Lentogenic field isolates of Newcastle disease virus isolated in Canada and Hungary are identical with the vaccine type used in the region

Lentogenic field isolates of Newcastle disease virus were examined by restr iction enzyme analysis of RT-PCR products generated from the matrix protein gene that discriminates between strains LaSota and B-1, the two most widel y used lentogenic vaccine viruses. Isolates were derived from regions where , exclusively or predominantly, only one type of vaccine was employed. Viru ses collected in Hungary for two decades were exclusively of LaSota-type wh ile the Canadian collection predominantly included B-l, which corresponded to the vaccine types used in the regions. Isolation of vaccine type lentoge nic viruses from unvaccinated flocks supports the occurrence of area spread of these lentogenic viruses.