Experimental Pathogenicity of the Newly Emerging North American Avian Metapneumovirus Subgroup B in Chickens and...

The recent ongoing outbreak of avian metapneumovirus subgroup B (aMPV-B) in the United States represents a serious threat to turkey and chicken production. In this study, we used the aMPV-B/chicken/USA/SD-24/P6 isolate to test pathogenicity and to establish a challenge model in chickens and turkeys. Two studies were conducted in both species: in 7- and 14-day-old chickens and in 1-, 7-, and 14-day-old turkeys. We inoculated two challenge doses: 10³ and 10⁵ 50% tissue culture infectious doses. All challenged birds were monitored for clinical signs, gross lesions, histopathologic lesions, as well as virus shedding for 7 days postchallenge (DPC). Each challenged chicken and turkey showed clear clinical signs, as well as gross and histopathologic lesions, along with virus shedding with no noticeable variations related to age or challenge dose. The inoculated virus demonstrated high pathogenicity in both chickens (the host of origin) and turkeys, indicating no host specificity. The highest virus shedding occurred at 3 and 5 DPC in all challenge groups across both species. To our knowledge, this study is the first in the United States to fulfill the Koch's postulates for aMPV-B by experimentally reproducing clinical disease in both chickens and turkeys using a U.S. field strain, followed by successful reisolation of the virus from infected tissues. These challenge models will be helpful for evaluation of the efficacy of imported and new vaccines.