Experimental Infection of Local Domestic and Feral (Columba livia domestica) Pigeons with Local Isolate of H9N2 Influenza Virus: Virological and Histopathological Study

A local isolate of low pathogenic Avian Influenza Virus (AIV) H9N2 subtype was used in experimental infection of 50 domestic and 50 feral pigeons (Columba livia domestica) to determine the susceptibility of these birds to H9N2 infections and to study its histopathological effects on vaccinated and unvaccinated pigeons with H9N2 commercial vaccine. The birds were divided into five groups. Groups A and C contained 20 feral pigeons, B and D contained 20 domesticated pigeons. Group E contained 10 feral and 10 domesticated pigeons that were used as unvaccinated controls. Groups A and B were vaccinated with H9N2 and Newcastle disease virus commercial vaccines. Group C and D were vaccinated with Newcastle disease virus vaccine only. All groups except E were challenged with a local isolate of H9N2 serotype. Antibodies titers against AIV were estimated pre and post-vaccination using ELISA. The results indicated low antibody titers against AIV in all groups in pre-vaccination that ranged between 152.83 ± 42.01 and 337.00 ± 150.76 with no significant differences between groups. Post-vaccination antibody evaluation indicated high titers of anti-AIV antibodies in groups A and B (740.13 ± 214.38 and 673.00 ± 242.40, respectively) in comparison to pre-vaccination levels. Clinical signs appeared on 5th day post-vaccination that included mild respiratory signs, digestive disorders, and conjunctivitis in some birds of all groups. Histopathological changes in affected tissues appeared as moderate to severe multifocal necrosis diffused in the parenchymal cells of lung tissues. Infiltration with mononuclear inflammatory cells was detected in some lung tissue areas. Necrotic foci and mononuclear cell infiltration were also observed in trachea and liver of infected pigeons but mild changes were observed in intestine. The challenge virus was re-isolated in embryonated hen's eggs of nine days old by inoculation in allantoic cavity using samples collected from tissues and cloaca of infected pigeons showing clear clinical signs. The re-isolated virus was detected by the haemagglutination test using chicken RBCs and identified by haemagglutination inhibition test using a locally prepared hyperimmune serum to H9N2 in rabbits. In conclusion, pigeons are susceptible to AIV (H9N2) that might facilitate the transmission of the virus to other bird species.