Scientific Opinion on the risk of transmission of classical scrapie via in vivo derived embryo transfer in ovine animals

The risk of transmission of classical scrapie via the transfer of in vivo derived embryo in ovines was assessed, taking into account the scientific information made available since the last EFSA opinion on this topic (2010) (see http://www.efsa.europa.eu/en/efsajournal/pub/1429.htm). The potential impact of PrP genotype of the embryo and/or of the ram and donor ewe on this risk was also assessed. The new data made available over the last three years further reinforce the view that classical scrapie could be vertically transmitted in sheep. Since the possibility of such vertical transmission was already considered in the previous opinion, its conclusions and recommendations relating to the risk of classical scrapie transmission via embryo transfer remain valid. In ovines, the susceptibility to classical scrapie infection in sheep is strongly influenced by certain polymorphisms of the PrP gene. Under natural exposure conditions, animals that are heterozygous or homozygous A₁₃₆R₁₅₄R₁₇₁ display respectively a low or negligible risk of being infected. The genetic control of the susceptibility to classical scrapie is also likely to impact on the risk of transmitting the disease via embryo transfer. Irrespective of the embryo’s genotype, embryos derived from rams and dams carrying at least one ARR allele would significantly decrease this risk (compared to an embryo from parents of unknown genotypes). The use of homozygous ARR embryos would provide the highest level of safety regarding the risk of transmitting classical scrapie through embryo transfer (in vivo derived embryos). The use of heterozygous ARR embryos would ensure a higher level of safety compared to Q₁₇₁/Q₁₇₁ embryos. Finally, it was concluded that, providing the OIE recommendations and procedures relating to embryo transfer are adhered to, the risk of transmitting classical scrapie due to the transfer of homozygous or heterozygous ovine ARR embryos can be considered negligible.