INTRODUCTION

KAROLY ERD ELYI

Central Agriculture Office, Veterinary Diagnostic Directorate, Budapest, Hungary

Both papillomaviruses and polyomaviruses are small, non­enveloped, icosahedral viruses, 50—55 nm and 40—45 nm in diameter, respectively.

Papillomaviruses are epitheliotropic viruses, causing predominantly benign mucosal or cutaneous tumours. Lesions vary from papillomas consisting almost exclusively of epithelial cells to Abropapillomas and fibromas contain­ing a very pronounced connective tissue component. Pap­illomaviruses are highly host-specific and infect a wide range of mammals, but only a few avian and reptile species. The Papillomaviridae family currently contains 16 genera. The phylogeny of papillomaviruses reflects their host- linked evolutionary history, but it has been also influenced by other evolutionary processes such as adaptive radiation, host-switching events (switching of a pathogen to a novel host) and genetic recombination. As papillomaviruses cannot be isolated in tissue culture, their characterization is based on the analysis of their genomic sequence, their pathogenesis and tissue preference.

Polyomaviruses have been isolated from both mammals and birds; they are less host-species-specific than papillo­maviruses and they also infect a wider range of cell types and tissues. A common feature of most polyomaviruses is that they are capable of maintaining latent, chronic infec­tions in some host species while inducing acute disease in others. Polyomaviruses may cause a wide range of lesions and have high affinity for the kidney, which probably plays a significant role in the maintenance of latency. There are currently 12 recognized species belonging to the Polyoma- viridae family, but similarly to papillomaviruses, there is also an increasing number of novel candidate species. Captive breeding of birds (especially psittacines and pas­serines) provides an excellent opportunity for the intensive circulation and evolution of polyomaviruses, which pose a potential threat for indigenous wild bird populations and hinder ex-situ conservation efforts.