Experimental infection of calves with two strains of bovine virus diarrhoea virus: virus recovery and clinical reactions.

Fifteen calves were inoculated with a mixture of two strains of bovine virus diarrhoea virus (BVDV), the cytopathogenic NADL strain which had been passaged over 20 times n vitro, and the non-cytopathogenic FCS strain, passaged only once after isolation from fetal calf serum. In a second experiment, seven calves received the NADL strain, and eight the FCS strain. The clinical and virological results of the two experiments were compared. In dual infections, the NADL strain interfered with the replication of the FCS strain resulting in less severe disease than the FCS strain alone. The FCS-BVDV was recovered from nasopharyngeal swabs and buffy coat cells whereas the NADL-BVDV was recovered only from nasopharyngeal swabs. The cytopathogenicity of the two strains did not change after passage in vivo. The differences observed are discussed in relation to cultural history and cytopathogenicity.