Overview of Avian Metapneumovirus Outbreak in Pennsylvania

Avian metapneumovirus (aMPV) is a virus in the <i>Pneumoviridae</i> family associated with respiratory and reproductive disease in commercial poultry. In fall of 2023, the United States experienced an outbreak of subtypes A and B in commercial poultry flocks that has persisted through 2025. This case series report describes epidemiologic, clinical, and pathologic aspects of the cases submitted to the Pennsylvania Animal Diagnostic Laboratory System (PADLS) from January 10, 2024, to January 10, 2025. Of the 883 PADLS cases that requested aMPV PCR and/or ELISA testing during the study period, 405 were positive Pennsylvania cases: 169 on PCR, 221 on ELISA, and 15 on both PCR and ELISA. Lancaster County, which is the state's most agriculturally dense area, had the first reported aMPV case in the state and the highest incidence of positive cases. Cases were organized by five primary production types: broiler breeders, commercial broilers, layer breeders, commercial layers, and commercial turkeys. The greatest numbers of positive cases submitted to PADLS were associated with commercial turkey flocks (42%), followed by commercial layer flocks (27%), broiler breeder flocks (17%), commercial broiler flocks (13%), layer breeder flocks (3%), and then other/uncategorized poultry flocks (2%). In total, 118 aMPV-positive necropsy cases were assessed for prevalence of co-infections with other agents. An average of 2.4 co-infections were found per aMPV-positive necropsy case. In all production types, <i>Escherichia coli</i> was the most common co-infection. In chickens, <i>Avibacterium paragallinarum</i>, <i>Gallibacterium anatis</i>, and infectious bronchitis virus were other common co-infections. In commercial turkeys, <i>Ornithobacterium rhinotracheale</i> was a common co-infection. A retrospective analysis of aMPV serology was conducted, and 90% of serum samples from commercial layers, 85% from broiler breeders, 79% from commercial turkeys, 57% from commercial broilers, and 100% of serum samples from layer breeders were positive for aMPV antibodies assessed via commercial ELISA kits. Titer values varied based on production type, with older birds/longer-lived production types producing the highest titer values and greater proportions of positive samples. The data collected from this retrospective study provide opportunities for future analysis of the first year of infection, particularly regarding titer values of naïve <i>vs</i>. vaccinated or previously exposed birds, as well as potential sources/reservoirs of infection, including wild birds.