Abstract
To determine histopathological damage in the respiratory tract, ducks were inoculated with five different influenza A viruses, including viruses virulent for other avian hosts. Lungs were collected for detection of virus and histopathological examination. Small amounts of infectious virus were recovered from lungs, and viral antigens were demonstrated by immunoperoxidase staining with monoclonal antibodies to the viral nucleoprotein. Although clinical signs were not detected, lungs of ducks infected with both virulent and avirulent viruses had mild pneumonia characterized by infiltrates of lymphocytes and macrophages. These findings show that although clinical signs are not evident, ducks may have damage to the respiratory tract during influenza.