Phylogenetic, antigenic and clinical characterization of type 2 BVDV from North America

Bovine viral diarrhea virus (BVDV) infection continues to have a significant impact upon US cattle producers despite the availability of more than 140 federally licensed vaccines. Detection and control is hampered by viral heterogeneity that results in differences in neutralizing epitopes, cytopathology and virulence. Recently it was found that there are two different genotypes, BVDV1 and BVDV2, among BVDV. BVDV2 isolates make up a significant proportion of the BVDV isolated in North America. Serologically BVDV2 viruses can be distinguished from BVDV1 and border disease viruses. Mab binding also distinguishes between BVDV1, BVDV2 and BDV. Like the BVDV1 viruses, BVDV2 viruses may exist as one of two biotypes, cytopathic or noncytopathic, based on their activity in cultured cells. Cytopathogenic effects on cultured cells does not correlate with virulence in vivo, as BVDV2 associated with hemorrhagic syndrome (HS) are noncytopathic. Variation among BVDV1 and BVDV2 in the 5' UTR is similar. Phylogenetic analysis and differences in virulence suggest that BVDV2 are heterogeneous. Symptoms resulting from BVDV2 infections may range from clinically inapparent to clinically severe. Recently, disease outbreaks associated with acute uncomplicated BVDV infection have been reported in the US and Canada. These outbreaks of clinically severe disease, termed HS, were all associated with viruses from the BVDV2 genotype. Not all BVDV2 isolates cause clinically severe disease. Avirulent BVDV2 isolates do exist and may predominate over virulent BVDV2 in nature. When virulent BVDV2 viruses are inoculated into calves they induce a disease characterized by fever, diarrhea, leukopenia, lymphopenia, neutropenia, thrombocytopenia, and death. Infection with avirulent BVDV2 results in a reduction of luekocytes that may be accompanied by a low-grade fever. These viruses do not cause clinical disease or a clinical leukopenia.