Development and application of a one-step real-time Taqman RT-PCR assay for detection of Duck hepatitis virus type1

A rapid simultaneous detection of duck hepatitis A virus 3 and novel duck reovirus based on RPA CRISPR Cas12a/Cas13a

The mixed infection of duck hepatitis A virus 3 (DHAV-3) and novel duck reovirus (NDRV) has caused significant losses to the global duck farming industry. On-site point-of-care testing of viruses plays a crucial role in the early diagnosis, prevention, and disease control. Here, we proposed an RPA-CRISPR Cas12a/Cas13a one-pot strategy (DRCFS) for rapid and simultaneous detection of DHAV-3 and NDRV. This method integrated the reaction of RPA and CRISPR Cas12a/Cas13a in a single tube, eliminating the need to open the lid during the intermediate processes and thereby avoiding aerosol contamination. On this basis, we proposed a dual RPA-CRISPR strategy coupled with a lateral flow analysis platform (DRC-LFA). This circumvented the necessity for complex instruments, enabling direct visual interpretation of results, making the test more accessible and user-friendly. Our findings demonstrated that the DRCFS method could detect DHAV-3 and NDRV at concentrations as low as 100 copy/μL, while DRC-LFA achieved limit of 101 copies/μL within 35 min. Furthermore, when DRCFS, DRC-LFA, and qPCR were employed collectively for clinical samples analysis, all three methods yielded consistent results. The specificity, sensitivity, and user-friendly of these methods rendered them invaluable for on-site virus detection.

HSP70 positively regulates translation by interacting with the IRES and stabilizes the viral structural proteins VP1 and VP3 to facilitate duck hepatitis A virus type 1 replication

The maintenance of viral protein homeostasis depends on the interaction between host cell proteins and viral proteins. As a molecular chaperone, heat shock protein 70 (HSP70) has been shown to play an important role in viral infection. Our results showed that HSP70 can affect translation, replication, assembly, and release during the life cycle of duck hepatitis A virus type 1 (DHAV-1). We demonstrated that HSP70 can regulate viral translation by interacting with the DHAV-1 internal ribosome entry site (IRES). In addition, HSP70 interacts with the viral capsid proteins VP1 and VP3 and promotes their stability by inhibiting proteasomal degradation, thereby facilitating the assembly of DHAV-1 virions. This study demonstrates the specific role of HSP70 in regulating DHAV-1 replication, which are helpful for understanding the pathogenesis of DHAV-1 infection and provide additional information about the role of HSP70 in infection by different kinds of picornaviruses, as well as the interaction between picornaviruses and host cells.

The role of turmeric and black pepper oil nanoemulsion in attenuating cytokine storm triggered by duck hepatitis A virus type I (DHAV-I)-induced infection in ducklings

The cytokine storm induced by duck hepatitis A virus type 1 (DHAV-1) infection significantly contributes to severe, rapid deaths and economic losses in the duck industry in Egypt. This study aimed to investigate the potential inhibitory effect of a nanoemulsion containing turmeric and black pepper oil on the immune response and pathogenesis of DHAV-1 in ducklings. A total of 105 ducklings from nonvaccinated breeders were divided into 5 experimental groups, each comprising 21 birds. The negative control group (G1) remained noninfected with DHAV-1 and nontreated with nanoemulsion, while the positive control group (G2) was infected with DHAV-1 but not treated with nanoemulsion. The other 2 groups (G3, the supplemented group which was noninfected with DHAV-1), and group 4 (the prophylactic group G4) which was infected with DHAV-1, both received nanoemulsion throughout the experiment. Group 5 (G5, the therapeutic group), on the other hand, which was infected with DHAV-1 received nanoemulsion only from the onset of clinical signs. At 5 days old, the ducklings in the positive control (G2), the prophylactic (G4), and the therapeutic group (G5) were infected with DHAV-1. All the ducklings in the infected groups exhibited depression, anorexia, and opisthotonos, and their livers displayed various degrees of ecchymotic hemorrhage, liver enlargement, and microscopic pathological lesions. Notably, the positive control group (G2) experienced the most severe and pronounced effects compared to the other infected groups treated with the nanoemulsion. Meanwhile, the viral RNA loads were lower in the liver tissues of the infected ducklings treated with the nanoemulsion (G4, and G5) compared to the positive control group G2. Additionally, the nanoemulsion effectively modulated proinflammatory cytokine expression, antioxidant enzymes, liver enzymes, and lipid profile of treated ducklings. In conclusion, the turmeric and black pepper oil nanoemulsion has the potential to be a therapeutic agent for regulating and modulating the immune response, decreasing DHAV-1-induced cytokine storms, and minimizing mortality and economic losses in the duck business. More research is needed to understand how turmeric and black pepper oil nanoemulsion alleviates DHVA-1-induced cytokine storms and lowers duckling mortality.

DHAV 3CD targets IRF7 and RIG-I proteins to block the type I interferon upstream signaling pathway

Duck hepatitis A virus type 1 (DHAV-1) is an acute, highly lethal infectious agent that infects ducklings and causes up to 95% mortality in ducklings up to 1 week of age, posing a significant economic threat to the duck farming industry. Previous studies have found that the proteolytic enzyme 3 C encoded by DHAV-1 can inhibit the IRF7 protein from blocking the upstream signaling pathway of the type I interferon to promote viral replication. However, there are still few studies on the mechanism of DHAV-1 in immune evasion. Here, we demonstrate that the DHAV-1 3CD protein can interact with IRF7 protein and reduce IRF7 protein expression without directly affecting IRF7 protein nuclear translocation. Further studies showed that the 3CD protein could reduce the expression of RIG-I protein without affecting its transcription level. Furthermore, we found that the 3CD protein interacted with the N-terminal structural domain of RIG-I protein, interfered with the interaction between RIG-I and MAVS, and degraded RIG-I protein through the proteasomal degradation pathway, thereby inhibiting its mediated antiviral innate immunity to promote DHAV-1 replication. These data suggest a novel immune evasion mechanism of DHAV-1 mediated by the 3CD protein, and the results of this experiment are expected to improve the understanding of the biological functions of the viral precursor protein and provide scientific data to elucidate the mechanism of DHAV-1 infection and pathogenesis.

DHAV-1 Blocks the Signaling Pathway Upstream of Type I Interferon by Inhibiting the Interferon Regulatory Factor 7 Protein

Duck hepatitis A virus (DHAV), which mainly infects 1- to 4-week-old ducklings, has a fatality rate of 95% and poses a huge economic threat to the duck industry. However, the mechanism by which DHAV-1 regulates the immune response of host cells is rarely reported. This study examined whether DHAV-1 contains a viral protein that can regulate the innate immunity of host cells and its specific regulatory mechanism, further exploring the mechanism by which DHAV-1 resists the host immune response. In the study, the dual-luciferase reporter gene system was used to screen the viral protein that regulates the host innate immunity and the target of this viral protein. The results indicate that the DHAV-1 3C protein inhibits the pathway upstream of interferon (IFN)-β by targeting the interferon regulatory factor 7 (IRF7) protein. In addition, we found that the 3C protein inhibits the nuclear translocation of the IRF7 protein. Further experiments showed that the 3C protein interacts with the IRF7 protein through its N-terminus and that the 3C protein degrades the IRF7 protein in a caspase 3-dependent manner, thereby inhibiting the IFN-β-mediated antiviral response to promote the replication of DHAV-1. The results of this study are expected to serve as a reference for elucidating the mechanisms of DHAV-1 infection and pathogenicity.

