N-GoAstV belongs to the genus Avastrovirus and was first reported in 2017. Since then it has been detected in multiple provinces across China, and its importance to poultry production has grown steadily. Infection in goslings produces typical gout symptoms, with mortality rates exceeding 40 percent in affected flocks. Pathological examination of diseased birds reveals extensive urate deposition in the viscera, articular capsules, and cartilage, often accompanied by renal enlargement and related lesions. In recent years, the incidence of N-GoAstV infection on some waterfowl farms has exceeded 50 percent. The virus spreads through both horizontal and vertical transmission routes, posing an increasing threat to farmed populations and generating considerable economic losses for the waterfowl industry.

Astroviruses themselves are non-enveloped viruses carrying single-stranded, positive-sense RNA genomes of roughly 6 to 7 kilobases, with icosahedral symmetry and capsids approximately 30 nanometers in diameter. Members of the genus Mamastrovirus infect mammals, while Avastrovirus members infect birds. The complete N-GoAstV genome contains three open reading frames, ORF1a, ORF1b, and ORF2, with partial sequence overlap between them. ORF1a and ORF1b primarily encode nonstructural proteins involved in viral replication and transcription: the ORF1a polyprotein carries a 3C-like serine protease that cleaves the polyprotein into functional fragments, while ORF1b encodes a highly conserved RNA-dependent RNA polymerase that catalyzes the synthesis of a negative-sense RNA intermediate and, from it, positive-sense genomic and subgenomic RNAs.

ORF2, by contrast, encodes the viral capsid, the protein shell essential for virion assembly and host receptor binding. It also serves as an important antigenic determinant that elicits host immune responses. Notably, the C-terminal region of ORF2 contains a highly variable segment that acts as a source of antigenic variation and host adaptation and as a hotspot for recombination events. These properties make the ORF2 coding region a suitable target for genotyping and epidemiological studies of N-GoAstV, and a key focus for investigating genetic relationships and advancing vaccine development. ORF1a and ORF1b, being generally less variable and less frequently used for astrovirus genotype demarcation, were not chosen as the basis for the new framework.

Previous research on N-GoAstV has concentrated mainly on viral isolation, pathogenicity, and epidemiology. Although evolutionary analyses recognizing two major genetic groups, N-GoAstV-1 and N-GoAstV-2, have been reported, a standardized and reproducible genotyping criterion has been lacking, with classifications resting largely on subjective interpretation of phylogenetic tree topology. PASC addresses this gap by calculating pairwise sequence divergence, the p-distance, and plotting the frequency distribution of genetic distances to identify intra- and inter-genotype peaks. A threshold value is then selected at the valley between these peaks as the criterion for genotype demarcation. The method has been widely employed for genotyping other viruses, including duck circovirus and porcine circovirus, and provides a quantitative, reproducible complement to tree-based classification.

In the new work, the team led by Zhibin Zhang of Nanyang Normal University, together with colleagues at South China Agricultural University, screened the retrieved genomes according to predefined inclusion criteria: complete genome sequences, available host information, and the absence of obvious sequence gaps or incomplete genomic regions. No additional geographic or temporal sampling quotas were imposed, leaving 179 complete genomes for analysis. Pairwise amino acid p-distances among the ORF2 sequences were calculated in MEGA X, with the p-distance defined as the proportion of amino acid differences per site between each pair of sequences; ambiguous positions and alignment gaps were excluded using the pairwise deletion option. Frequency and relative-frequency distributions were generated in OriginPro 2022 with automatic binning, which produced a bin width of 0.0016 for the dataset.

The resulting distance distribution showed several distinct components. A major transition appeared around a p-distance of 0.18, which, evaluated together with the corresponding neighbor-joining tree topology, was associated with the separation of the two major phylogenetic groups, N-GoAstV-1 and N-GoAstV-2. Additional empirical boundaries emerged around 0.05 within N-GoAstV-1 and around 0.033 within N-GoAstV-2. Because the separation between some intra- and inter-subtype distance distributions was not sharply defined, the authors treated these values as empirical demarcation boundaries interpreted in conjunction with phylogenetic clustering, rather than as strict, topology-independent cutoffs. On this combined basis, N-GoAstV-1 was subdivided into N-GoAstV-1a, N-GoAstV-1b, and N-GoAstV-1c, while N-GoAstV-2 was subdivided into N-GoAstV-2a and N-GoAstV-2b.

