Lentogenic avian paramyxovirus 1 of both class I and class II are circulating in danish wild and commercial waterfowl.

Newcastle disease, a notifiable avian disease, is caused by virulent avian paramyxovirus 1 (APMV-1). In this study, we screened samples collected through the active and passive surveillance for avian influenza virus in Denmark between 2014 and 2022 for APMV-1, detecting 32 APMV-1 in wild and commercial waterfowl. Phylogenetic analysis identified them as class I genotype 1.2 and class II genotype I.2 viruses, all with monobasic fusion (F) cleavage sites (¹¹²ERQERL¹¹⁷ and ¹¹²GKQGRL¹¹⁷), characterizing them as lentogenic. Nucleotide and amino acid similarities ranged from 95 to 99.8% and 98.2 to 100% for class I viruses and 95.6-99.8% and 98.7-100% for class II viruses. Genetic comparisons of F genes showed divergence from vaccine strains, ruling out vaccine spillover. Selection analysis revealed neutral and negative selection pressures, suggesting limited adaptive evolution. These findings indicate that lentogenic APMV-1 circulated in Denmark from 2014 to 2022 without strong selection pressures driving significant genetic changes.