Risks of transmitting scrapie and bovine spongiform encephalopathy by semen and embryos

This paper reviews current knowledge on transmission of scrapie and bovine spongiform encephalopathy (BSE) by semen and embryos. In sheep, in particular, it is difficult to distinguish between the genetic transmission of susceptibility to scrapie and vertical transmission of the infection. Nevertheless, there is evidence that vertical transmission of infection does occur, probably across the placenta, but none to suggest a significant scrapie risk from semen. Two teams have studied scrapie transmission from experimentally infected sheep using embryo transfer. Whereas one team found no evidence for transmission, the results from the other team suggest that embryos, even after washing, might carry the disease into the offspring. In regard to goats, although genetic differences in susceptibility exist, they are much less obvious than in sheep. There is no evidence for vertical transmission or for transmission through semen and embryos. With regard to BSE, although it appears that genetic differences in susceptibility are absent or unimportant, some recent work does suggest that the disease may be passed from cow to calf. The route of transmission and stage or stages when this takes place are unclear, however. In conclusion, despite growing evidence to indicate that scrapie and BSE are unlikely to be transmitted through semen and embryos, more research is needed to confirm this. Furthermore, until all possibility of risk is ruled out, risk reduction methods must be considered, especially when semen and embryos are being imported into countries where the diseases do not exist.