1. Introduction
Gyrovirus galga (GyVg) 1 is a member of the genus Gyrovirus of the family
Anelloviridae. The GyVg1 genome comprises circular, single-stranded DNA [
1,
2]. Until now, Gyrovirus chicken anemia, one member of the Gyrovirus family, has been reported as an important pathogen causing a contagious disease in chickens. In addition, the structure of the GyVg1 genome is closely related to Gyrovirus chicken anemia and other Gyroviruses. Therefore, it is difficult to differentiate it from other related viral diseases like Gyrovirus chicken anemia and other Gyroviruses. Co-circulation of GyVg1 and other viruses (Newcastle disease virus, Gyrovirus chicken anemia) Hs been reported in chickens and vaccines [
3,
4,
5]. GyVg strains may be associated with clinical manifestations in commercial chickens in the fields [
6].
Recently,
Anelloviridae have been reported to consist of 31 genera containing 155 species. The Gyrovirus genus includes Gyrovirus chicken anemia and 11 recognized species: HgyV/AVG2, GyV3 to GyV11, and GyV13. These 11 viral strains have been reported in some hosts, such as cats, chickens, dogs, ferrets, humans, mice, pigeons, seabirds, snakes, and wild passerine birds [
7,
8,
9,
10,
11,
12,
13,
14].
The viral genome consists of 2.3 kb encoding the three VPs 1, 2, and 3 proteins [
2,
15]. In detail, its untranslated region (UTR) length is approximately 490 nucleotides, while three partially overlapping open reading frames (ORFs), including ORF1, ORF2, and ORF3, code for a nonstructural protein VP2, the nonstructural protein VP3, and the viral protein VP1, respectively [
15,
16]. VP1 contributes to exhibiting viral antigenicity, pathogenicity, and replication [
2]. Of the remaining two nonstructural proteins, VP2 plays a role in viral replication and pathogenicity, while VP3 induces apoptosis in infected cells [
17].
Currently, GyVg1 infection has been globally reported in some regions of South America (Brazil), Asia (Japan, China), Europe (The Netherlands), and South Africa [
6,
15,
17,
18,
19,
20]. In Japan, GyVg1 was first reported in cryopreserved chicken tissue samples in 1997. The positive rate was 47.5% (19/40) [
19]. Based on the complete genome and full-length VP1 gene sequences, the four Japanese GyVg1 belonged to genotypes I and II [
19]. The positive rates were lower than that, ranging from 11.69% to 22.46% in China [
20]. Zhang et al. reported that the 10 Chinese GyVg1 strains belonged to genotype I and genotype III by analysis of the complete genome [
20]. In addition, GyVg1 was considered to infect humans, chickens, and other species [
2,
7,
14,
21,
22]. Specifically, in chickens, infections by the GyVg1 viruses commence with damage to the brain and declines in mental acuity and weight [
5].
In Vietnam, Northern Vietnam is one of the main areas for chicken production (
https://www.gso.gov.vn/), where a wide range of infectious diseases may have an affect. Information on GyVg1 infection is limited, with the epidemiology of this virus in chickens and other animals still unknown. Therefore, since GyVg1 has been reported globally in several countries, there is a need to investigate GyVg1 infection in chickens and identify the genetic characterization of GyVg1 strains to gain insight into the GyVg1 infection situation in Vietnam. Consequently, the current study was carried on to determine the molecular characteristics of GyVg1 strains from chickens raised in Northern Vietnam.
2. Materials and Methods
2.1. Sample Collection
Sample collection was performed from March 2023 to April 2024, and 126 samples were collected from chicken flocks in Hanoi (HN), Thainguyen (TN), Bacninh (BN), Bacgiang (BG), and Haiduong (HD) in Northern Vietnam. Tissue samples of the liver, spleen, lungs, and brain were collected from each suspected sick chicken based on signs of stunted growth, weaknesses, and diarrhea. Tissue samples were homogenated at 10% in phosphate-buffered saline supplemented with gentamicin (10 mg/mL). The homogenates were stored at −80 °C.
2.2. Extraction of Total DNA and Polymerase Chain Reaction (PCR)
Extraction of viral DNA from the supernatant of the homogenized samples was performed using Viral Gene-spin™ Viral DNA/RNA Extraction Kits (iNtRON Biotechnology; Seoul, Korea). The extraction protocol followed the manufacturer’s instructions.
For detection, PCR was run using GyVg1-F1/R1 (
Table 1), as described elsewhere [
18], and Gotaq
TM Green Master Mix (Promega, USA). The primers were used to amplify a fragment located in overlapping VP2 and VP3 genes. For sequencing, three primer pairs were previously designed to amplify the 801 or 802 bp, 733 bp, and 981 bp fragments of the GyVg1 genome (
Table 1) [
15]. The following thermal conditions were applied: a denaturing step at 95 °C for 5 min, then 40 cycles at 95 °C for 30 s, 60 °C for 30 s, and 72 °C for 40 s, and a final extension at 72 °C for 10 min. The PCR product was run in a 1.2% agarose gel and was observed using UV light.
2.3. Nucleotide Sequencing and Phylogenetic Analysis
Purification of the PCR products was performed using GeneClean
® II Kits (MP Biomedicals; Santa Ana, CA, USA). The GyVg1 genome was sequenced by 1st BASE, Singapore (
https://base-asia.com/).
The nucleotide sequence data were compared with other available sequences using GENETYX v.10 software (GENETYX Corp.; Tokyo, Japan) and BLAST (
https://blast.ncbi.nlm.nih.gov/Blast.cgi, accessed on 1 September 2024). The Clustal W algorithm supplemented in the BioEdit v.7.2.5 software [
23,
24] was used to compare the deduced amino acid sequences. The maximum likelihood method with 1000 bootstrap replicates in MEGA version 6.0 software [
25] was used to establish phylogenetic trees.
