Identification of a new group of bovine viral diarrhea virus strains associated with severe outbreaks and high mortalities.

New BVDV strains associated with very high mortalities, which killed about 25% of the veal calves in Quebec in 1993, have been isolated. In this study, characterization of the last two-thirds of the 5' untranslated region (5'UTR) of their genome and virus neutralization experiments with polyvalent antisera raised in different animals both demonstrated that these strains formed a distinct group. Despite a difference of about 25% in the 5'UTR sequence with that of the classical strains, these 5'UTRs maintained the same secondary structure albeit with a higher stability. Serological crossreactivity between the classical and new BVDV strains was relatively low and suggest that new strains should also be included to obtain efficient BVDV vaccines. Based upon the distinct characteristics of these new BVDV strains, we propose to divide BVDV into two groups. Group I comprises the classical BVDV isolates including commonly used laboratory and vaccine strains, and group II comprises the newly described BVDV strains and those associated with thrombocytopenia and hemorrhaging.