Twenty-one strains of avian paramyxovirus type 1 of low virulence for chickens were isolated in Denmark between 1996 and the beginning of 2003. The low virulence of the strains was demonstrated by sequencing the fusion (F) gene at the cleavage site motif and in some cases by determining the intracerebral pathogenicity index in day-old chicks. By using a panel of monoclonal antibodies it was shown that the isolates belonged to four different antigenic groups (five C2 isolates, six E isolates, six H isolates and four G/Q isolates). They were placed in three distinguishable genetic groups by phylogenetic analysis of a partial sequence of the F gene. The origin of the six E isolates was probably contaminated vaccines; the other viruses were isolated from wild birds and from poultry which probably came into contact with wild birds.
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