A viroporin-like 2B protein of duck hepatitis A virus 1 that induces incomplete autophagy in DEF cells

Duck hepatitis A virus 1 (DHAV-1) can cause high morbidity and fatal acute infectious hepatitis in ducklings, which seriously endangers animal husbandry. Viroporin is a small molecular weight hydrophobic transmembrane protein encoded by the virus, that has been suggested to induce autophagy in host cells by increasing the membrane permeability through disturbing the ion balance. In this study, we aimed to investigate whether the DHAV-1 2B protein can induce autophagy in DEF cells with a viroporin-like function. Bioinformatics analysis has indicated that the 2B protein is characterized by a viroporin domain, which is consistent with the type IA viroporin transmembrane protein. We experimentally confirmed that the 2B protein disturbed the Ca2+ balance of infected cells by elevating the intracellular Ca2+ concentration. Eukaryotic expression of the 2B protein upregulates the expression of microtubule-associated protein 1 light chain 3 II (LC3-II) and the number of autophagosomes in the cell. Interestingly, the Western Blot (WB) results showed that 2B protein expression induced less protein degradation of the autophagic substrate sequestosome 1 (SQSTM1/p62) than the positive control, while microscopy observations showed that the autophagosomes did not colocalize with the lysosomes. In summary, 2B protein expression induced autophagy in host cells, but the autophagic flow was incomplete. The results of this experiment are expected to provide reference scientific data for elucidating the infective and pathogenic mechanism of DHAV-1.

Establishment of duplex SYBR Green I-based real-time PCR assay for simultaneous detection of duck hepatitis a virus-1 and duck astrovirus-3

Duck viral hepatitis (DVH) mainly affects ducklings under 1 month of age, causes liver necrosis, enlargement, and hemorrhage, and is highly lethal, seriously jeopardizing the duck industry. The prevalence of serotypes DHAV-1 and DAstV-3 is increasing, and co-infection is common. Moreover, the similar clinical characteristics of the DHAV-1 and DAstV-3 infections and high frequency of co-infection make diagnosis difficult. In this study, to establish a method for the rapid simultaneous detection of DHAV-1 and DAstV-3 , two pairs of specific primers were designed according to their conserved gene regions. A SYBR Green I-based real-time PCR assay was successfully established that can quickly and differentially detect the two viruses. Moreover, the assay is highly specific and does not show cross-reaction with other common viruses. The detection limit of the method is 7.34 × 10 1 copies/μL and 3.78 × 10 1 copies/μL for DHAV-1 and DAstV-3, respectively, indicating high sensitivity. A total of 34 clinical samples were tested using the established method; the positive rates for DHAV-1 and DAstV-3 was 14.71% and 8.82%, respectively, and that for co-infection was 2.94% (1/34), which was better than that obtained with conventional PCR. In summary, the SYBR Green I-based real-time PCR assay established in this study has high specificity, good sensitivity and accuracy, high feasibility, and is rapid. Thus, it can be a powerful tool for the co-infection detection of DHAV-1 and DAstV-3 and for future epidemiological studies.

Duck hepatitis A virus 1 has lymphoid tissue tropism altering the organic immune responses of mature ducks

Duck hepatitis A virus 1 (DHAV-1) is a highly prevalent pathogen within adult ducks causing acute as well as chronic hepatitis which closely emulates the progression of human hepatitis. However, the underlying mechanisms of DHAV-1 persistence and the pathogenesis of chronic liver disease are not well defined. The association between hematopoietic reservoirs of virus and persistent infection is increasingly concerning. Here, we explored the ability of lymphoid replication of DHAV-1 and the effect on immunity. We found that DHAV-1 was able to infect and replicate productively in the lymphoid organs of model ducks, persisting over 6 months. Moreover, a significant correlation of viral loads between these organs and blood was found, documenting a major contribution of lymphoid replication to DHAV-1 viraemia. Along with viral replication, the mRNA of PRRs and immune-related cytokines was up-regulated in these organs during the early phase of infection, showing tissue-dependent expression patterns but all inclining towards Th2 responses due to the consistently higher level of IL-4 than IL-2 and IFN-γ. Additionally, the expression of CCL19, CCL21, MHC-I and MHC-II, which are involved in T cell homing to the periphery and priming, was dysmodulated. Our data indicate that DHAV-1 possesses lymphoid tissue tropism, contributing to persistent infection and chronic hepatitis via altering the early endogenous transcription of immune-related genes and thereby perturbing organic immunity. These results may be useful to develop novel strategies to treat chronic viral hepatitis based on stimulation of the early innate system and regulation of T-cell trafficking.

The VP3 protein of duck hepatitis A virus mediates host cell adsorption and apoptosis

Duck hepatitis A virus (DHAV) causes an infectious disease that mainly affects 1- to 4-week-old ducklings, resulting in considerable loss to the duck industry. Although there have been many studies on DHAV in recent years, the effects on host infection and pathogenesis of DHAV-1 remain largely unknown. This study investigated the effects of the DHAV-1 structural protein VP3 on DHAV-1 virus adsorption and apoptosis to explore the role of VP3 in the viral life cycle. The effects of DHAV-1 VP3 and an antibody against the protein on virion adsorption was analyzed by qRT-PCR. The results showed that the virus copy number for the rabbit anti-VP3 IgG-treated group was significantly lower than that for the negative control group but higher than that for the rabbit anti-DHAV-1 IgG-treated group. This result indicates that VP3 mediates DHAV-1 virus adsorption but that it is not the only protein that involved in this process. In addition, a eukaryotic recombinant plasmid, pCAGGS/VP3, was transfected into duck embryo fibroblasts (DEFs), and the apoptotic rate was determined by DAPI staining, the TUNEL assay and flow cytometry. DAPI staining showed nucleus fragmentation and nuclear edge shifting. TUNEL assay results revealed yellow nuclei, and flow cytometry indicated a significant increase in the apoptotic rate. In addition, qRT-PCR revealed increased in the transcriptional levels of the apoptotic caspase-3, −8 and −9, with the largest increase for caspase-3, followed by caspase-9 and caspase-8. Enzyme activity analysis confirmed these results. Furthermore, the VP3 protein decreased the mitochondrial membrane potential, and the transcriptional levels of the proapoptotic factors Bak, Cyt c and Apaf-1 in the mitochondrial apoptotic pathway were significantly upregulated. These data suggest that expression of VP3 in DEFs induces apoptosis and may primarily activate caspase-3-induced apoptosis through mitochondrion-mediated intrinsic pathways. The findings provide scientific data to clarify DHAV-1 infection and pathogenesis.

