Influence of the route of infection of Pasteurella anatipestifer on the clinical and immune responses of White Pekin ducks

The clinical, pathological and immunological responses were compared in ducklings infected by the intramuscular, oral and intranasal routes with virulent Pasteurella anatipestifer. Intramuscular challenge resulted in clinical signs of infection and caused 100 per cent mortality within three days. No disease signs or death were observed in the orally challenged ducks. Whereas intranasal inoculation caused no deaths, signs of infection were observed in two of 12 birds four days later. In the orally challenged group, low concentrations of antibodies (0·17 log2 to 4·5 log2) were detected in the tracheal washes of five of nine birds examined using an enzyme-linked immunosorbent assay. Humoral antibodies were detected in only one of these birds. In the intranasally infected group, serum antibody levels ranging in titre from 0·62 log2 to 6·2 log2 were found in four of nine birds examined over seven to 14 days following infection. Nine of the birds in this group were shown to have low concentrations of antibodies (0·50 log2 to 6·33 log2) in the tracheal washings. The demonstration of antibodies in the tracheal washings, but not in the serum of nine birds examined, suggested that a local immune response had occurred. However, these studies have shown that antibodies present on the tracheal surface can also be derived from antibodies given intraperitoneally.