Background
Porcine circovirus type 2 (PCV2) is a small, non-enveloped, single-strand circular DNA (ssDNA) virus with 1766–1768 nucleotides (nt) in length, classified under the genus
Circovirus in the family
Circoviridae [
1]. PCV2 was first isolated from tissues of pigs in western Canada in 1998, many more PCV2 isolates have been reported worldwide since then, posing a continuing threat to veterinary public health [
2]. PCV2 can cause a group of diverse multi-factorial syndromes in domestic pigs and wild boars across the globe, collectively named PCV-associated diseases (PCVADs), such as post-weaning multisystemic wasting syndrome (PMWS), porcine dermatitis and nephropathy syndrome (PDNS), porcine respiratory disease complex (PRDC), enteritic disease, and reproductive failure [
3‐
6].
PCV2 is known for its high rates of infection, transmission, and mutation together with inter- and intra-genotype recombination, which is considered to be an important evolutionary mechanism for the emergence of new genotypes [
7‐
10]. Compared with other DNA viruses, PCV2 has a higher evolutionary rate (1.21 × 10
−3 to 6.57 × 10
−3 substitutions/site/year) [
4,
11]. Currently, PCV2 has been classified into eight genotypes, PCV2a to PCV2e, and the newly reported PCV2f, PCV2g, and PCV2h [
12,
13], of which only three genotypes (PCV2a, PCV2b, and PCV2d) have a persistent and broad worldwide distribution, especially in pig-producing countries, causing significant economic losses and veterinary public health issues [
12,
14‐
16].
Domestic pigs and wild boars are generally considered as the natural reservoirs of PCV2. But currently, the known host range of this virus has expanded to humans [
17] and other non-porcine mammals (such as bovids, minks, foxes, dogs, raccoon dogs, goats, rats, and mice) [
18‐
25], making it more conducive to virus transmission and prevalence. Experimental mice are generally used as the model to investigate the role of rodents in carrying, replicating, and transmitting PCV2 [
19,
26‐
29]. It has been reported that PCV2 (genotypes PCV2a and PCV2b) can frequently spillover from pigs to rodents on pig farms [
20,
30,
31]. However, there was no report on the presence of the currently predominant genotype PCV2d in wild rats and PCV2 infection in rats outside pig farms. In this study, two PCV2d strains were identified from wild rats (
Rattus norvegicus) captured far from pig farms in Jiangsu province, China. This finding provided the first evidence that genotype PCV2d has the capacity to naturally infect rats.
Discussion
Rodents rank as the largest mammalian species (approximately 43% of all mammal species). They are widely distributed and the natural reservoirs of a diverse group of pathogenic viruses [
43]. In our study, the classified eukaryotic viral reads were mainly related to the genus
Picornavirales occupying 97.31% (n = 595,373) of the total reads, while most of which were assigned to the family
Dicistroviridae (n = 313,196) and picorna-like viruses (n = 236,984) of probable insect and environmental origin. A total of 21 viral sequences were subsequently characterized in the two rat pools after extension of contigs and nPCR amplification (Additional file
1: Table S1 and Additional file
2). Pairwise-sequence comparisons showed that these sequences shared sequence identities with their closest genetic relatives ranging from 42.2% to 99.8% at the nucleotide level, and their lengths ranged from 431 to 9787 nt. Apart from invertebrate, plant and uncultured environmental viruses, several vertebrate-infecting viral sequences were detected, including anellovirus, picornavirus, astrovirus, and retrovirus sharing > 80% nucleotide identity with previously reported viruses in rats or rat cells [
44‐
46], together with porcine circovirus 2, known as pathogenic to pigs [
47].
At present, porcine epidemic virus, PCV2, is one of the most economically important swine pathogens that has a significant impact on animal performance and production [
48]. Prior to 2003, PCV2 was dominated by the PCV2a genotype [
49]. On a global scale, the first genotype shift from PCV2a to PCV2b occurred around 2003 [
50]. Since 2009, there has been a second genotype shift in the predominant prevalence of PCV2 [
51]. Until now, PCV2d has been the predominant genotype in swine populations in China, North America, South Korea, and Uruguay [
52,
53]. Since China is known for only importing swine, the reason for this genotype's global popularity remains unclear. In recent years, several studies have investigated the epidemiology of PCV2d in pigs in China: Henan, where 1283 (72.90%) of the 1760 tested samples were PCV2 positive and 47.06% (8/17) of the discovered strains belonged to PCV2d [
53], Yunnan, where the percentage of PCV2 positive samples was 60.93% (170/279) and 80% (12/15) of the isolates were PCV2d [
54], shanghai, in which 104 out of 199 (52.26%) were screened positive for PCV2d [
52], and Jiangsu, where 34 of the 120 (28.33%) tested samples were PCV2 positive and PCV2d accounted for 47.06% of the 34 isolates [
55]. The epidemiological data reveal that PCV2d has been circulating in pig-producing provinces of China for many years and recognized as a severe threat to the Chinese pig industry.
Phylogenetic trees constructed based on the full genome and ORF2 sequences showed that the two rat-associated PCV2 strains in this study belonged to the genotype PCV2d. When using js2021-Rt001 and js2021-Rt002 as query sequences, the closest hits in the BLASTn search were both the porcine-origin PCV2 isolates in Vietnam. Meantime, the detection of other genotypes in rodents inhabiting PCV2-infected pig farms [
20,
30,
31] makes possible cross-species transmission of the PCV2d between porcine and rodent hosts. PCV2 ORF2 gene encodes the capsid protein, the major immunogenic protein involved in virus attachment to the host cellular receptor(s) and immune responses [
40]. No aa changes were found in previously reported antibody recognition domains, an immunodominant decoy epitope, and a heparin sulfate binding motif of the rat-associated PCV2d Cap proteins [
40‐
42]. Meantime, the ORF2 sequences of js2021-Rt001 and js2021-Rt002 were 100.00% aa identical to the Vietnam isolates, Han8 and PCV2/PhuTho/G40312/2018, respectively. Even with these evidence, the origin of the viruses remains elusive and further studies are required to confirm the potential cross-species transmission of diverse genotypes PCV2 existing between rat and porcine hosts.
It has been demonstrated that PCV2 could replicate in mice with distribution in multiple organs [
26,
27,
31]. In this study, multiple tissue samples were found positive for PCV2, indicating that the two PCVs were capable of infecting the wild rats rather than only passing through the gut. Of particular note, the two highly similar PCV2 strains were present in samples collected from two wild rat individuals on different dates at the adjacent sampling sites. Horizontal and vertical transmissions were confirmed to be efficient ways for PCV2 onward spread among rodent populations [
19]. This suggests the possibility of the long-term prevalence of PCV2 in the local rat populations.
PCV2 host jumps may also be a potential threat to human health. Zoonotic transmission of PCV2 has been proposed and reported in a few studies [
17,
56,
57]. Rodents on swine farms have a high potential for contact with humans, posing the possibility of zoonotic transmission of PCV2 from rodents to personnel with professional occupation with pigs indirectly via contamination of water or food products. Therefore, it is necessary to capture or kill rodents on swine farms to avoid virus spread and zoonotic transmission of PCV2.
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