Comparative pathogenesis of a subtype A with a subtype B avian pneumovirus in turkeys

This paper describes a study in which the pathogenesis of avian pneumovirus strains, isolated in Belgium, and belonging to the two subtypes A and B, were compared in 2-week-old turkeys. After oculonasal inoculation, animals were either observed for clinical signs or killed for pathological and virological examination. Virus titration and immunofluorescence were performed on the conjunctivae, turbinates, sinuses, upper and lower part of the trachea, lungs and air sacs. No differences were seen between the two subtypes concerning respiratory signs, or macroscopic and microscopic lesions in the respiratory tract. Slight variations were found in site and extent of virus replication. First, only subtype A was able to invade the lower parts of the respiratory tract (bronchi), whereas viral antigens were not detected in the lungs with subtype B. Secondly, the subtype A strain infected two times more epithelial cells at all levels of the upper respiratory tract compared to subtype B. Thirdly, the amount of virus produced at different sites along the respiratory tract was lower in subtype B-inoculated turkeys than in subtype A-inoculated ones.