1 | INTRODUCTION
Canine parvovirus causes severe and fatal epidemic diseases of hemorrhagic gastroenteritis and subacute myocarditis in dogs, cats, and several wild carnivore species around the world (Miranda & Thompson, 2016). As a member of the Genus Protoparvovirus Parvovirinae subfamily in the Parvoviridae family, parvovirus is a small (diameter of 25 nm), nonenveloped virus (Cotmore et al., 2019). It has a linear, single-stranded, negative-sense DNA genome, which consists of approximately 5200 nucleotides (nt), including two large gene cassettes. One of them encodes two structural (VP1 and VP2) proteins and the other encodes two nonstructural (NS1 and NS2) proteins by alternative splicing of the same mRNAs (Reed, Jones, & Miller, 1988). VP1 contains the full-length VP2 sequence. However, the most abundant structural protein, VP2, accounts for 90% of the viral capsid, representing the major determinant of host range and virus-host interactions, and it is cleaved to VP3 by host proteases (Decaro & Buonavoglia, 2012).
It has been estimated that this virus has a significantly higher nucleotide substitution rate with values of approximately 10-4 substitutions/site/year, considering the genes of VP2, NS1 and the full-length genome (Hoelzer, Shackelton, Parrish, & Holmes, 2008). The frequent variation and evolution of CPV changes its host range and forms virus strains with different geographical characteristics. Amino acid substitutions in the VP2 gene have been responsible for changes in its genetic and antigenic properties.
CPV was first reported in the late 1970s as the original type 2 (CPV-2) and rapidly spread worldwide (Appel, Scott, & Carmichael, 1979). Within a few years of its emergence in dogs, five amino acid mutation in VP2 (Met87Leu, Ile101Thr, Ala300Gly, Asp305Tyr, and Val555Ile) occurred, resulting in a distinct antigenic type CPV-2a, and it completely replaced CPV-2 as the main epidemic strain in the world in 1980 (Shackelton, Parrish, Truyen, & Holmes, 2005; Stucker et al., 2012). Another early variant CPV-2b appeared in 1984, which has the Asn426Asp substitution in VP2 compared with CPV-2a (Parrish et al., 1991; Pratelli et al., 2001). In 1990, the Ser297Ala mutation appeared in VP2 of CPV-2a and CPV-2b, which were designated as new CPV-2a and new CPV-2b accordingly (Ohshima et al., 2008). In 2000, another antigenic variant having an amino acid substitution (Asp426Glu), named CPV-2c, was first reported in Italy (Buonavoglia et al., 2001), and then distributed to many parts of the world rapidly, even becoming the predominant variant in some European and American countries (Calderón et al., 2011; H. Wang et al., 2016). Currently, new CPV-2a and new CPV-2b appear to have replaced the prototype CPV-2a and CPV-2b in many countries (Zhuang et al., 2019).
In China, recent epidemiological surveys showed that the new CPV-2a and new CPV-2b strains have been in cocirculation. Although the CPV-2c strain has also been increasingly found since it was first discovered in 2010, the new CPV-2a and new CPV-2b strains are still the prominent CPV genotypes in many parts of China (H. Wang et al., 2016; Wu, Li, Wang, Liu, & Tian, 2018; Zhao et al., 2017). To further understand the prevalence and genetic evolution of CPV-2 in Jilin province, 44 positive samples were collected from animal hospitals in Changchun and Liaoyuan. The VP2 genes were amplified and analyzed to determine whether the epidemic CPV genotype has changed to provide a scientific basis for epidemic surveillance, control and vaccine research of CPV-2.