This study documents the clinical signs, necropsy findings and viral antigen distribution of the highly pathogenic avian influenza (HPAI) H5N1 virus infection in domestic poultry (a backyard farm) and the phylogenetic analysis of the virus. On January 29, 2015, an outbreak of HPAI H5N1 in domestic poultry was reported on a backyard farm in Bulgaria. Out of the twenty-two chickens with clinical signs, twenty died while the remaining two were destroyed. The morbidity was 100%, whereas the overall

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... mortality and lethality were 90.91%. The clinical observations made were sudden death, high mortality, weakness, and recumbency. Although multisystemic lesions were observed occasionally, the main pathologic findings were observed in the nervous, circulatory, respiratory, and gastrointestinal systems. An influenza virus nucleocapsid protein was identified by an immunohistochemical analysis in all the analysed organs: brain 3/3, trachea 3/3, lung 3/3, intestine 3/3, heart 3/3, which confirmed the systemic infection. The phylogenetic analyses of the virus showed a close genetic relationship with the H5N1 viruses of Asian origin, isolated in 2012 and 2013, belonging to the clade 2.3.2.1c. The HA-gene genetically clusters with HPAI H5N1 viruses isolated from wild pelicans in Romania and Bulgaria, thereby demonstrating the link between wild and domestic birds in the epidemiology of avian influenza. The contact between the affected chickens and migrating water birds over Bulgaria's territory was suspected as a reason for the outbreak in the backyard farm. In addition, the detection of the virus in wild bird populations in Bulgaria three days earlier strongly supports the hypothesis of migrating wild birds spreading HPAI H5N1.