Development and evaluation of reverse transcription-insulated isothermal PCR assay to detect duck hepatitis A virus type A in liver samples using the POCKITTM system

Duck hepatitis A virus (DHAV) infection is characterized by severe hepatitis. In recent years, DHAV-A has become widespread in Asia and has led to economic losses. Conventional methods of DHAV-A detection must often be performed in the laboratory with inconvenience equipment. We have developed a rapid reverse transcription insulated isothermal (RT-iiPCR) technique for the on-site detection of DHAV-A based on the POCKIT^TM system in a convenient minitype device. We optimized the PCR primers and probes for the amplification of the DHAV-A 3C/3D genes, and successfully amplified a specific fragment of DHAV-A, but no fragment from 18 other duck pathogens. The limit of detection for viral RNA was 49 copies per reaction, and the sensitivity and specificity were each 100% in the analysis of 60 liver samples. By comparison, the sensitivities of RT-iiPCR was comparable in sensitivity to existing rRT-PCR. Furthermore, the RT-iiPCR results were 98.3% in agreement with those of the rRT-PCR, with a kappa value of 0.938. In conclusion, this new method not only offers a higher sensitivity and specificity than existing techniques, but also time-saving and better suited to field diagnoses because device is portable.

Improved one-tube RT-PCR method for simultaneous detection and genotyping of duck hepatitis A virus subtypes 1 and 3

Background

The cocirculation of duck hepatitis A virus subtypes 1 (DHAV-1) and 3 (DHAV-3) in ducklings has resulted in significant economic losses. Ducklings with DHAV-1 or DHAV-3 infection show similar clinical signs and gross lesions; hence, it is important to identify the viral subtypes in infected ducklings as early as possible for better clinical management.

Methods and results

Based on multiple 5’ noncoding region (5’-NCR) sequences of DHAV-1 and DHAV-3 strain alignments, universal and type-specific primers were designed and synthesized. With three primers in one-tube reverse transcription-PCR (RT-PCR), reference DHAV-1 and DHAV-3 isolates ranging over 60 years and across many different countries were successfully amplified, indicating that the primer sequences were completely conserved. The sequence results and the sizes of amplicons from reference DHAV-1 and DHAV-3 isolates are completely correlated with their subtypes. Moreover, with this one-tube RT-PCR system, amplicon sizes from liver samples of reference DHAV-1- or DHAV-3-infected birds fit closely with their subtypes, which was determined by virus isolation and neutralization testing. No other duck-origin RNA viruses were detected. The sensitivity of viral RNA detection was 10 pg. With this system, 20% subtype 1, 45% subtype 3, and 9% coinfection of two subtypes were detected in 55 clinical samples.

Conclusions and significance

This novel approach could be used for rapidly typing DHAV-1 or DHAV-3 infection in routine clinical surveillance or epidemiological screening.

Comparative pathogenicity of two subtypes (hepatitis-type and pancreatitis-type) of duck hepatitis A virus type 1 in experimentally infected Muscovy ducklings

Duck hepatitis A virus type 1 (DHAV-1) causes acute hepatitis with high morbidity and mortality in ducklings of the genera Cairina and Anas and is characterized by ecchymotic haemorrhage and necrosis of the liver surface. Since September 2011, a new subtype of DHAV-1 (named pancreatitis-type DHAV-1) has been isolated. This new subtype is characterized by yellowish or haemorrhagic pancreatitis, but with no significant pathological changes in the liver. To further investigate the difference in pathogenicity between hepatitis-type DHAV-1 and pancreatitis-type DHAV-1, we infected Muscovy ducklings with a hepatitis-type DHAV-1 strain, FZ86, or a pancreatitis-type DHAV-1 strain, MPZJ1206, and then compared the resulting gross lesions, histopathological changes, viral distribution and cellular apoptosis in the liver and pancreas of Muscovy ducklings. The results suggested that FZ86 induced a more efficient viral propagation in the liver than MPZJ1206, and the gross and histopathological lesions were also limited to the liver. However, MPZJ1206 induced more effective viral replication in the pancreas than FZ86. The MPZJ1206-infected Muscovy ducklings showed an obviously yellowed and haemorrhagic pancreas, but with no significant pathological changes in the liver. Furthermore, FZ86 induced notable hepatocyte apoptosis and increased the expression of caspase-3 in the liver, whereas MPZJ1206 caused apoptosis in a large number of acinar epithelial cells and elevated the expression of caspase-3 in the pancreas. Taken together, these results demonstrated that pancreatitis-type DHAV-1 has many new pathogenic features which distinguish it from the hepatitis-type DHAV-1. RESEARCH HIGHLIGHTS Pancreatitis-type DHAV-1 (MPZJ1206) was characterized by pancreatic haemorrhage and yellow discolouration, but with no obvious haemorrhage and necrosis in the liver. Pancreatitis-type DHAV-1 (MPZJ1206) exhibits many new pathogenic features which distinguish it from the hepatitis-type DHAV-1 (FZ86).

DHAV-1 Inhibits Type I Interferon Signaling to Assist Viral Adaption by Increasing the Expression of SOCS3

Duck hepatitis A virus type 1 (DHAV-1) is one of the most lethal pathogens in the duck industry. The attenuated vaccine (the CH60 strain) is cultivated through serial passage in chicken embryos and is widely used for the prevention and control of the disease. However, the specific mechanism underlying its adaptation in chicken embryos has not been fully elucidated. In this study, we first infected chicken embryo fibroblasts (CEFs) with the DHAV-1 CH60 strain. The peak of viral proliferation occurred within 36–48 h post-infection. The different DHAV-1 strains significantly induced the expression of IFNα, IFNγ, and Suppressor of cytokine signaling 3 (SOCS3) in CEFs, and we found that SOCS3 overexpression significantly promoted viral replication. Furthermore, SOCS3 overexpression significantly inhibited the expression of IFNα but promoted the expression of IFNγ. In addition, SOCS3 overexpression clearly decreased the mRNA levels of STAT1 and STAT3 in the Janus kinase (JAK)-STAT signaling pathway and inhibited the expression of the antiviral proteins MX1 and OASL. Immune-precipitation assays indicated that SOCS3 and IFNα do not physically interact. Subcellular localization of SOCS3 and IFNα revealed that SOCS3 was mainly located in the nucleus and cytoplasm, while IFNα was located only in the cytoplasm. Co-localization of these two proteins was not observed in the cytoplasm. In conclusion, the DHAV-1 CH60 strain may inhibit the expression of IFNα by increasing the SOCS3 protein and SOCS3 can in turn decrease STAT1 and STAT3 mRNA levels, thereby inhibiting the antiviral protein MX1 and ultimately promoting viral proliferation, indirectly assisting in viral adaptation in chicken embryos.