2.4. Analysis of Selection Profiles and Recombination Events
The recombination detection program (RDP) version 5.0 was used to analyze the recombination events [
26]. Evolutionary selection profiles were examined using Datamonkey (
http://www.datamonkey.org/), according to the fast unconstrained Bayesian approximation (FUBAR) method [
27].
2.5. GenBank Accession Numbers
Deposition of the complete GyVg1 genome sequences obtained in this study was made into GenBank. The accession numbers were PQ154631–PQ154635.
2.6. Statistical Analysis
Significant differences in the positive GyVg1 genome detection rate between geographical regions (at the individual and flock levels) were detected using Fisher’s exact test. A significant difference was considered statistically with a value of p < 0.05.
4. Discussion
GyVg1 was considered a newly discovered Gyrovirus in 2011 [
21], even though the presence of this virus was reported in Brazil in 2008 [
2]. GyVg1 has been reported in poultry production globally [
8,
9,
12,
17]. The current study was the first to report GyVg1 in commercial broiler chickens in Northern Vietnam.
In this study, the GyVg1 infection was identified in 20.63% of the chicken samples in the five provinces/cities in 2023–2024. This rate was considered higher than the percentage of GyVg1 infection in chickens farmed in China, which reported about 12.28% from 2015 to 2016 [
28], but less than that of 60.32% in the southern Netherlands or 84.41% in Brazil. This further supports the hypothesis that geographical differences and the environment may result in percentage differences [
6]. Although, the pathogenicity of GyVg1 has not been clearly described, notably, all the current samples were collected in chickens showing weakness, stunted growth, and diarrhea. Therefore, the pathogenicity of GyVg1 needs to be further explored.
The complete genomes obtained in the current study were each 2375 nucleotides in length, while other GyVg1 sequences available in GenBank were approximately 2380 bp in length, with the difference being due to the varying length of the G and poly C regions [
15]. Additionally, all five current GyVg1 sequences had a serine (S) deletion at position 162 in the VP2 protein, while some novel GyVg1 sequences had an S insertion at this site of the VP2 protein [
15]. The genetic characterization of the GyVg1 strains in Northern Vietnam was investigated based on the complete genome sequence of five GyVg1 strains obtained in the study. Based on the results, the five current strains had a >90% nucleotide identical rate at the nucleotide sequence level with other available GyVg1 sequences in the GenBank. This is consistent with other reports that the genome of current GyVg1 strains from chickens in Vietnam is highly conserved [
19,
29].
The phylogenetic analysis of the amino acid and nucleotide sequences did not perfectly match with each other [
15], perhaps due to synonymous mutations. In the current study, the Vietnamese GyVg1 strains were also divided into several genotypes when comparing the VPs 1, 2, and 3 proteins. This indicated that various genotypes have been prevalent in Vietnamese chicken flocks in the 21st century. Furthermore, the sequences of the VPs 1, 2, and 3 proteins of the five Vietnamese GyVg1 strains remained well conserved [
15].
All VP1 protein sequences have been reported as being 460 amino acids in length [
15,
19], including those detected in Vietnam. The function of the VP1 protein may be mainly to induce a host immune response to Gyroviruses [
15]. Although there was a series of amino acid substitutions in the VP1 protein identified in the study, the function of the VP1 protein of GyVg1 has not been well understood. It has been suggested that the hypervariable region is detected in positions 228 to 314 of the VP1 protein [
19,
20]. One amino acid substitution at position 288 was observed in the hypervariable region of all five current GyVg1 strains. In addition, the positive sites were reported with amino acids at positions 36, 288, and 293 [
29]. Based on the current results, there were three amino acid-positive sites (including 288 and 293) in the VP1 protein of all five Vietnamese GyVg1 strains. This finding further confirms the evolutionary advantages of GyVg1 strains. Furthermore, these positive selection positions are speculated to help GyVg1 evade immune responses in animals.
The VP2 protein of GyVg1 was mostly conserved among the five current GyVg1 strains. In addition, the role of VP2 protein phosphatase activity was important for Gyrovirus chicken anemia [
30]. Furthermore, some amino acid substitutions were found in the current Vietnamese GyVg1 strains. The importance of the phosphatase motif in GyVg1 VP2 regarding its effect on GyVg1 requires further investigation, although GyVg1 VP2 may have the same function as that of Gyrovirus chicken anemia.
The GyVg1 VP3 protein has been reported to induce apoptosis of tumor cells [
16]. The VP3 protein may play a critical role in inducing apoptosis in infected cells [
31,
32]. The domains proposed were a putative leucine-rich domain at positions 38 to 51, a putative nuclear export signal domain at positions 102 to 110, and other sites at 84–88 and 116–124 [
16]. In the current study, some amino acid substitutions were found in the NES and NLS2 domains and other positions in the VP3 of the GyVg1 strains. These might affect the function of VP3 GyVg1. Furthermore, positive selection sites were detected in the VP3 protein of the current Vietnamese VP3, which may provide evolutionary advantages to GyVg1. Notably, GyVg1 and Gyrovirus chicken anemia have contaminated various vaccines [
4].
Other studies have reported evidence of recombination events among Gyroviruses [
3,
29,
33,
34]. This study’s results are evidence of a recombination event in Vietnamese GyVg1 strains in the VP1 protein. This is strongly consistent with other findings that the genomes of GyVg1 have some recombination regions, including the VP1 protein and others [
29].