Pathological characterization of H5N8 and H5N1 Avian Influenza Virus in mute swans during two epizootics in Serbia

Abstract

Multiple outbreaks with highly pathogenic avian influenza virus, including H5N8 and H5N1, have occurred in Serbia since 2016. This report describes the pathological lesions among mute swans (Cygnus olor) that succumbed to a highly pathogenic avian influenza virus (H5N8 and H5N1) infections during an outbreak in Northern Serbia. The pathological examinations were carried out on 15 mute swans naturally infected with HPAIV H5N8 in winter 2016/2017 and 7 mute swans infected with HPAIV H5N1 in winter 2021/2022. To determine tissue tropism, imunohistochemical demonstration of HPAI viral antigen was performed in H5N8 infected swans. The most frequently recorded postmortem lesions were pancreatic necrosis, haemorrhages in heart, petechial haemorrhages in abdominal fat, congestion of lungs. Microscopically, lesions were consisted of focal pancreas necrosis, haemorrhages, congestion and oedema of lungs, non-supurative encephalitis, hemorrhages and necrosis in heart, renal tubular necrosis, haemorrhages and necrosis of spleen. In H5N8 infected mute swans viral antigen was detected in all examined tissues, except in intestines. Influenza virus antigen was expressed in epithelial, endothelial and brain neuronal cells. The observed pathological lesions and virus expression were compatible with findings of highly pathogenic avian influenza infections of other wild bird species reported in the literature. Overall, the character and severity of pathological lesions in H5N8 infected mute swans were similar to those caused by H5N1 infection, with the exception of intestinal lesions, which were not present in H5N1 infection.