Dynamic Expression Pattern of SERPINA1 Gene from Duck (Anas platyrhynchos)

SERPINA1 is a member of serine protease inhibitors and is increasingly considered to be a regulator of innate immunity in human and animals. However, the expression and function of SERPINA1 gene in immune defense against viral infection remain unknown in ducks. The full-length du SERPINA1 cDNA sequence was obtained using reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). It contained 1457 nucleotide, including 47-bp 5' UTR, 135-bp 3' UTR, and 1275-bp open reading frame (ORF), and encodes a 424-amino acid protein. Then, the tissue expression profile of du SERPINA1 gene was determined. Real-time quantitative polymerase chain reaction (real-time qPCR) analysis revealed that du SERPINA1 mRNA is ubiquitous in various tissues, but higher expression levels were observed in lung and liver tissues. In addition, the expression pattern was investigated when the ducklings were challenged with duck hepatitis virus 1(DHV-1) and polyriboinosinic polyribocytidylic acid (poly I:C). After DHV-1 injection or poly I:C treatment, du SERPINA1 mRNA was up-regulated in the liver and kidney tissues. However, the peak time in two tissues was not consistent. In kidney, the expression lever of SERPINA1 increased immediately after the treatment while in liver tissue it kept steady until 12 h post-infection. Our results indicate that SERPINA1 has an active role in the antiviral response, and thus improve our understanding of the role of this protein.

Comparative pathogenicity of different subtypes of duck hepatitis A virus in Pekin ducklings

Duck hepatitis A virus (DHAV) is a major pathogen of viral hepatitis in ducks, which is a fatal and contagious disease of young ducklings. Despite the identification of numerous DHAV strains (e.g. DHAV-3, DHAV-2, DHAV-1 and DHAV-1a), the pathogenic differences among the different subtypes have not been evaluated. The objective of this study was to compare the pathogenic properties of three epidemic strains DHAV-3, DHAV-1, and DHAV-1a in mainland China, in a Pekin duckling infection model. We evaluated the pathogenicity of these different subtypes by investigating clinical signs, macroscopic and microscopic lesions, immunohistochemical examination, and viral RNA detection after experimental inoculation of Pekin ducklings with the three different DHAV strains. There was no significant difference in pathogenicity between DHAV-3 and DHAV-1. Pathogenicity of DHAV-1a differed significantly from that of classical duck hepatitis A (DHAV-3 or DHAV-1), in that there were no clinical signs of opisthotonos. More importantly, pancreatic bleeding or yellowing, and spleen swelling and bleeding were the predominant lesions in the DHAV-1a group, while liver and spleen lesions were the main signs in classical hepatitis (DHAV-1/3). Our findings indicate that there are differences in the pathogenicity of different subtypes of DHAV in ducklings, which may be useful for understanding the biological characteristics of the different subtypes of DHAV in ducks.

Transcriptomic Characterization of a Chicken Embryo Model Infected With Duck Hepatitis A Virus Type 1

Duck hepatitis A virus type 1 (DHAV-1) is one of the most common and lethal pathogens in young ducklings. Live-attenuated DHAV vaccine (CH60 strain) developed by passaging in chicken embryos provided effective immune protection for ducklings. However, the accurate mechanism for such adaption in chicken embryos is not fully revealed. Here, we utilize RNA-sequencing to perform global transcriptional analysis of DHAV-1-innoculated embryonated livers along with histopathological and ultrastructural analysis. This study revealed that infection with DHAV-1 strain CH60 is associated with enhanced type I and II interferon responses, activated innate immune responses, elevated levels of suppressor of cytokine signaling 1 and 3 (SOCS1 and SOCS3) accompanied with abnormalities in multiple metabolic pathways. Excessive inflammatory and innate immune responses induced by the CH60 strain are related to severe liver damage. Our study presents a comprehensive characterization of the transcriptome of chicken embryos infected with DHAV-CH60 and provides insight for in-depth exploration of viral adaption and virus–host interactions.

Incompatible Translation Drives a Convergent Evolution and Viral Attenuation During the Development of Live Attenuated Vaccine

Live attenuated vaccines are widely used to protect humans or animals from pathogen infections. We have previously developed a chicken embryo-attenuated Duck Hepatitis A Virus genotype 1 (DHAV-1) vaccine (CH60 strain). This study aims to understand the mechanisms that drive a virulent strain to an attenuated virus. Here, we systematically compared five DHAV-1 chicken embryo attenuated strains and 68 virulent strains. Phylogenetic analysis indicated that duck virulent strains isolated from different geographic regions of China undergo a convergent evolution in the chicken embryos. Comparative analysis indicated that the codon usage bias of the attenuated strains were shaped by chicken codons usage bias, which essentially contributed to viral adaption in the unsuitable host driven by incompatible translation. Of note, the missense mutations in coding region and mutations in untranslated regions may also contribute to viral attenuation of DHAV-1 to some extent. Importantly, we have experimentally confirmed that the expression levels of four viral proteins (2A3pro, 2A3pro, 3C^pro, and 3D^pro) in the liver and kidney of ducks infected with an attenuated strain are significantly lower than that infected with a virulent strain, despite with similar virus load. Thus, the key mechanisms of viral attenuation revealed by this study may lead to innovative and easy approaches in designing live attenuated vaccines.

Cytokine storms are primarily responsible for the rapid death of ducklings infected with duck hepatitis A virus type 1

Duck hepatitis A virus type 1 (DHAV-1) is one of the most harmful pathogens in the duck industry. The infection of adult ducks with DHAV-1 was previously shown to result in transient cytokine storms in their kidneys. To understand how DHAV-1 infection impacts the host liver, we conducted animal experiments with the virulent CH DHAV-1 strain and the attenuated CH60 commercial vaccine strain. Visual observation and standard hematoxylin and eosin staining were performed to detect pathological damage in the liver, and viral copy numbers and cytokine expression in the liver were evaluated by quantitative PCR. The CH strain (10^8.4 copies/mg) had higher viral titers than the CH60 strain (10^4.9 copies/mg) in the liver and caused ecchymotic hemorrhaging on the liver surface. Additionally, livers from ducklings inoculated with the CH strain were significantly infiltrated by numerous red blood cells, accompanied by severe cytokine storms, but similar signs were not observed in the livers of ducklings inoculated with the CH60 strain. In conclusion, the severe cytokine storm caused by the CH strain apparently induces hemorrhagic lesions in the liver, which might be a key factor in the rapid death of ducklings.

Live attenuated duck hepatitis virus vaccine in breeder ducks: Protective efficacy and kinetics of maternally derived antibodies

Duck viral hepatitis type I is a rapidly spreading infection lethal in young ducklings, caused by the duck hepatitis A virus (DHAV). Vaccination of breeder ducks is a common practice to control DHAV. However, maintaining proper maternal antibody levels in large flocks is difficult. Therefore, a simple vaccination strategy that can induces stable high antibody levels through mass vaccination is desirable. We evaluated a DHAV vaccination strategy for breeder ducks involving oral administration under field conditions, and examined the kinetics of antibody response in the ducks and their progeny. The strategy included a primary intramuscular vaccination, followed by secondary and tertiary oral vaccinations. Five weeks after the primary vaccination, virus-neutralizing antibody titers increased by 8.4 ± 1.3 log₂. The titers remained stable at around 9.0 ± 1.1 log₂ for up to 36 weeks. None of the progeny died when challenged with virulent DHAV at 1, 7 or 14 days of age. The transfer percentage of antibodies from the breeder ducks to their progeny was 12.8 ± 3.0%. When antibody levels of the progeny were measured from the day of hatching to 20 days of age, the levels steadily declined, reaching a mean titer of 0 log₂ at 20 days. The half-life of the maternally derived antibodies against DHAV was 3.4 ± 1.1 days. Our vaccination strategy might be effective in breeder ducks because it can be easily applied and induced strong immunity. Moreover, our results might provide a foundation for the mechanistic study of maternally derived antibodies in passive protection.

