Clinical evaluation of a temperature-sensitive bovine viral diarrhea vaccine strain

A naturally occurring strain of bovine viral diarrhea (BVD) virus was chemically treated to produce a genetically stable, temperature-sensitive mutant, designated RIT 4350. The RIT 4350 strain had a restrictive growth temperature of 39.5 C, so that systemic replication or fetal infection was not detected after parenteral administration in cattle. The RIT 4350 strain was tested as a cell culture-adapted vaccine in healthy heifers, immunodepressed calves, and pregnant cows. In a pathogenicity test in 5 healthy heifers, vaccination with 10 times the field dose resulted in seroconversion, but produced no clinical signs of disease or leukopenia. In a pathogenicity test in immunodepressed calves, 7 test animals were treated with dexamethasone and inoculated with 10 times the field dose. The calves developed mild enteric signs, but virus isolation attempts were negative. Following vaccination, the immunodepressed vaccinated calves were challenge exposed with the Osloss strain of BVD virus; all these vaccinated calves remained healthy. Two of 68 postchallenge serum samples from vaccinated calves were positive for BVD virus, compared with 9 of 20 samples from control calves. In another pathogenicity test, 7 pregnant cows vaccinated with 4 times the field dose seroconverted, remained clinically healthy, and delivered healthy calves.