COMPARATIVE PATHOLOGY OF CHICKENS EXPERIMENTALLY INOCULATED WITH AVIAN INFLUENZA-VIRUSES OF LOW AND HIGH PATHOGENICITY

Pathologic changes and distribution of viral antigen as determined by immunohistochemistry were compared among 4-wk-old specific-pathogen-fr ee chickens inoculated intratracheally with avian influenza virus (AIV ) isolates of either low or high pathogenicity. Viruses of low pathoge nicity, previously characterized as mildly pathogenic (MP), included A /chicken/Pennsylvania/21525/83 (H5N2) (MP-Penn) and A/chicken/Alabama/ 7395/75 (H4N8) (MP-Alab). Viruses of high pathogenicity included A/chi cken/Pennsylvania/1370/83 (H5N2), A/chicken/Victoria/A185/85 (H7N7), a nd A/turkey/Ontario/7732/66 (H5N9). Extremely variable clinical signs ranging from mild respiratory distress to high mortality were present among chickens inoculated with these viruses. Chickens inoculated with highly pathogenic (HP) virus had histologic lesions of necrosis and i nflammation in cloacal bursa, thymus, spleen, heart, pancreas, kidney, brain, trachea, lung, and skeletal muscle, whereas chickens inoculate d with MP virus had histologic lesions most frequently in lung and tra chea or lacked histologic lesions. Immunospecific staining for avian i nfluenza viral proteins was most common in cells within heart, lung, k idney, brain, and pancreas of chickens inoculated with HP viruses, but immunospecific staining was present only and infrequently in trachea and lung of chickens inoculated with MP-Penn AIV. MP-Alab did not prod uce lesions nor have viral antigen in inoculated chickens but did prod uce serologic evidence of infection. The pattern of organ involvement and viral antigen distribution in chickens intratracheally inoculated with HP AIV isolates indicates a common capability to spread beyond th e respiratory tract and confirms the pantrophic replicative, pathobiol ogic, and lethal nature of the viruses. However, variability in severi ty and lesion distribution exists between different HP AIVs. By contra st, MP viruses had the ability to replicate in respiratory or enteric tracts or both and produce lesions within the respiratory tract. These MP viruses exhibited a restricted ability to replicate or produce les ions or both in nonrespiratory or nonenteric tissues; such effects wer e associated with only sporadic deaths.