Avian Metapneumovirus: Virology, Epidemiology, and Insights from a Comparative Analysis with Human Metapneumovirus—A Review

Metapneumoviruses comprise a genus of negative-sense RNA viruses that cause significant respiratory disease across human and avian hosts. Human metapneumovirus (hMPV) is a globally prevalent pathogen associated with acute lower respiratory tract infections in infants, older adults, and immunocompromised individuals. Avian metapneumovirus (aMPV) imposes substantial economic losses on the poultry industry through respiratory disease, reproductive impairment, and high mortality in the presence of secondary infections. Despite their distinctive host ranges, hMPV and aMPV share a conserved genomic architecture and encode homologous structural and non-structural proteins that mediate viral entry, replication, assembly, and evasion of host innate immunity. Comparative analysis highlights that both have deeply conserved polymerase and nucleocapsid functions, and yet have a wide range of diversity in the attachment glycoprotein (G) and small hydrophobic protein (SH), reflecting divergent evolutionary pressures in human versus avian hosts that have led to such distinctive differences. The recent emergence and detection of aMPV/A and aMPV/B across the previously aMPV-free United States beginning in late 2023, combined with rising cases globally of hMPV post-SARS-CoV-2 pandemic, underscore the continued challenges of metapneumovirus surveillance and control in humans and animals. This review aims to highlight the current knowledge on the history, molecular virology, pathogenesis, epidemiology, diagnostics, and control strategies for aMPV while drawing mechanistic parallels to hMPV. By contextualizing shared biology and structure alongside host-specific adaptations, we aim to identify key gaps that shape vaccine design, antiviral development, and future research priorities aimed at mitigating the health and economic burden posed by metapneumoviruses found in both birds and humans.