According to the latest report from the International Committee on Taxonomy of Viruses (ICTV), the Circoviridae family comprises two genera of Cyclovirus and Circovirus (1). Duck circovirus (DuCV), which belongs to the genus circovirus, is the non-enveloped, icosahedral virus with a diameter of 15 to 16 nm (2)(3)(4). The DuCV's genome is a ~2 kb single-stranded circular DNA harboring three open reading frames (ORFs): ORF1 encodes the virus replicase, ORF2 encodes the major antigenic capsid protein, and the ORF3 encodes apoptosis-related protein (5)(6)(7). Furthermore, the genomic region between the ORF1 and ORF3 harbors a stem-loop that acts as a cisacting element for viral replication initiation (8). To date, three main genotypes of the DuCV have been classified: DuCV-1 (1a~d), DuCV-2 (2a~c), and DuCV-3 which was first discovered in China in 2022 (9). The DuCV-1/2 may evolve from the genetically similar ancestors through different pathways. Despite being the most recently identified, DuCV-3 exhibits a relatively low genomic identity (62.3-63.7%) with other known DuCVs (9).DuCV was initially found in two 6-week-old female ducks from a German farm in 2003 and has since spread worldwide (8). In China, the virus was first reported in Taiwan province in 2006, and its infection has since been detected in all poultry-raising regions (10). According to accumulating evidence, DuCV could infect the ducks of all breeds and ages, mainly affect the immune system negatively and cause severe immunosuppression, thereby making infected ducks more susceptible to secondary infections (11)(12)(13)(14). Clinically, infected ducks exhibit irregular plumage, stunted growth, lymphocytopenia, and hepatic/splenic necrosis (15). The pathogenic characteristics of the DuCV are systemic infection, persistent infection, and horizontal transmission (16,17). Emerging investigations have documented the detection of DuCV in Chinese geese, indicating an expansion of its host range (15). Therefore, persistent epidemiological surveillance of DuCV in duck and goose populations is of significant clinical importance for understanding and controlling DuCV infection.From January 2024 to July 2025, 750 tissue samples were collected from birds exhibiting the clinical signs of extensive dorsal feather loss and lethargy from 75 farms, including 23 farms (8 duck farms and 15 goose farms) from Hebei Province, 23 farms (6 duck farms and 17 goose farms) from Hubei Province, and 29 farms (10 duck farms and 19 goose farms) from Henan Province. All the birds used for sampling died from cachexia or infection of unknown cause. All the farms were single species farms, exclusively raised either geese or ducks, and were not in contact with other farms. Due to the absence of specific clinical signs in some waterfowl infected with DuCV, a sampling strategy was employed wherein 10 birds were selected from each farm. All birds were transported to the laboratory under refrigerated conditions for necropsy.Stringent measures were implemented during sample processing to prevent crosscontamination between individual birds.Liver and spleen tissues from each bird were collected, and equal masses (500 mg each) of the two tissue types were combined and then homogenized. After snap-freezing in liquid nitrogen and grinding, 20 mg of the resulting powder was taken, and viral nucleic acid was extracted using the Virus DNA/RNA Isolation Kit (YALEPIC BIOTECH, Suzhou, China) according to the manufacturer's instructions. After quantifying with the NanoDrop ND-1000 spectrophotometer (Thermo Fisher Scientific, Waltham, MA), the extracted viral DNA was stored at -80 ℃ until further use.PCR was performed according to established methods as described in previous study (15). All samples were screened for DuCV by PCR using extracted DNA as the template. and DuCV-R2: 5′-CCAGGCTCTTCCTCCCAGCKWCTCTT-3′), previously reported to amplify the full-length genomes of both DuCV and GoCV as universal primers (18).All cycling conditions for each PCR assay were listed in Supplementary Table 1.Subsequently, the PCR amplicon with correct molecular weight (complete genome) as visualized by gel electrophoresis was cloned into the pMD-18T vector and submitted for Sanger sequencing (Tongyong, Anhui, China). The obtained whole genome sequences were assembled via the SeqMan modules (DNASTAR, Wisconsin, US) using the default settings and verified based on the sequence-similarity through BLAST (Basic Local Alignment Search Tool) in NCBI database.To understand the evolutionary relationship between these newly identified DuCVs and Putative recombination events identified by RDP (v4.83) were further analyzed using SimPlot v3.5.1 software to validate recombination breakpoints and characterize recombination patterns using bootscan analysis (15).3 Descriptive resultsThrough DuCV screening tests, 9 out of 24 duck farms (the positivity rate for birds within each farm ranged from 40% to 100%), and 4 out of 51 goose farms tested positive for DuCV (the positivity rate for birds within each farm ranged from 20% to 40%). The provincial distribution showed 5 positive farms in Henan (4 duck farms and 1 goose farm), 4 in Hubei (2 duck farms and 2 goose farms) and 4 in Hebei (3 duck farms and 1 goose farm). These results suggested that DuCV can infect not only ducks but also geese, which was supported by both this study and our prior work (15). By suppressing the host immune system, DuCV predisposes infected birds to secondary infections and increases their susceptibility to other viral pathogens (20,21). This exacerbates clinical disease severity, leading to significant economic losses in the poultry industry and implementing preventive measures against DuCV is recommended (11). In addition to our findings regarding the expanding host diversity of DuCV, infection in wild ducks has also been documented in other studies (22). Therefore, epidemiological investigation in wild waterfowl and other poultry is warranted to prevent the complex and extensive transmission of DuCV (2). Given that DuCV is known to be transmitted horizontally among birds, rigorous monitoring of water and feed sources is essential to prevent exposure to contaminated materials (17).Concurrently, implementing regular health surveillance and prompt isolation of sick or deceased birds are critical measures to contain viral spread within flocks.Referring to sequencing results, all the DuCV-positive samples from the initial screening were also positive for the full genome screening. The nucleotide sequences obtained from all positive samples within the same farm exhibited 100% similarity. The phylogenetic tree based on whole-genome alignments of the 13 newly-obtained sequences (Supplementary Table 2) and 104 reference sequences (Supplementary Table 3) is displayed in Recombination is regarded as the principal driving force behind viral evolution and the primary origin of the majority of viral genetic diversity (23). Based on recombination analysis according to the complete genome sequences of the newly obtained and reference DuCV strains, a total of 17 recombination events were predicted. Notably, among them, 10 were related to DuCV strains identified in geese. Figure 2 presented that the majority of these goose-origin strains participated as major/minor parents participated in recombination events with duck-derived strains. In contrast, the remaining two goose-origin strains (G240117/DuCV-1b and G250408/DuCV-1b) were recombinants from duck-derived DuCV strains (GQ423747.1/DuCV-1a, OM176555.1/DuCV-1b, MF627688.1/DuCV-1b and MN928799.1/DuCV-1b) identified as their parental donors. These intra-genotype and cross-species recombination events collectively contribute to the genetic variability of DuCV. Owing to the lack of a cell culture system for propagating the requisite virus amounts in vitro, the isolation of strains from different host species and the execution of challenge experiments are both restricted (15). Therefore, phylogenetic analyses and predicted recombination events can only serve as preliminary references for inferring cross-host transmission.In conclusion, this study demonstrates a relative high genetic identity between gooseand duck-derived DuCV strains. The recombination events observed between duck-and goose-derived strains of different genotypes also further hint at the potential horizontal transmission between duck and goose populations. However, the underlying transmission characteristics still require confirmation through extensive epidemiological investigations and experimental infection studies.
Wang et al. (Mon,) studied this question.