Avian metapneumovirus infections in poultry flocks in Poland – molecular evidence for emerging divergence from vaccine strains

Abstract

Despite available vaccination strategies, respiratory disease caused by avian metapneumovirus (aMPV) has increased in Polish poultry in recent years. Between 2022 and 2025, 48 aMPV outbreaks were confirmed in chicken and turkey flocks, with positive vaccination status reported for 25 flocks. Subtyping and phylogenetic analyses based on partial glycoprotein (G) gene sequences revealed a predominance of aMPV subtype B (n = 47) and a single subtype A. Forty-two aMPV-B strains clustered in a distinct “Polish field” group, whereas four associated with vaccine strains. Full-length genomes were obtained for two chicken-origin aMPV-B viruses, enabling comprehensive phylogenetic and molecular characterization of individual viral genes. Phylogenetic analyses indicated that these strains formed a separate branch within the cluster of early European and vaccine strains, whereas American and Asian strains form distinct clusters. Considerable evolutionary divergence from other European aMPV-B strains within this cluster was evident. A gap of over 30 years separates the first European aMPV-B strains (1980s–1990s) from Polish strains from 2022–2023. Analysis of amino acid sequences revealed numerous substitutions in the surface G and SH proteins, including changes in regions implicated in antigenicity, receptor binding, and immune recognition. These alterations may modify viral epitopes, potentially contributing to immune escape and reducing the efficacy of current vaccines. Overall, the presented study underscores the importance of continuous genomic surveillance of circulating strains, particularly given the limited availability of complete European aMPV genomes, to better inform vaccine design and disease control measures.