Three cases of Newcastle disease virus (NDV) found in nature had the lentogenic motif 112G-R-Q-G-R-L117 in their fusion protein cleavage sites. However, both intracerebral pathogenicity and intravenous pathogenicity indexes showed that these NDV isolates were virulent. In comparison with the LaSota live virus vaccine, these viruses had significant genetic variations in the hemagglutinin-neuraminidase gene.
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