**2. Taxonomic classification of PVs**

Originally, PVs were grouped with the polyomaviruses in the family *Papovaviridae*, which was justified based on such shared traits as morphologically similar non-enveloped cap‐ sids and circular double-stranded DNA genomes. Because the genomes of both groups were later found to exhibit different sizes and organizations, as well as a low similarity between their nucleotide (nt) and aminoacid (aa) sequences, PVs are currently classified in the family *Papillomaviridae* [4,5].

PVs are traditionally designated as "viral types". Each viral type represents a complete genome with the L1 gene nt sequence – which encodes the main capsid protein – exhibit‐ ing at least 10% dissimilarity compared with the same sequence from any other previous‐ ly identified PV [5].

The classification of PVs into genera unites several phylogenetically related species that differ with respect to their biological properties, whereas classification based on species groups phylogenetically close viral types that also exhibit similar biological and pathological traits. In terms of nt sequence identity, these taxonomic relationships are expressed as follows: i) different genera exhibit less than 60% similarity in their L1 ORFs (open reading frames) and less than 23% to 43% when their full genomic sequences are compared; ii) different species within the same genus exhibit 60% to 70% similarity in their L1 ORFs (Table 1) [5].

Currently, the family *Papillomaviridae* contains at least 29 genera that include more than 200 PV types. The Greek alphabet is used to name the genera, which thus range from *Alphapapil‐ lomavirus* to *Dyoiotapapillomavirus*. Each species is designated according to the viral type that


**Table 1.** Relationship between diverse taxonomic levels and the identity observed on the L1 ORF nt sequences. Source: [5].

**2. Taxonomic classification of PVs**

114 Current Issues in Molecular Virology - Viral Genetics and Biotechnological Applications

in the family *Papillomaviridae* [4,5].

ly identified PV [5].

Originally, PVs were grouped with the polyomaviruses in the family *Papovaviridae*, which was justified based on such shared traits as morphologically similar non-enveloped cap‐ sids and circular double-stranded DNA genomes. Because the genomes of both groups were later found to exhibit different sizes and organizations, as well as a low similarity between their nucleotide (nt) and aminoacid (aa) sequences, PVs are currently classified

**Figure 1.** Electron micrograph of bovine papillomavirus type 1 virions (BPV1) (diameter of 55 nm). Source: [1].

PVs are traditionally designated as "viral types". Each viral type represents a complete genome with the L1 gene nt sequence – which encodes the main capsid protein – exhibit‐ ing at least 10% dissimilarity compared with the same sequence from any other previous‐

The classification of PVs into genera unites several phylogenetically related species that differ with respect to their biological properties, whereas classification based on species groups phylogenetically close viral types that also exhibit similar biological and pathological traits. In terms of nt sequence identity, these taxonomic relationships are expressed as follows: i) different genera exhibit less than 60% similarity in their L1 ORFs (open reading frames) and less than 23% to 43% when their full genomic sequences are compared; ii) different species

Currently, the family *Papillomaviridae* contains at least 29 genera that include more than 200 PV types. The Greek alphabet is used to name the genera, which thus range from *Alphapapil‐ lomavirus* to *Dyoiotapapillomavirus*. Each species is designated according to the viral type that

within the same genus exhibit 60% to 70% similarity in their L1 ORFs (Table 1) [5].

best represents it, whereas the remaining PV types classified within a single species are named as virus strains (Table 2).



**Table 2.** Classification of *Papillomaviridae* family. Source: [3].

PVs isolated from vertebrates are classified into 24 genera, whereas viral species that occur exclusively in birds and reptiles are grouped into three genera and one genus, respectively. The taxonomic nomenclature of animal PV types is based on the scientific name of their hosts according to the genus and species. For example, FdPV1 is the name given to PV of the domestic cat (*Felis domesticus*) type 1 [3]. An exception occurs in the case of the bovine papillomavirus, which was named *Bos taurus* papillomavirus but by consensus is usually referred to as BPV. Table 3 describes the genera and species of PVs identified in various species of domestic animals.

Because PVs are not amenable to isolation using classic cell culture techniques and do not induce a strong humoral immune response in their hosts, the taxonomic terms "strain" and "serotype" were not originally applied to this virus family. Consequently, the family classifi‐ cation is based on the similarities between nt sequences and a limited number of biological and medical properties [5,6].
