Polyomaviruses are non-enveloped DNA viruses with a diameter of 45 nm and an icosahedral capsid (similar to papillomaviruses). Polyomaviruses are latently present in mammalian cells and mostly cause persistent infections there. They usually form typical intranuclear inclusions in infected cells and after infection of heterologous cells lead to their transformation. They are therefore regarded as oncogenic. Polyomaviruses have a circular double-stranded DNA.
The highly contagious budgerigar fledgling disease virus (BFDV) is considered the first avian polyomavirus (APV). BFDV causes an infection that may be fatal for psittacine nestlings; in adult birds, septicaemia and hepatitis are observed. In chronic cases, feather malformation and inability to fly occur; affected animals usually hop or run around. Particularly budgerigars are affected. The disease is also called French moult.