OBSERVATIONS ON THE RELATIONSHIP IN CHICKENS BETWEEN THE VIRULENCE OFSOME AVIAN INFLUENZA-VIRUSES AND THEIR PATHOGENICITY FOR VARIOUS ORGANS

Comparative histological and immunocytochemical studies were conducted on formalin-fixed tissues from chickens infected with avian influenza viruses of varying virulence. Results showed a distinct pattern of di sease that depended on the virulence of the virus and the susceptibili ty of the birds. At 3 days post-intranasal inoculation with a highly v irulent H7N7 virus, all 6-to-8-week-old specific-pathogen-free (SPF) b irds were affected, and all developed pancreatic necrosis and encephal itis associated with specific immunoperoxidase staining. Other same-ag ed SPF birds were only occasionally affected 6 to 8 days after intrave nous inoculation with almost avirulent H4N4, H6N2, or H3N8 virus. Spec ific lesions and immunoperoxidase staining were noted in the kidneys o nly. The H7N7 virus in older commercial birds and an H7N3 virus in you ng SPF and older commercial birds caused intermediate mortality rates at 4 to 11 days postinoculation, and there was a broad range of lesion s and specific immunoperoxidase staining in the pancreas, brain, kidne y, heart, and skeletal muscle. Two exceptional birds had immunostainin g of small blood vessels throughout their bodies with or without lesio ns or staining in the tissues, which may have represented a transitory pre-localizing phase occurring in many birds. There was necrosis with out virus antigen detection in the bursae, thymuses, and cecal tonsils , possibly secondary to stress or only transitory infection of virus. These data indicate that rapid, retrospective diagnosis of avian influ enza in fixed tissues is possible by using an immunoperoxidase test on pancreas, brain, and kidney.