Papillomaviridae is a family of non-enveloped DNA viruses whose members are known as papillomaviruses.[1] Several hundred species of papillomaviruses, traditionally referred to as "types",[2] have been identified infecting all carefully inspected mammals,[2] but also other vertebrates such as birds, snakes, turtles and fish.[3][4][5] Infection by most papillomavirus types, depending on the type, is either asymptomatic (e.g. most Beta-PVs) or causes small benign tumors, known as papillomas or warts (e.g. human papillomavirus 1, HPV6 or HPV11). Papillomas caused by some types, however, such as human papillomaviruses 16 and 18, carry a risk of becoming cancerous.[6]
Papillomaviruses are usually considered as highly host- and tissue-tropic, and are thought to rarely be transmitted between species.[7] Papillomaviruses replicate exclusively in the basal layer of the body surface tissues. All known papillomavirus types infect a particular body surface,[2] typically the skin or mucosal epithelium of the genitals, anus, mouth, or airways.[8] For example, human papillomavirus (HPV) type 1 tends to infect the soles of the feet, and HPV type 2 the palms of the hands, where they may cause warts. Additionally, there are descriptions of the presence of papillomavirus DNA in the blood and in the peripheral blood mononuclear cells.
Papillomaviruses were first identified in the early 20th century, when it was shown that skin warts, or papillomas, could be transmitted between individuals by a filterable infectious agent. In 1935 Francis Peyton Rous, who had previously demonstrated the existence of a cancer-causing sarcoma virus in chickens, went on to show that a papillomavirus could cause skin cancer in infected rabbits. This was the first demonstration that a virus could cause cancer in mammals.
There are over 100 species of papillomavirus recognised,[9] though the ICTV officially recognizes a smaller number, categorized into 53 genera, as of 2019.[10][11][12] All papillomaviruses (PVs) have similar genomic organizations, and any pair of PVs contains at least five homologous genes, although the nucleotide sequence may diverge by more than 50%. Phylogenetic algorithms that permit the comparison of homologies led to phylogenetic trees that have a similar topology, independent of the gene analyzed.[13]
Phylogenetic studies strongly suggest that PVs normally evolve together with their mammalian and bird host species, but adaptive radiations, occasional zoonotic events and recombinations may also impact their diversification.[13] Their basic genomic organization appears maintained for a period exceeding 100 million years, and these sequence comparisons have laid the foundation for a PV taxonomy, which is now officially recognized by the International Committee on Taxonomy of Viruses. All PVs form the family Papillomaviridae, which is distinct from the Polyomaviridae thus eliminating the term Papovaviridae. Major branches of the phylogenetic tree of PVs are considered genera, which are identified by Greek letters. Minor branches are considered species and unite PV types that are genomically distinct without exhibiting known biological differences. This new taxonomic system does not affect the traditional identification and characterization of PV "types" and their independent isolates with minor genomic differences, referred to as "subtypes" and "variants", all of which are taxa below the level of "species".[14] Additionally, phylogenetic groupings at higher taxonomic level have been proposed.[15]
This classification may need revision in the light of the existence of papilloma–polyoma virus recombinants.[16] Additional species have also been described. Sparus aurata papillomavirus 1 has been isolated from fish.[17]