The present invention is directed to a bovine viral diarrhea virus comprising at least one helicase domain amino acid mutation wherein the mutation in the NS3 domain results in a loss of recognition by a monoclonal antibody raised against wild-type NS3 but wherein viral RNA replication and the generation of infectious virus is retained. The present invention is useful, for example, to produce a marked bovine viral diarrhea virus vaccine or to differentiate between vaccinated and infected or unvaccinated animals.