Viral-host interaction in kidney reveals strategies to escape host immunity and persistently shed virus to the urine

Hepatitis A virus is one of five types of hepatotropic viruses that cause human liver disease. A similar liver disease is also identified in ducks caused by Duck Hepatitis A virus (DHAV). Notably, many types of hepatotropic viruses can be detected in urine. However, how those viruses enter into the urine is largely unexplored. To elucidate the potential mechanism, we used the avian hepatotropic virus to investigate replication strategies and immune responses in kidney until 280 days after infection. Immunohistochemistry and qPCR were used to detect viral distribution and copies in the kidney. Double staining of CD4+ or CD8+ T cells and virus and qPCR were used to investigate T cell immune responses and expression levels of cytokines. Histopathology was detected by standard HE staining. In this study, viruses were persistently located at scattered renal tubules. No CD4+ or CD8+ T cells were recruited to the kidney, which was only accompanied by transient cytokine storms. In conclusion, the extremely scattered infection was the viral strategy to escape host immunity and may persistently shed virus into urine. The deletion of Th or Tc cell responses and transient cytokine storms indeed provide an advantageous renal environment for their persistent survival.

Virologic and Immunologic Characteristics in Mature Ducks with Acute Duck Hepatitis A Virus 1 Infection

Duck hepatitis A virus 1 (DHAV-1) infection in mature ducks has previously been proposed as a small-animal model for human hepatitis A. However, basic research on the outcome of DHAV-1 infection in mature ducks is limited. Here, we examined the course of viremia, the characteristics of antibody responses, and the profiles of plasma cytokines in mature ducks infected with DHAV-1. During the course of infection, the viremia was detectable soon after infection and persisted for 196 days, however, the ducks presented as clinically asymptomatic. Specific and timely immunoglobulin G (IgG), IgM, and IgA1 responses were elicited. At the same time, extensive inhibition of viral replication was observed with increasing IgG concentration. With respect to pattern-recognition receptors, TLR-7 was mainly involved in triggering the innate defense against the DHAV-1 infection. In addition, plasma immune analytes were measured and were determined to have bidirectional roles in virus clearance. It was concluded that DHAV-1 spreads quickly in blood. The spontaneous clearance of DHAV-1 during asymptomatic infection in mature ducks depends on the cooperation of timely antibody responses and alert innate immune responses. Moreover, the delayed clearance may be associated with a weak interferon-γ-producing CD8+ T cell response. This study allows us to reveal the mechanism of clearance and persistence of DHAV-1 infection in mature ducks. We anticipate that it will provide a basis for future studies focused on defining the nature mechanisms involved in the clearance and persistence of human hepatitis virus.

Hepatitis Virus Infections in Poultry

Viral hepatitis in poultry is a complex disease syndrome caused by several viruses belonging to different families including avian hepatitis E virus (HEV), duck hepatitis B virus (DHBV), duck hepatitis A virus (DHAV-1, -2, -3), duck hepatitis virus Types 2 and 3, fowl adenoviruses (FAdV), and turkey hepatitis virus (THV). While these hepatitis viruses share the same target organ, the liver, they each possess unique clinical and biological features. In this article, we aim to review the common and unique features of major poultry hepatitis viruses in an effort to identify the knowledge gaps and aid the prevention and control of poultry viral hepatitis. Avian HEV is an Orthohepevirus B in the family Hepeviridae that naturally infects chickens and consists of three distinct genotypes worldwide. Avian HEV is associated with hepatitis-splenomegaly syndrome or big liver and spleen disease in chickens, although the majority of the infected birds are subclinical. Avihepadnaviruses in the family of Hepadnaviridae have been isolated from ducks, snow geese, white storks, grey herons, cranes, and parrots. DHBV evolved with the host as a noncytopathic form without clinical signs and rarely progressed to chronicity. The outcome for DHBV infection varies by the host's ability to elicit an immune response and is dose and age dependent in ducks, thus mimicking the pathogenesis of human hepatitis B virus (HBV) infections and providing an excellent animal model for human HBV. DHAV is a picornavirus that causes a highly contagious virus infection in ducks with up to 100% flock mortality in ducklings under 6 wk of age, while older birds remain unaffected. The high morbidity and mortality has an economic impact on intensive duck production farming. Duck hepatitis virus Types 2 and 3 are astroviruses in the family of Astroviridae with similarity phylogenetically to turkey astroviruses, implicating the potential for cross-species infections between strains. Duck astrovirus (DAstV) causes acute, fatal infections in ducklings with a rapid decline within 1-2 hr and clinical and pathologic signs virtually indistinguishable from DHAV. DAstV-1 has only been recognized in the United Kingdom and recently in China, while DAstV-2 has been reported in ducks in the United States. FAdV, the causative agent of inclusion body hepatitis, is a Group I avian adenovirus in the genus Aviadenovirus. The affected birds have a swollen, friable, and discolored liver, sometimes with necrotic or hemorrhagic foci. Histologic lesions include multifocal necrosis of hepatocytes and acute hepatitis with intranuclear inclusion bodies in the nuclei of the hepatocytes. THV is a picornavirus that is likely the causative agent of turkey viral hepatitis. Currently there are more questions than answers about THV, and the pathogenesis and clinical impacts remain largely unknown. Future research in viral hepatic diseases of poultry is warranted to develop specific diagnostic assays, identify suitable cell culture systems for virus propagation, and develop effective vaccines.

The neglected avian hepatotropic virus induces acute and chronic hepatitis in ducks: an alternative model for hepatology

Duck Hepatitis A Virus (DHAV) belongs to the Avihepatovirus, which is also classified into Picornaviridae with Hepatovirus, Hepatitis A Virus (HAV). In humans, the pathogenesis of HAV is not well understood because of limited work with animal models. Here, we investigated the progress of duck viral hepatitis caused by DHAV and their potential for dissecting the pathogenesis of HAV. During the course of infection, the duck model had undergone hepatocellular lesions (vacuolation, acidophilic degeneration and steatosis), lymphocytes recruitment (neutrophil granulocytes, heterophilic granulocytes and T cells or plasm cells) and repair (activation of hepatic stellate cells, fibrosis and regeneration). Coincident with liver injury, the serum biomarkers, aspartate aminotransferase and alanine transaminase were significantly increased. Moreover, comparatively lower CD4+ and CD8+ T-cells were recruited to the liver, which might lead to a persistent infection (40 wk). Because DHAV and HAV have similar genomic structure, biological phenotypes and can easily replicate in liver. And half of fibrosis-related genes had high homology between humans and ducks. Considering these similarity in pathological and virological phenotypes, we proposed that the ducks might be an alternatively small animal model that would provide insight into the pathogenesis of viral hepatitis, fibrosis and liver regeneration.

