Histopathological examinations of tissue samples collected on 4 dpi revealed similar microscopic findings in all APMVs. Specifically, the trachea showed mild lymphocytic tracheitis, mild to moderate multifocal mucosal attenuation, and loss of tracheal alveolar mucous glands (Fig. 7A) . Lung sections exhibit moderate, multifocal, lymphocytic to lymphohistiocytic bronchitis with mild to moderate perivascular and peribronchial interstitial inflammation and focal perivascular cuffing with varying severity (Fig. 7B) . Minimal lymphoid depletion was detected in the spleen, and microscopic lesions were not found in any of the brain tissues (data not shown). The presence of virus antigens in various tissues of infected chickens was further evaluated by immunohistochemistry analysis. Deparaffinized sections of the virus-infected and uninfected control tissue were immunostained using polyclonal antibodies against the N protein of the respective APMV. The presence of antigens for most APMVs was detected in the epithelial lining of trachea (Fig. 7C) . However, no antigen was detected in any tissues harvested from chickens infected with APMV-5. In the lungs, the viral antigens were mostly localized in the epithelium surrounding the medium and small bronchi (Fig. 7D) . Lung tissues showed extensive presence of antigens for APMV-1 strain BC followed by APMV-1 strain LaSota, APMV-3, and APMV-2. However, presence of antigens for other serotypes was not detected in lung tissues. Our results suggested that APMV 2-9 were avirulent, and their replication was mostly restricted to trachea and lungs of infected chickens.