scrapie genotypes. Fifty-seven lambs were produced from embryos collected from donor ewes. These donor ewes were in a scrapie-infected flock and became clinically sick with scrapie or developed clinical scrapie after embryo collection. Fifty of the generated lambs were confirmed as having scrapie-susceptible genotypes with 48 animals identified as ARQ/ARQ and 2 as ARQ/ARH. The respective mean and median ages for the 57 scrapie-susceptible experimental offspring in the study were 1565 and 1826 days. Forty-one of these animals survived to the end point of the study at 5 years of age. In a negative control group the mean and median ages at death for the 17 sheep were 1500 and 1826 days, respectively, and 12 survived to 5 years of age. Post-mortem examinations were carried out on all animals derived by embryo transfer and in none was histological or immunohistochemical evidence of scrapie found. In contrast, in the originating infected flock the majority of scrapie cases occurred in ARQ/ARQ genotyped animals with 56% mortality from scrapie in animals of this genotype. Thus, the study provides no evidence for transmission of scrapie and reinforces published evidence that vertical transmission of scrapie may be circumvented by embryo transfer procedures.

This study was commissioned and financially supported by Defra. Amongst the many involved in the study the authors particularly wish to acknowledge Jon Hunton, James Mylne, Tony Wrathall and Chris Lewis. Keith Chalmers Watson is thanked for his interest and for the land rental that made the work feasible.