Comparative analysis of virus-host interactions caused by a virulent and an attenuated duck hepatitis A virus genotype 1

Because of their better immunogenicity and the improved protection they afford, live attenuated vaccines derived from serial passaging in an abnormal host are widely used to protect humans or animals from certain pathogens. Here, we used a virulent and a chicken embryo-attenuated duck hepatitis A virus genotype 1 to compare the different regulated immune responses induced by viruses with differing virulence. In this study, the attenuated strains had lower protein expression levels than the virulent strains as identified by immunohistochemistry. This may be caused by apparent codon usage bias selected during passage. Furthermore, lower translation efficiency led to decreased viral replication, which is highly dependent on non-structural viral protein expression. Although the two strains had differing levels of virulence, both could induce strong innate immune responses and robust Tc or Th cell populations during the early stages of the immune response. However, due to fixed single nucleotide polymorphisms (SNPs) selected by passage, the virulent and attenuated strains may induce differing immune responses, with stronger Tc cell immunity induced by the attenuated strain in the spleen and thymus and stronger Tc cell immunity induced by the virulent strain in the liver, lung, bursa of Fabricius and Harderian gland. Four immune related genes (RIG-1, MDA5, IFN-β, and IL-6) were highly differentially expressed in the Harderian gland, bursa of Fabricius and thymus. This study has provided further information about differences in virus-host interactions between duck hepatitis A viruses of differing virulence.

The 3D protein of duck hepatitis A virus type 1 binds to a viral genomic 3' UTR and shows RNA-dependent RNA polymerase activity

To explore the RNA-dependent RNA polymerase (RdRP) function of the 3D protein of duck hepatitis A virus type 1 (DHAV-1), the gene was cloned into the pET-32a(+) vector for prokaryotic expression. The 3' untranslated region (3' UTR) of DHAV-1 together with a T7 promoter was cloned into the pMD19-T vector for in vitro transcription of 3' UTR RNA, which was further used as a template in RNA-dependent RNA polymerization. In this study, three methods were applied to analyze the RdRP function of the 3D protein: (1) ammonium molybdate spectrophotometry to detect pyrophosphate produced during polymerization; (2) quantitative reverse transcription PCR (RT-qPCR) to investigate the changes in RNA quantity during polymerization; and (3) electrophoresis mobility shift assay to examine the interaction between the 3D protein and 3' UTR. The results showed the 3D protein was successfully expressed in bacteria culture supernatant in a soluble form, which could be purified by affinity chromatography. In 3D enzymatic activity assays, pyrophosphate and RNA were produced, the amounts of which increased based on approximative kinetics, and binding of the 3D protein to the 3' UTR was observed. These results indicate that prokaryotically expressed soluble DHAV-13D protein can bind to a viral genomic 3' UTR and exhibit RdRP activity.

Ferritin heavy polypeptide 1 mediates apoptosis-related gene expression of duck (Anas platyrhynchos domesticus)

Duck hepatitis virus type 1 (DHV-1) infection of ducklings causes hepatitis and is associated with high morbidity and mortality. Virus infection may induce apoptosis and inhibit proliferation. In humans, ferritin heavy polypeptide 1 (FTH1) has been reported to affect the development of hepatitis and inhibit apoptosis. However, the effect of duck FTH1 (duFTH1) on apoptosis in DHV-1 infected ducklings has not been investigated. Therefore, we measured duFHT1 expression in tissues of DHV-1 infected ducklings and characterized the functional effects of ectopic overexpression and endogenous downregulation of FTH1 in duck embryo fibroblasts (DEF) to elucidate possible mechanisms involved. In the present study, the expression of duFTH1 was decreased in liver and spleen after DHV-1 infection. The effects of altered FTH1 expression on expression of pro- and anti-apoptotic genes were evaluated by qPCR arrays. Decreased expressions of Caspase-3 and Caspase-8 were observed in FTH1-overexpressing DEF cells, while decreased expression of Bcl-2 was detected in FTH1 knocked down DEF cells. Our findings suggest that the regulation of FTH1 expression indirectly mediated the expression of apoptosis-related genes; the protective effect of FTH1 was associated with the inhibition of apoptosis in DEF.

Live Attenuated Vaccine Based on Duck Enteritis Virus against Duck Hepatitis A Virus Types 1 and 3

As causative agents of duck viral hepatitis, duck hepatitis A virus type 1 (DHAV-1) and type 3 (DHAV-3) causes significant economic losses in the duck industry. However, a licensed commercial vaccine that simultaneously controls both pathogens is currently unavailable. Here, we generated duck enteritis virus recombinants (rC-KCE-2VP1) containing both VP1 from DHAV-1 (VP1/DHAV-1) and VP1 from DHAV-3 (VP1/DHAV-3) between UL27 and UL26. A self-cleaving 2A-element of FMDV was inserted between the two different types of VP1, allowing production of both proteins from a single open reading frame. Immunofluorescence and Western blot analysis results demonstrated that both VP1 proteins were robustly expressed in rC-KCE-2VP1-infected chicken embryo fibroblasts. Ducks that received a single dose of rC-KCE-2VP1 showed potent humoral and cellular immune responses and were completely protected against challenges of both pathogenic DHAV-1 and DHAV-3 strains. The protection was rapid, achieved as early as 3 days after vaccination. Moreover, viral replication was fully blocked in vaccinated ducks as early as 1 week post-vaccination. These results demonstrated, for the first time, that recombinant rC-KCE-2VP1 is potential fast-acting vaccine against DHAV-1 and DHAV-3.

A one-step duplex rRT-PCR assay for the simultaneous detection of duck hepatitis A virus genotypes 1 and 3

Duck hepatitis A virus (DHAV) is a highly infectious pathogen that causes significant bleeding lesions in the viscera of ducklings less than 3 weeks old. There are three serotypes of DHAV: serotype 1 (DHAV-1), serotype 2 (DHAV-2) and serotype 3 (DHAV-3). These serotypes have no cross-antigenicity with each other. To establish an rRT-PCR assay for the rapid detection of a mixed infection of DHAV-1 and DHAV-3, two pairs of primers and a pair of matching TaqMan probes were designed based on conserved regions of DHAV-1 VP0 and DHAV-3 VP3. Finally, we established a one-step duplex rRT-PCR assay with high specificity and sensitivity for the simultaneous detection of DHAV-1 and DHAV-3. This method showed no cross-antigenicity with the other pathogens tested, including duck plague virus, Muscovy duck parvovirus, Riemerella anatipestifer, and pathogenic E. coli from ducks. Sensitivity tests identified the minimum detection limits of this method as 98 (DHAV-1) and 10 (DHAV-3) copies/reaction. To validate the method, thirty-eight clinical samples and thirty artificially infected samples collected from dead duck embryos were studied. Thirty-seven samples were positive for DHAV-1, seventeen samples were positive for DHAV-3, and fourteen samples were positive for a mixed infection using the duplex rRT-PCR method. The method established in this study is specific, sensitive, convenient and timesaving and is a powerful tool for detecting DHAV-1, DHAV-3, and their mixed infection and for conducting surveys of pandemic virus strains.

