LOSSES OVER A 2-YEAR PERIOD ASSOCIATED WITH FETAL INFECTION WITH THE BOVINE VIRAL DIARRHEA VIRUS IN A BEEF COW-CALF HERD IN SASKATCHEWAN

In 1992, significant calf losses occurred between birth and weaning in a 650-cow Saskatchewan beef herd. These losses occurred subsequent to ill-thrift and disease, and every calf necropsied was found to be per sistently infected with bovine viral diarrhea virus (BVDV). The object ives of this study were to describe the losses associated with fetal i nfection with BVDV in this herd and to determine why they occurred. Fo r investigative purposes, blood samples were collected from the entire cow herd and the surviving calves at pregnancy testing in 1992, and t ested by virus isolation for BVDV. Between 51 and 71 persistently infe cted calves were born in 1992. Bovine viral diarrhea virus was only is olated from calves. The only confirmed fetal infections with BVDV were recorded as the birth of persistently infected calves. However, abort ions, reduced pregnancy rates, and delayed calvings were also recorded in the cow herd and may have been the result of fetal infections. The herd was monitored again in 1993. Fetal infections with BVDV were rec orded as the birth of stunted, deformed, and persistently infected cal ves. The greatest losses due to fetal infection with BVDV in the 2 yea rs of this study occurred in cows that were 3-years-old at calving (se cond calvers). Bovine viral diarrhea virus appears to have remained en demic in this herd by transmission from persistently infected calves o n young 3- and 4-year-old cows to naive calved 2-year-old cows that we re mingled with them annually for rebreeding. Significant numbers of t he 2-year-old cows remained naive to BVDV, because they were segregate d from persistently infected calves at weaning, preventing cross-infec tion with BVDV.