Isolation and characterization of avian metapneumovirus subtypes A and B associated with the 2024 disease outbreaks among poultry in the USA.

Avian metapneumovirus (aMPV) is currently classified into four subtypes (aMPV-A, -B, -C, and -D). In late 2023 and early 2024, aMPV-A and aMPV-B were detected in US poultry for the first time, causing significant economic losses. This study analyzed aMPV RT-PCR data from 2,204 samples (1,158 turkey, 936 chicken, and 110 other breeds) submitted to a US veterinary diagnostic laboratory between January and November 2024. A higher percentage of turkey samples (51.04%) tested PCR-positive for aMPV-A and/or aMPV-B compared to chicken samples (15.6%), with aMPV-A showing an overall higher positive rate than aMPV-B, although the positive rates varied by state. Additionally, four aMPV-A and three aMPV-B isolates were successfully recovered from clinical samples using primary chicken embryo lung and/or fibroblast cells. Two aMPV-A isolates (USA/IA55601-6/2024 and USA/IA56509-5/2024) and two aMPV-B isolates (USA/NC20487-GA/2024 and USA/NC23734-GA/2024) were adapted to grow efficiently in a Vero cell line, reaching titers of ~10⁴-10⁶ TCID₅₀/mL between passages 4 and 10 for aMPV-A and between passages 1 and 10 for aMPV-B. Whole genome sequencing of the two aMPV-A and two aMPV-B isolates at different passages revealed that the viruses progressively acquired several nucleotide changes, some of which led to amino acid substitutions in different viral proteins during 10 passages in cell culture. Comparative analysis with 46 aMPV-A, -B, -C, and -D GenBank sequences showed that US aMPV-A and aMPV-B strains were genetically closely related within their subtypes. These cell culture-adapted US aMPV-A and aMPV-B isolates provide valuable tools for further characterization of aMPV and vaccine development.

Importance: Avian metapneumovirus (aMPV) subtypes A and B were first detected in US poultry in late 2023 and early 2024, rapidly spreading nationwide and posing a significant threat to the industry. This study analyzed RT-PCR data from 2,204 clinical samples (January to November 2024) to determine aMPV-A and aMPV-B detection rates across poultry species, age groups, and states, providing insights into their epidemiology in the USA. Modified live vaccines are urgently needed to control aMPV but are hindered by the lack of US isolates growing efficiently in cell culture. We successfully isolated aMPV-A and aMPV-B in primary chicken embryo cells and adapted them to a Vero cell line. Their infectious titers and genetic stability were characterized over serial passages. These US aMPV-A and aMPV-B cell culture isolates provide valuable tools for studying pathogenesis, determining virus infectious doses, evaluating disinfectants and antivirals, and developing vaccines.