Risks of transmitting ruminant spongiform encephalopathies (prion diseases) by semen and embryo transfer techniques

Early experiments suggested that scrapie transmission via sheep embryos was a possibility, and gave rise to much controversy. However, when account is taken of the complex genetic effects on ovine susceptibility to scrapie, and of the several different scrapie strains with different clinical and pathological effects, the overall conclusion now is that transmission of classical scrapie by embryo transfer is very unlikely if appropriate precautions are taken. Recent embryo transfer studies have confirmed this. Other studies in sheep have shown that from about the middle of pregnancy the placental trophoblast is liable to scrapie infection in genetically susceptible ewes if the fetus is also susceptible. Since the contrary is also true, use of resistant ewes as embryo recipients could add to the safety of the embryo transfer, at least for classical scrapie. There has been little recent research on scrapie transmission via semen in sheep, and, with hindsight, the early studies, though negative, were inadequate. There is scant information on scrapie transfer via goat semen or embryos, although one study did find that bovine spongiform encephalopathy (BSE) was not transmitted via goat embryos. In cattle it has been shown that, if appropriate precautions are taken, the risks of transmitting BSE via semen and in vivo-derived embryos are negligible, and this conclusion has gained worldwide acceptance. Research on TSE transmission via reproductive technologies in deer has not yet been done, but information on the pathogenesis and epidemiology of chronic wasting disease (CWD) of deer, and on transmission risks in other species, provides optimism that transmission of CWD via semen and embryos of deer is unlikely. The presence of TSE infectivity in blood and various other tissues of infected animals, particularly sheep, gives rise to concerns that certain biological products currently used in reproductive technologies, e.g. pituitary gonadotrophins for superovulation, and certain tissue and blood products used in semen and embryo transfer media, could carry TSE infectivity. Instruments such as laparoscopes used for insemination, and for collection and transfer of embryos, especially in small ruminants, are also a concern because effective decontamination can be very difficult.