INVESTIGATION OF AN EPIZOOTIC OF BOVINE VIRAL DIARRHEA VIRUS-INFECTION IN CALVES

Eight of 19 calves born to bovine viral diarrhea virus (BVDV)-negative and -immunocompetent dams were determined to be infected with a noncytopathic strain of BVDV. Six of the 8 calves had diarrhea and 2 had no clinical signs of disease. In 3 euthanatized calves, lesions consistent with mucosal disease were found throughout the gastrointestinal tract, and the virus was isolated from the spleen, lymph nodes, and small intestine. In 5 calves, BVDV was isolated from mononuclear cells in blood samples obtained 21 days apart, indicating persistent infection. The virus was not isolated from sera obtained from 2 calves, with chronic nonclinical infections, that had neutralizing antibody titers greater-than-or-equal-to 1:512 against bovine viral diarrhea-Singer virus and titers greater-than-or-equal-to 1:256 against the persistent BVDV. Twenty-one days after vaccination with a vaccine that contained inactivated noncytopathic and cytopathic biotypes of BVDV, 4 of 5 persistently infected calves had neutralizing antibody titers less-than-or-equal-to 1:4 against the bovine viral diarrhea-Singer virus and their persistent virus. Prior to vaccination, 2 of 11 virus-negative calves had neutralizing antibody titers less-than-or-equal-to 1:128 against the bovine viral diarrhea-Singer virus, and after vaccination, only 1 virus-negative calf had a titer less-than-or-equal-to 1:512. At 149 days after revaccination and prior to weaning, 4 virus-negative calves had neutralizing antibody titers less-than-or-equal-to 1:512 (range, 1:16 to 1:384). Under the specific conditions in this herd, we were not able to detect a beneficial effect of vaccination. Colostral origin BVDV-specific antibody, capable of neutralizing the persistently infective BVDV strain, most likely interfered with isolation of the virus from the sera of 2 persistently infected calves.