Stratified pairwise-distance statistics reinforced the picture. The minor peaks located at approximately 0.19 to 0.22 in the overall distribution were primarily attributable to inter-subtype comparisons within N-GoAstV-1: distances between N-GoAstV-1a and N-GoAstV-1b ranged from 0.1875 to 0.1946, whereas comparisons involving N-GoAstV-1c ranged from 0.2102 to 0.2164. In stark contrast, pairwise distances between N-GoAstV-1 and N-GoAstV-2 ranged from 0.5783 to 0.6065, revealing a broad genotype-level genetic gap. Within N-GoAstV-2, however, the distribution between subtypes 2a and 2b showed partial overlap, with inter-subtype distances ranging from 0.0071 to 0.0554, indicating that the 0.033 boundary should be read as an empirical reference supported by phylogenetic clustering rather than an absolute cutoff.

Phylogenetic analyses provided independent support for the scheme. A maximum likelihood tree built from the ORF2 amino acid sequences under the optimal substitution model JTT+G, with branch reliability assessed through 1000 bootstrap replications and a human astrovirus strain (NC_001943.1) as the external outgroup, formed five subtype-level groups matching the proposed assignments. Although the placement of strain MN894548 differed slightly between the neighbor-joining and maximum likelihood trees, PASC showed an average pairwise p-distance of 0.013 between MN894548 and N-GoAstV-2a reference strains, supporting its classification within N-GoAstV-2a. An additional unrooted maximum likelihood tree built from only the 179 N-GoAstV sequences retained the membership of the major groups, indicating that the clustering pattern was not substantially affected by the choice of outgroup.

Recombination screening with RDP4, using seven detection methods and requiring support from at least four, identified 27 unique sequences with putative recombination signals. Eleven of these events overlapped the ORF2 coding region. In a sensitivity analysis excluding sequences with ORF2-overlapping recombination, the major N-GoAstV-1/N-GoAstV-2 split and the multi-sequence subtype groupings were largely retained, indicating that the classification framework was not substantially influenced by recombinant sequences. The N-GoAstV-1c lineage, however, could not be independently evaluated because its sole representative strain, OR907134.1, was excluded due to an ORF2-overlapping recombination signal, and the authors emphasize that its designation remains provisional until additional non-recombinant sequences become available.

Among the GenBank records analyzed, N-GoAstV-2a was represented throughout 2016 to 2025 and accounted for the largest number of records, with most collected during 2018 to 2021. N-GoAstV-1a appeared among sequences collected during 2017 to 2023, while N-GoAstV-2b was represented primarily among sequences from 2014 to 2019. N-GoAstV-2a also spanned the largest number of provinces in the dataset. The authors caution that because the dataset was drawn from publicly available GenBank records without predefined sampling quotas, these patterns describe sequence availability rather than population-level prevalence or geographic incidence of the subtypes.

Several further limitations frame the conclusions. Beyond the sampling issue, the proposed genetic-distance values are empirical demarcation references rather than strict cutoffs, particularly for N-GoAstV-2, where intra- and inter-subtype distance distributions partially overlap. The biological consequences of N-GoAstV genetic diversification also remain unclear: while genotype-associated variation in the ORF2-encoded capsid, including differences in antigenic epitopes, has been reported, and recent experimental studies have demonstrated genetic and pathogenic diversity among GAstV-2 strains, future work integrating comparative genomics, structural analyses, and experimental characterization will be needed to determine what the observed diversification means for viral fitness and disease.

Even with these caveats, the study delivers what the field has lacked: a quantitative, reproducible framework for N-GoAstV genotype and subtype delineation grounded in empirical genetic-distance patterns and complementary phylogenetic support. By confirming the two major genotypes and refining the picture into five proposed subtypes, the PASC-based classification complements purely tree-based approaches and provides a reference point for future molecular epidemiological investigations, vaccine development, and disease prevention and control efforts in the waterfowl industry.

Subject of Research: PASC-based genotyping of novel goose astrovirus using ORF2 amino acid sequences

Article Title: Novel PASC-based genotyping of goose astrovirus using ORF2 amino acid sequences

Article References: Zhang, Z., Wang, H., Xu, X., Li, D., Ji, J., Yao, L., Bi, Y., & Xie, Q. (2026). Novel PASC-based genotyping of goose astrovirus using ORF2 amino acid sequences. Frontiers in Veterinary Science, 13, 1732594. https://doi.org/10.3389/fvets.2026.1732594

Image Credits: AI Generated

DOI: 10.3389/fvets.2026.1732594

Keywords: goose astrovirus, PASC, genotyping, ORF2, capsid protein, phylogenetics, subtypes, avian viruses, recombination, waterfowl disease, Novel, PASC-based