Abstract
Field isolates of BVDV which do not show the exaltation of Newcastle disease virus (END) phenomenon (END(-)) are rarely reported. In this study, 45 BVDV field isolates from cattle in Hokkaido prefecture in Japan were analyzed by the reverse plaque formation method, the END method and observation of cytopathic effects. END(-) virus was detected in 34 of 45 isolates (75.6%), although 35 of 45 field isolates contained END phenomenon positive virus as the predominant virus population. We propose that END(-) viruses are widely distributed in the field and that it is possible that the mixture of biologically distinct BVDV correlates with the appearance of disease in infected animals.