Evaluation of a novel inactivated vaccine against duck circovirus in muscovy ducks

Epidemiological Investigation and Genetic Analysis of Duck Circovirus in Korea from 2013 to 2022

Duck circovirus (DuCV) infections cause immunosuppression in ducks, potentially leading to significant economic losses for the duck farming industry. This study investigates the prevalence, genetic characteristics, and evolutionary trends of DuCV in Korea between 2013 and 2022. Samples from 184 farms across seven provinces were analyzed using polymerase chain reaction (PCR). The overall DuCV infection rate was 29.4% (54/184), with Jeollanam-do showing the highest prevalence (37.5%, 15/40). Ducks aged 3-6 weeks were most susceptible to infection, while ducklings younger than one week were rarely infected. Whole-genome sequencing was performed on 24 positive samples with phylogenetic analysis revealing that DuCV-1b is the predominant subtype in Korea (23/24 strains). Notably, a Korean DuCV-1a subtype strain was identified for the first time, showing close genetic relatedness to Chinese DuCV 1a strains. Novel subtype-specific amino acid variations in ORF1 and ORF2 were statistically analyzed and classified. Recombination analysis suggested some Korean DuCV-1b strains may have resulted from recombination events involving strains from different countries. This comprehensive study provides crucial insights into the current prevalence, genetic diversity, and evolutionary dynamics of DuCV in Korea, offering valuable data for developing effective control strategies and understanding the global epidemiology of this economically important avian pathogen.

Duck circovirus regulates the expression of duck CLDN2 protein by activating the MAPK-ERK pathway to affect its adhesion and infection

Duck circovirus (DuCV) is widely recognized as a prominent virus in China's duck farming industry, known for its ability to cause persistent infections and significant immunosuppression, which can lead to an increased susceptibility to secondary infections, posing a significant threat to the duck industry. Moreover, clinical evidence also indicates the potential vertical transmission of the virus through duck embryos to subsequent generations of ducklings. However, the limited availability of suitable cell lines for in vitro cultivation of DuCV has hindered further investigation into the molecular mechanisms underlying its infection and pathogenicity. In this study, we observed that oral DuCV infection in female breeding ducks can lead to oviduct, ovarian, and follicular infections. Subsequently, the infection can be transmitted to the fertilized eggs, resulting in the emergence of virus-carrying ducklings upon hatching. In contrast, the reproductive organs of male breeding ducks were unaffected by the virus, thus confirming that vertical transmission of DuCV primarily occurs through infection in female breeding ducks. By analyzing transcriptome sequencing data from the oviduct, we focused on claudin-2, a gene encoding the tight junction protein CLDN2 located on the cell membrane, which showed significantly increased expression in DuCV-infected oviducts of female breeding ducks. Notably, CLDN2 was confirmed to interact with the unique structural protein of DuCV, namely capsid protein (Cap), through a series of experimental approaches including co-immunoprecipitation (co-IP), GST pull-down, immunofluorescence, and adhesion-blocking assays. Furthermore, we demonstrated that the Cap protein binds to the extracellular loop structural domains EL1 and EL2 of CLDN2. Subsequently, by constructing a series of truncated bodies of the CLDN2 promoter region, we identified the transcription factor SP5 for CLDN2. Moreover, we found that DuCV infection triggers the activation of the MAPK-ERK signaling pathway in DEF cells and ducks, leading to an upregulation of SP5 and CLDN2 expression. This process ultimately leads to the transportation of mature CLDN2 to the cell surface, thereby facilitating increased virus adherence to the target organs. In conclusion, we discovered that DuCV utilizes host CLDN2 proteins to enhance adhesion and infection in oviducts and other target organs. Furthermore, we elucidated the signaling pathways involved in the interaction between DuCV Cap proteins and CLDN2, which provides valuable insights into the molecular mechanism underlying DuCV's infection and vertical transmission.

IMPORTANCE

Although duck circovirus (DuCV) poses a widespread infection and a serious hazard to the duck industry, the molecular mechanisms underlying DuCV infection and transmission remain elusive. We initially demonstrated vertical transmission of DuCV through female breeding ducks by simulating natural infection. Furthermore, a differentially expressed membrane protein CLDN2 was identified on the DuCV-infected oviduct of female ducks, and its extracellular loop structural domains EL1 and EL2 were identified as the interaction sites of DuCV Cap proteins. Moreover, the binding of DuCV Cap to CLDN2 triggered the intracellular MAPK-ERK pathway and activated the downstream transcription factor SP5. Importantly, we demonstrated that intracellular Cap also interacts with SP5, leading to upregulation of CLDN2 transcription and facilitating enhanced adherence of DuCV to target tissue, thereby promoting viral infection and transmission. Our study sheds light on the molecular mechanisms underlying vertical transmission of DuCV, highlighting CLDN2 as a promising target for drug development against DuCV infection.

From obscurity to urgency: a comprehensive analysis of the rising threat of duck circovirus

Duck circovirus (DuCV) is a small, nonenveloped, single-stranded DNA virus with immunosuppressive effects on ducks that leads to slow growth and elevated mortality following mixed infections. Its infection manifests as feather loss, slow growth, swelling of respiratory tissue, and damage to immune organs in ducks. Although single infections with DuCV do not cause noticeable clinical symptoms, its ability to compromise the immune system and facilitate infections caused by other pathogens poses a serious threat to duck farming. Given the prevalence of this disease and the increasing infection rates in recent years, which have resulted in significant economic losses in duck farming and related sectors, research and control of DuCV infection have become especially important. The aim of this review is to provide a summary of the current understanding of DuCV, serving as a reference for subsequent research and effective control of the virus. We focus mainly on the genetics and molecular biology, epidemiology, clinical symptoms, and pathology of DuCV. Additionally, topics such as the isolation and culture of the virus, vaccines and antiviral therapies, diagnostics, and preventative measures are discussed.

