Avian Metapneumovirus Subtype A Infection in Commercial Chickens (Gallus gallus domesticus) and Turkeys (Meleagris gallopavo) in California

Avian metapneumovirus (aMPV) is a disease that has become economically significant in commercial poultry worldwide. This virus has been reported to affect ducks, layers, broilers, and turkeys and causes upper respiratory disease, drops in egg quality and production, and increases in secondary infections. aMPV subtype C was the only subtype previously reported in the United States from 1997 to 2007. However, in October 2023, the California Animal Health and Food Safety Lab (CAHFS) received the first reported case of aMPV subtype A in the country. Since then, aMPV subtype A has been found in commercial broiler and turkey operations throughout California and has more recently spread to Midwestern states. Given the potential economic impact on commercial poultry operations, this case series aims to determine patterns in poultry type, age, location, temporal distribution, and comorbidities of poultry diagnosed with aMPV subtype A throughout the CAHFS system. During the period October 2023 to March 2024, 93 submissions were tested for aMPV subtype A, which comprised 33 commercial meat turkey cases and 60 chicken cases (broiler: 57/60, layer: 2/60, backyard: 1/60). Of those cases tested, 58.1% (54/93) were positive for aMPV subtype A via reverse transcription polymerase chain reaction and included 29 broiler cases and 25 meat turkey cases. The age of affected meat turkeys ranged from 16 to 105 days with a median age of 42 days. The top three most common comorbidities were colibacillosis (96.0%, 24/25), salmonellosis (20.0%, 5/25), and <i>Ornithobacterium rhinotracheale</i> (ORT) infection (20.0%, 5/25), with some turkeys being infected by more than one of these pathogens. The age of affected broiler chickens ranged from 25 to 70 days with a median age of 37 days, and the most common comorbidities were colibacillosis (93.1%, 27/29), infectious bronchitis virus infection (58.6%, 17/29), and ORT infection (37.9%, 11/29). Coinfection with more than one pathogen was also noted in chickens. Statistical analysis showed a significant difference in comorbidity frequency in commercial broilers during the outbreak period when compared to the year prior to the outbreak (<i>P</i> < 0.001). Ultimately, this case report provides baseline information on the first detection of aMPV subtype A in California and aids in our understanding of this emerging disease in the United States.