# Porcine Parvovirus 7: Evolutionary Dynamics and Identification of Epitopes toward Vaccine Design
## 1 Introduction  
The family _Parvoviridae_ contains two subfamilies: _Parvovirinae_ and _Densovirinae_, whose hosts are vertebrates and arthropods, respectively [1]. The subfamily _Parvovirinae_ is further divided into nine genera: _Tetraparoovirus_, _Copiparvovirus_, _Ecrythroparvovirus_, _Dependarvovirus_, _Aveparvovirus_, _Bocaparoovirus_, _Amodparvovirus_, _Protoparoovirus_ and _Chapparvovirus_[1,2]. To date, seven genotypes of porcine parvoviruses (PPV) have been discovered in pigs, and belong to four genera: _Protoparoovirus_ (PPV1), _Tetraparoovirus_ (PPV-3), _Copiparvovirus_ (PPV4-6) and _Chapparvovirus_ (PPV7), based on the similarity of non-structural protein 1 (NS1) [2,3]. PPV1, first identified in 1965 in Germany [4], is a major cause of reproductive failure in pigs, characterized by mummified fetuses, infertility, early embryonic death, stillbirths and delayed returns to the estrus [5,6]. Subsequently, six other PPV genotypes (PPV2-PPV7) were identified using novel techniques, including next-generation sequencing [2,7,8,9,10,11,12,13,14,15,16,17,18,19].  
In 2016, PPV7 was first identified by the metagenomic sequencing of rectal swab samples from pigs in the USA [2]. Subsequently, PPV7 infections were reported in Sweden, Poland, China, South Korea and Brazil [12,13,14,15,16,17,18,19]. PPV7 is a small, single-stranded linear and non-enveloped DNA virus with a genome of \(\sim\)4 kb that contains two major open reading frames (ORFs): ORF1 encodes non-structural protein 1 (NS1) responsible for viral replication, and ORF2 encodes the major structural capsid protein (Cap) or VP2 protein [2]. PPV Cap is also the major antigenic component, with an important role in eliciting neutralizing antibodies against viral infection [20]. In addition, it is a prospective antigen for subunit vaccine design and serological diagnosis. A vaccine based on the Cap could elicit antibodies toneutralize virus infection by blocking virus entry [21]. Thus, the Cap could be regarded as an effective antigenic component for PPV7 vaccine design and development. Currently, there is no, or very limited, information about its antigenic structure and immunogenic profiles, thus, the immune profiles of PPV7 Cap are required to develop PPV7 Cap-based vaccines against PPV7 infection.  
Despite limited knowledge regarding PPV7 pathogenicity, its presence in aborted pig fetuses suggests that it causes reproductive failure [17]. Co-infections are more frequent than single infection in swine herd; multiple infectious pathogens such as PCV2 and PPV can impact the outcome of respiratory infections and deserve [22]. In addition, co-infections with PPV7 and PCV2 were recently reported [23], although whether this co-infection enhanced the severity of porcine cicovirus associated diseases (PCVADs) has yet to be determined. To better understand the molecular evolution and genetic diversity of this newly emerging PPV7, we analyzed the phylogeny and estimated the time of origin and evolutionary dynamics of PPV7. Importantly, computer-based immune-informatics were used to assess the potential B cell and CD8 T cell epitopes of the PPV7 Cap, which will greatly facilitate the development of new generation vaccines against PPV7 infection and contribute to the development of effective serological diagnostic methods.