Molecular Detection and Genetic Characterization of Vertically Transmitted Viruses in Ducks

To investigate the distribution and genetic variation in four vertically transmitted duck pathogens, including duck hepatitis B virus (DHBV), duck circovirus (DuCV), duck hepatitis A virus 3 (DHAV-3), and avian reoviruses (ARV), we conducted an epidemiology study using PCR and RT-PCR assays on a duck population. We found that DHBV was the most prevalent virus (69.74%), followed by DuCV (39.48%), and then ARV (19.92%) and DHAV-3 (8.49%). Among the 271 duck samples, two, three or four viruses were detected in the same samples, indicating that the coinfection of vertical transmission agents is common in ducks. The genetic analysis results showed that all four identified DuCV strains belonged to genotype 1, the DHAV-3 strain was closely clustered with previously identified strains from China, and the ARV stain was clustered under genotype 1. These indicate that different viral strains are circulating among the ducks. Our findings will improve the knowledge of the evolution of DuCV, DHAV-3, and ARV, and help choose suitable strains for vaccination.

Generation of a monoclonal antibody against duck circovirus capsid protein and its potential application for native viral antigen detection

Introduction

Duck circovirus (DuCV) infection is currently recognized as an important immunosuppressive disease in commercial duck flocks in China. Specific antibodies against DuCV viral proteins are required to improve diagnostic assays and understand the pathogenesis of DuCV infection.

Methods and results

To generate DuCV-specific monoclonal antibodies (mAbs), a recombinant DuCV capsid protein without the first 36 N-terminal amino acids was produced in Escherichia coli. Using the recombinant protein as an immunogen, a mAb was developed that reacted specifically with the DuCV capsid protein, expressed in E. coli and baculovirus systems. Using homology modeling and recombinant truncated capsid proteins, the antibody-binding epitope was mapped within the region of ¹⁴⁴IDKDGQIV¹⁵¹, which is exposed to solvent in the virion capsid model structure. To assess the applicability of the mAb to probe the native virus antigen, the murine macrophage cell line RAW267.4 was tested for DuCV replicative permissiveness. Immunofluorescence and Western blot analysis revealed that the mAb recognized the virus in infected cells and the viral antigen in tissue samples collected from clinically infected ducks.

Discussion

This mAb, combined with the in vitro culturing method, would have widespread applications in diagnosing and investigating DuCV pathogenesis.

Development and application of a multiplex qPCR assay for the detection of duck circovirus, duck Tembusu virus, Muscovy duck reovirus, and new duck reovirus

A multiplex qPCR assay was developed to simultaneously detect duck circovirus (DuCV), duck Tembusu virus (DTMUV), Muscovy duck reovirus (MDRV), and novel duck reovirus (NDRV), but it did not amplify other viruses, including duck virus enteritis (DVE), infectious bursal disease virus (IBDV), avian reovirus (ARV), H5 avian influenza virus (H5 AIV), H7 avian influenza virus (H7 AIV), H9 avian influenza virus (H9 AIV), Newcastle disease virus (NDV), and Muscovy duck parvovirus (MDPV), and the detection limit for DuCV, DTMUV, MDRV, and NDRV was 1.51 × 10¹ copies/μL. The intra- and interassay coefficients of variation were less than 1.54% in the repeatability test with standard plasmid concentrations of 1.51 × 10⁷, 1.51 × 10⁵, and 1.51 × 10³ copies/μL. The developed multiple qPCR assay was used to examine 404 clinical samples to verify its practicability. The positivity rates for DuCV, DTMUV, MDRV, and NDRV were 26.0%, 9.9%, 4.0%, and 4.7%, respectively, and the mixed infection rates for DuCV + DTMUV, DuCV + MDRV, DuCV + NDRV, MDRV + NDRV, DTMUV + MDRV, and DTMUV + NDRV were 2.7%, 1.2%, 1.2%, 1.0%, 0.5%, and 0.7%, respectively.

Effects of duck circovirus on immune function and secondary infection of Avian Pathogenic Escherichia coli

Duck circovirus (DuCV) infection occurs frequently in ducks in China and is generally believed to lead to immunosuppression and secondary infection, though there has been a lack of detailed research and direct evidence. In this study, one-day-old Cherry Valley ducklings were artificially infected with DuCV alone and co-infected with DuCV and Avian Pathogenic Escherichia coli (APEC). The immune indexes at 32 d old were systematically monitored, including immune organ weight, lymphocyte transformation rate, IL-10, IL-12, soluble CD4 (sCD4), soluble CD8 (sCD8), IFN-γ, viral loads in each organ, APEC colonization, and so on. The results showed the development of immune organs in ducklings was affected, resulting in a decrease in the lymphocyte transformation rate (LTR), IL-12, sCD4, sCD8, IFN-γ and an increase in IL-10 content at 8 to 32 d postinfection (dpi). In the detection of virus loads in some organs, it was found that 8 dpi, DuCV existed stably in various organs, suggesting the importance of preventing and controlling the virus in the early stage of culture. The results of exploring the DuCV infection that shows some influence on secondary infection by APEC. The results showed that DuCV infection could significantly enhance the pathogenicity of APEC and the colonization ability of APEC in vivo. DuCV can induce more serious APEC infection in 24 dpi than in 14 dpi. Based on the above results, it can be concluded that DuCV infection will affect the immune system, cause immunosuppression, and lead to more serious secondary infection.