Avian Metapneumovirus: A Review of Pathogenesis and Its Effect on Diagnosis

Avian metapneumovirus (aMPV) is a highly contagious upper respiratory pathogen of poultry globally that emerged in the United States in January 2024 and rapidly spread to flocks throughout North America. In turkeys, the disease (turkey rhinotracheitis) initiates as an upper respiratory infection with high morbidity and frequent complication by bacterial coinfection that can lead to high mortality and significant economic loss. In chickens (swollen head syndrome), uncomplicated clinical disease is less severe than in turkeys. Chickens are less susceptible to the virus, although, once infected, bacterial complication is common. This results in cellulitis and fasciitis of the head and face giving rise to the "swollen head" presentation, often accompanied by neurological signs and elevated mortality. In turkey breeders, broiler breeders, and laying hens, aMPV causes a decrease in egg production, which may become permanent in turkey breeders and may affect eggshell quality causing significant economic impact. Given its similar clinical appearance to other respiratory pathogens, definitive diagnosis requires identification of the virus by isolation, RT-PCR/RT-qPCR, immunohistochemical staining, or in situ hybridization. Exposure to vaccinal- or wild-type aMPV can be monitored using ELISA and virus neutralization. Stringent biosecurity and vaccination protocols along with accurate diagnosis and epidemiological surveillance are required for control of aMPV.