Development of an indirect ELISA method based on the VP3 protein of duck hepatitis A virus type 1 (DHAV-1) for dual detection of DHAV-1 and DHAV-3 antibodies

An indirect enzyme-linked immunosorbent assay (I-ELISA) based on the recombinant VP3 protein of duck hepatitis A virus type 1 (DHAV-1) was developed and evaluated in this study. The optimal antigen, serum and enzyme-labeled antibody dilutions were 1:160 (0.94μg), 1:160 and 1:2000, respectively. The optimal blocking buffer was 1% gelatin. The cutoff value was determined to be 0.332, and the analytical sensitivity was 1:1280 (OD450-630=0.37). The results of the specificity evaluation showed that no cross-reactivity existed between DHAV-1 antiserum and other common duck-sensitive pathogens, except for duck hepatitis A virus type 3 (DHAV-3), suggesting that this could be a common approach for the simultaneous detection of DHAV-1 and DHAV-3 antibodies. The coefficients of variation (CVs) for all of the tested samples were lower than 10%. The concordance between the I-ELISA based on the VP3 subunit of DHAV-1 and that based on the whole DHAV-1 particle was 96%. These results indicate that the VP3-based I-ELISA method has high sensitivity, specificity, and repeatability and is as effective as the DHAV-1-based I-ELISA method for sero-surveillance. Thus, it may be a convenient and novel method for DHAV antibody detection and epidemiological surveillance of DHAV prevalence.

Determine the structure of phosphorylated modification of icariin and its antiviral activity against duck hepatitis virus A

BACKGROUND

Our previous research showed that icariin (1) and its phosphorylated structural modification (2) improved the survival and attenuated oxidative stress and liver dysfunction induced by duck virus hepatitis. In this paper, we were one step closer to determine the structure of phosphorylation icariin (2) by the FT-IR, HRESIMS and (13)C NMR. Anti-DHAV activities of 1 and 2 were compared in duck embryonic hepatocytes (DEHs) cultured in vitro and by artificial infection method in vivo. Additionally, the antiviral mechanisms of replication/release in vitro and the DHAV gene expression in vivo of 1 and 2 were analyzed.

RESULTS

Compound 2's molecular formula was C33H42O18P. The results indicated that 1 and 2 effectively resisted DHAV invading DEHs, that they decreased the mortality of ducklings challenged with DHAV, and that 2 performed more effectively. 1 and 2 performed evenly on DHAV release; however, 2 restrained virus replication far more effectively. Since the anti-DHAV mechanisms of 1 and 2 in vitro probably involve suppression of replication and release, 2's better performance in anti-DHAV may result from its far more effectively inhibiting virus replication.

CONCLUSIONS

The compound 2's chemical structure was defined as 8-prenylkaempferol-4'-methylether-3-rhamnosyl-7-(6'''-phosphate)-glycoside. 1 and 2 exhibited anti-virus activity on DHAV. Our results suggest that 1 and 2 might become an anti-virus plant material candidate.

Virulent and attenuated strains of duck hepatitis A virus elicit discordant innate immune responses in vivo

Previous studies of duck hepatitis A virus infection have focused only on the pathogenicity and host response of one strain. Here, we show that the virulent SH strain and the attenuated FC64 strain induced varied pathogenicity, apoptosis and immune responses in the livers of 1-day-old ducklings. SH infection caused apoptosis and visible lesions in the liver; serum aspartate aminotransferase, alanine transaminase, alkaline phosphatase, γ-glutamyltransferase and total bilirubin activities were markedly upregulated; and ducklings died at 36 h post-infection (p.i.). However, FC64 infection did not induce significant symptoms or impair liver function, and all of the infected ducklings remained healthy. In addition, both virus strains replicated well in the liver, spleen and intestine, whilst the SH strain replicated more efficiently than FC64. IFN-γ, IL-2, inducible nitric oxide synthase and nitric oxide were strongly induced by SH infection, and may be associated with the pathogenicity of the SH strain. IFN-α, IFN-β, IFN-stimulated transmembrane protein 1, IFN-stimulated gene 12, 2′,5′-oligoadenylate synthetase-like and IL-6 were moderately induced by SH infection at 24 h p.i., and dramatically induced by FC64 infection at 36 h p.i. The intensive induction of cytokines by FC64 may be involved in restriction of virus replication and stimulation of adaptive immune responses. Ducklings inoculated with FC64 produced high levels of antiviral antibodies within 45 days p.i. The low virulence and strong immune response of FC64 rendered this strain a good vaccine candidate, as confirmed by a protective assay in this study.

Detection, differentiation, and VP1 sequencing of duck hepatitis A virus type 1 and type 3 by a 1-step duplex reverse-transcription PCR assay

Duck hepatitis A virus (DHAV) is an infectious pathogen causing fatal duck viral hepatitis in ducklings. Although both the inactivated vaccines and live attenuated vaccines have been used to protect ducklings, DHAV-1 and DHAV-3 still cause significant serious damage to the duck industry in China and South Korea. For rapid detection, differentiation, and epidemic investigation of DHAV in China, a genotype-specific 1-step duplex reverse-transcription (RT) PCR assay was established in this study. The sensitivity and specificity of the developed RT-PCR assay was evaluated with nucleic acids extracted from 2 DHAV reference strains, and 9 other infectious viruses and bacteria. The genotype-specific primers amplified different size DNA fragments encompassing the complete VP1 gene of the DHAV-1 or DHAV-3. The assay detected the liver samples collected from experimentally infected ducklings and dead ducklings collected from different regions of China. Sequence analysis of these DNA fragments indicated that VP1 sequences of DHAV-1 can be used to distinguish wild type and vaccine strains. The phylogenetic analysis of VP1 sequences indicated that the developed RT-PCR assay can be used for epidemic investigation of DHAV-1 and DHAV-3. The developed RT-PCR assay can be used as a specific molecular tool for simultaneous detection, differentiation, and sequencing the VP1 gene of DHAV-1 and DHAV-3, which can be used for understanding the epidemiology and evolution of DHAV.

DNA methylation and regulation of the CD8A after duck hepatitis virus type 1 infection

Background

Cluster of differentiation 8 (CD8) is expressed in cytotoxic T cells, where it functions as a co-receptor for the T-cell receptor by binding to major histocompatibility complex class I (MHCI) proteins, which present peptides on the cell surface. CD8A is critical for cell-mediated immune defense and T-cell development. CD8A transcription is controlled by several cis-acting elements and trans-acting elements and is also regulated by DNA methylation. However, the epigenetic regulation of CD8A in the duck and its relationship with virus infection are still unclear.

