Emergence of a virulent type C avian metapneumovirus in turkeys in Minnesota

The objectives of the present study were to investigate the pathogenesis of a recent isolate of avian metapneumovirus (aMPV) in turkeys and to evaluate the quantitative distribution of the virus in various tissues during the course of infection. Seventy 2-week-old turkey poults were divided equally into two groups. One group was inoculated with aMPV (MN 19) with a titer of 10 ^5.5 TCID₅₀ oculonasally. Birds in the second group were maintained as sham-inoculated controls. Birds showed severe clinical signs in the form of copious nasal discharge, swollen sinus, conjunctivitis, and depression from 4 days postinoculation (PI) to 12 days PI. Samples from nasal turbinates, trachea, conjunctiva, Harderian gland, infraorbital sinus, lungs, liver, and spleen were collected at 1, 3, 5, 7, 9, 11, and 14 days PI. Histopathologic lesions such as a multifocal loss of cilia were prominent in nasal turbinate and were seen from 3 to 11 days PI. Immunohistochemistry revealed the presence of aMPV from 3 to 9 days PI in nasal turbinate and trachea. Viral RNA could be detected for 14 days PI from nasal turbinate and for 9 days from trachea. In situ hybridization demonstrated the presence of aMPV from 1 to 11 days PI in nasal turbinates and from 3 to 9 days PI in the trachea. Quantitative real-time polymerase chain reaction data showed the presence of a maximum amount of virus at 3 days PI in nasal turbinate and trachea. Clinically and histopathologically, the new isolate appears to be more virulent compared to the early isolates of aMPV in the United States.