152 TRANSMISSION OF BOVINE VIRAL DIARRHEA VIRUS BY IN VITRO-FERTILIZED EMBRYOS UNDER EXPERIMENTAL CONDITIONS

Abstract

Bovine viral diarrhea virus (BVDV) infection affects cattle throughout the world. It causes significant economic losses in the cattle industry. The potential for transmission of a cytopathic biotype of BVDV by in vivo-derived embryos has been thought to be negligible. However, there is no study to prove non-transmission of the most common field isolate of noncytopathic biotype (NCPB) of BVDV by IVF embryos. Here we report on the preliminary outcome of embryo transfer (ET) of IVF embryos exposed in vitro to type-1 (NY-1) and type-2 (P-131) genotypes of NCPB of BVDV. For this experiment, IVF embryos were generated using standard methods which briefly involve: maturation of cumulus–oocyte complexes in TCM medium, fertilization of oocytes with BVDV-free semen, and culture of zygotes to the blastocyst stage in SOF medium without somatic cells. Day 7 blastocysts were exposed for 1 h to NY-1 or P-131 (10³–10⁷ TCID₅₀ mL^–1) BVDV strains before being washed (without trypsin) as recommended by IETS. Two embryos were transferred on each occasion. Embryo recipients were virus-free and anti-BVDV antibody-free prior to ET. The recipients remained individually in isolation premises after ET. In total, 126 ET procedures were performed resulting in 57 pregnancies and 34 calves born free of the infectious virus and BVDV antibodies (5 pregnancies are still pending). In total, 23 pregnancies were lost after 30 days. Exposure of embryos to type-2 BVDV resulted in a loss of 46% (17/37) of pregnancies after 30 days post-ET and 20 recipients seroconverted to BVDV. Within seroconverted and pregnant animals (n = 14), only 2 recipients maintained pregnancy and delivered uninfected calves at term. In contrast, exposure of embryos to type-1 caused 30% (6/20) of the pregnancy losses after 30 days and did not cause any seroconversion in ET recipients. After washing, 33% (3/9) and 38% (17/44) single embryos from the infected pool of IVF embryos tested positive for the BVDV. In conclusion, under these experimental conditions, a proportion of recipients was apparently infected after receipt of BVDV-exposed embryos. However, all of the calves that survived to term were BVDV-free and anti-BVDV antibody free.