Highly Pathogenic Avian Influenza H5N8 Outbreak in Backyard Chickens in Serbia

Simple Summary A novel reassortant highly pathogenic H5N8 avian influenza virus spread through migratory birds in many countries in Europe in 2016 and 2017. In November 2016, the virus was first detected in wild birds in Serbia, and shortly thereafter, it spread to backyard poultry in Vojvodina province, in the north of Serbia. In 2021/2022, new cases of avian influenza infection in wild birds and backyard poultry were reported in our country, and this time the H5N1 subtype dominated. The aim of this study is to describe the outbreaks of highly pathogenic avian influenza H5N8 and H5N1 in backyard chickens, including a detailed description of the pathological changes, clinical signs of the disease, and identification of the virus. Our results are consistent with those previously reported for poultry naturally infected with HPAIV H5N8 and H5N1 viruses and confirm that infection in chickens results in severe systemic disease with neurological manifestations. In winter 2016/2017, the highly pathogenic avian influenza virus H5N8 was detected in backyard poultry in Serbia for the first time. The second HPAI outbreak case in backyard poultry was reported in 2022, caused by subtype H5N1. This is the first study that documents the laboratory identification and pathology associated with highly pathogenic avian influenza in poultry in Serbia during the first and second introduction waves. In both cases, the diagnosis was based on real-time reverse transcriptase PCR. The most common observed lesions included subepicardial hemorrhages, congestion and hemorrhages in the lungs, and petechial hemorrhages in coelomic and epicardial adipose tissue. Histologically, the observed lesions were mostly nonpurulent encephalitis accompanied by encephalomalacia, multifocal necrosis in the spleen, pancreas, and kidneys, pulmonary congestion, and myocardial and pulmonary hemorrhages. In H5N8-infected chickens, immunohistochemical examination revealed strong positive IHC staining in the brain and lungs. Following these outbreaks, strict control measures were implemented on farms and backyard holdings to prevent the occurrence and spread of the disease. Extensive surveillance of birds for avian influenza virus did not detect any additional cases in poultry. These outbreaks highlight the importance of a rapid detection and response system in order to quickly suppress outbreaks.