*Review* **Pathogenicity of Avian Polyomaviruses and Prospect of Vaccine Development**

**Chen-Wei Wang 1,2 , Yung-Liang Chen <sup>3</sup> , Simon J. T. Mao <sup>4</sup> , Tzu-Chieh Lin 1,2, Ching-Wen Wu 1,5 , Duangsuda Thongchan 1,6, Chi-Young Wang 7,8,\* and Hung-Yi Wu 1,\***


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**Abstract:** Polyomaviruses are nonenveloped icosahedral viruses with a double-stranded circular DNA containing approximately 5000 bp and 5–6 open reading frames. In contrast to mammalian polyomaviruses (MPVs), avian polyomaviruses (APVs) exhibit high lethality and multipathogenicity, causing severe infections in birds without oncogenicity. APVs are classified into 10 major species: Adélie penguin polyomavirus, budgerigar fledgling disease virus, butcherbird polyomavirus, canary polyomavirus, cormorant polyomavirus, crow polyomavirus, *Erythrura gouldiae* polyomavirus, finch polyomavirus, goose hemorrhagic polyomavirus, and Hungarian finch polyomavirus under the genus *Gammapolyomavirus*. This paper briefly reviews the genomic structure and pathogenicity of the 10 species of APV and some of their differences in terms of virulence from MPVs. Each gene's genomic size, number of amino acid residues encoding each gene, and key biologic functions are discussed. The rationale for APV classification from the *Polyomavirdae* family and phylogenetic analyses among the 10 APVs are also discussed. The clinical symptoms in birds caused by APV infection are summarized. Finally, the strategies for developing an effective vaccine containing essential epitopes for preventing virus infection in birds are discussed. We hope that more effective and safe vaccines with diverse protection will be developed in the future to solve or alleviate the problems of viral infection.

**Keywords:** avian polyomavirus; *Gammapolyomavirus*; genomic structure; pathogenicity; vaccine
