154 THE QUANTITY OF BOVINE VIRAL DIARRHEA VIRUS ASSOCIATED WITH SINGLE ZONA PELLUCIDA-INTACT IN VITRO-PRODUCED BOVINE EMBRYOS FOLLOWING ARTIFICIAL EXPOSURE

The primary objective of this study was to determine the percentage of individual, preimplantation, in vitro-produced bovine embryos which maintained association with virus despite washing following artificial exposure to a high affinity strain of bovine viral diarrhea virus (BVDV). Another objective of this study was to determine the quantity of virus associated with these embryos. A total of eighty-seven zona pellucida-intact, Day 7, in vitro-produced bovine embryos were exposed for 1 h to 2 � 106 cell culture infected doses per mL to the 50 percent endpoint (CCID50 mL–1) of a type 1 noncytopathic strain of BVDV (SD-1). Following exposure, the embryos were washed according to International Embryo Transfer Society standards for in vitro-produced bovine embryos; they then underwent sonication, RNA extraction, and freezing at –80�C until assayed for virus. A real-time quantitative polymerase chain reaction (QPCR) was run in duplicate on each of the 87 embryos. Forty-two percent (39/87) of the embryos assayed were determined to be positive for virus. The quantity of virus associated with the embryos averaged 0.55 viral copies per 5 µL (SD = 0.89 copies/5 µL, SEM = 0.14 copies/5 µL). Assessment of data using tolerance intervals (P = 0.05) indicates that 90% of contaminated embryos were associated with ≤2.40 viral copies per 5 µL while 99% of contaminated embryos were associated with ≤3.44 viral copies per 5 µL. These findings show that there is a low level of virus associated with in vitro-produced embryos but virus is associated with a significant number of exposed embryos. In conclusion, this study indicates that the potential for transmission of BVDV via embryo transfer of in vitro-produced embryos is small given the amount of virus that was found to associate with individual embryos.