Results

We investigated the epigenetic transcriptional regulatory mechanisms, such as DNA methylation, for the expression of the CD8A and further evaluated the contribution of such epigenetic regulatory mechanisms to DHV-I infection in the duck. Real-time quantitative polymerase chain reaction (RT-qPCR) revealed the highest level of CD8A expression to be in the thymus, followed by the lungs, spleen, and liver, and the levels of CD8A expression were very low in the kidney, cerebrum, cerebellum, and muscle in the duck. RT-qPCR also demonstrated that the CD8A mRNA was down-regulated significantly in morbid ducklings treated with DHV-1 and up-regulated significantly in non-morbid ducklings in all the tissues tested. In addition, hypermethylation of CD8A was detected in the morbid ducklings, whereas relatively low methylation of CD8A was evident in the non-morbid ducklings. The CD8A mRNA level was negatively associated with the CpG methylation level of CD8A and global methylation status.

Conclusions

We concluded that the mRNA level of the CD8A was negatively associated with the CpG methylation level of CD8A and global methylation status in the duck, suggesting that the hypermethylation of CD8A may be associated with DHV-1 infection. The first two CpG sites of the CD8A promoter region could be considered as epigenetic biomarkers for resistance breeding against duckling hepatitis disease in the duck.

Comparison of Bush Sophora Root polysaccharide and its sulfate's anti-duck hepatitis A virus activity and mechanism

In order to research the sulfating modification in enhancing the anti-duck hepatitis A virus (DHAV) activity of Bush Sophora Root polysaccharide (BSRPS), sulfated Bush Sophora Root polysaccharide (sBSRPS) was prepared by chlorosulfonic acid-pyridine method. KBr pellets method was applied to analyze their different structures. Anti-DHAV activity was studied by duck embryonic hepatocytes culture in vitro and artificial inoculation method in vivo. Direct immunofluorescence method and Real-time PCR were applied to study the antiviral mechanism of adsorption, replication and release in vitro and the dynamic change of virus content of blood in vivo. The results showed at the most effective content, sBSRPS (7.813 μg/mL) could inhibit both replication and release of DHAV in vitro, BSRPS (500 μg/mL) only inhibit replication. The relative expression of DHAV gene at the 8thh and the mortality rate of sBSRPS group were significantly reduced. These results indicated sBSRPS performed more effectively in anti-DHAV activity than BSRPS.

Rapid detection of duck hepatitis A virus genotype C using reverse transcription loop-mediated isothermal amplification

A one-step reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was used and optimized to develop a rapid and sensitive detection system for duck hepatitis A virus genotype C (DHAV-C) RNA. A set of four specific primers was designed against highly conserved sequences located within the 3D gene from DHAV (strain GX1201). Under optimal reaction conditions, the sensitivity of DHAV-C-specific RT-LAMP was 100-fold higher than that of reverse transcriptase-polymerase chain reaction (RT-PCR), with a detection limit of 0.3pg (6.59×10(4) copies) per reaction. No cross-reactivity was observed from the samples of other duck viruses, which is in good accordance with RT-PCR. Furthermore, a positive reaction can be visually inspected by observing turbidity or color change after the addition of SYBR green I dye. The DHAV-C-specific RT-LAMP assay was applied to the samples and compared with RT-PCR. The positive-sample ratios were 26.7% (12 of 45) by RT-LAMP and 20% (9 of 45) by RT-PCR. Therefore, the newly developed RT-LAMP assay is a rapid, specific, sensitive, and cost-effective method of DHAV-C detection. This assay has potential applications in both clinical diagnosis and field surveillance of DHAV-C infection.

Effect of age on the pathogenesis of DHV-1 in Pekin ducks and on the innate immune responses of ducks to infection

Duck hepatitis virus (DHV) affects 1-week-old but not 3-week-old ducks, and it causes a more severe disease in the younger ducks. These differences may be partially due to the host response to DHV infection. In order to understand this difference, we characterized the pathobiology of and innate immune response to DHV infection in 1-day-old (1D) and 3-week-old (3 W) ducks. Viral RNA was detected in duck livers at 24, 36 and 72 h after inoculation with DHV at a dose of 10(3) LD50. Virus-induced pathology ranged from no clinical signs to severe disease and death, and it was more severe in the 1D ducks. Infection with DHV induced up-regulation of gene expression of Toll-like receptor (TLR)-7, TLR3, retinoic-acid-inducible gene I (RIG-I), melanoma differentiation-associated gene 5 (MDA-5), interleukin (IL)-6, interferon (IFN)-α, interferon-induced transmembrane protein 1 (IFITM1), interferon-stimulated gene 12 (ISG12), and 2'-5' oligoadenylate synthetase-like gene (OASL) in the livers of 3 W ducks. Of these, IL-6, OASL and ISG12 mRNA levels were more than 100-fold higher in infected 3 W ducks than in mock-infected ducks of the same age. These genes were induced much less in infected 1D ducklings. We present evidence that a lower level of viral replication in the hepatocytes of 3 W ducks, whose basal level of cytokines is higher than that in 1D ducklings, may be related to the strong innate immunity induced. From our data, we conclude that duck age plays an important role in the pathogenicity of and innate immune responses to DHV.

Development and application of a reverse transcription loop-mediated isothermal amplification method for rapid detection of Duck hepatitis A virus type 1

We developed and evaluated a reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay for detecting Duck hepatitis A virus type 1 (DHAV-1). The amplification could be finished in 1 h under isothermal conditions at 63 °C by employing a set of four primers targeting the 2C gene of DHAV-1. The RT-LAMP assay showed higher sensitivity than the RT-PCR with a detection limit of 0.1 ELD(50) 0.1 ml(-1) of DHAV-1. The RT-LAMP assay was highly specific; no cross-reactivity was observed from the samples of other related viruses, bacteria, allantoic fluid of normal chicken embryos, or the livers of uninfected ducks. Thirty clinical samples were subjected to detection by RT-LAMP, RT-PCR, and virus isolation, which obtained completely consistent, positive results. As a simple, rapid, and accurate detection method, this RT-LAMP assay has important potential applications in the clinical diagnosis of DHAV-1.

Development of a real-time quantitative PCR for detecting duck hepatitis a virus genotype C

Recently, duck hepatitis A virus genotype C (DHAV-C), a causative agent of duck viral hepatitis, has been responsible for increasing economic losses in the duck industry in China and South Korea. In this study, a real-time PCR assay targeting the 2C gene for detecting DHAV-C was developed. The assay was confirmed to be specific and sensitive, and the minimum detection limit was 3.36 × 10(3) copies per reaction, making this assay suitable for rapid diagnosis of DHAV-C infection from clinical samples. In addition, the dynamics of the viral loads in tissues of specific-pathogen-free (SPF) ducklings infected with DHAV-C were investigated using this method. The DHAV-C could be detected earliest in the liver within 12 h postinfection. Moreover, high viral loads were identified in the heart, liver, spleen, lung, kidney, bursa of Fabricius, thymus, pancreas, brain, and small intestine after 24 h postinfection. Taking the data collectively, the study described in this report is the first to have developed a real-time PCR method for detection of DHAV-C and thus contributes to pathogenicity research.

Rapid detection of duck hepatitis virus type-1 by reverse transcription loop-mediated isothermal amplification

A reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay for the detection of duck hepatitis virus type-1 (DHV-1) was established. Using primers specific to the highly conserved 3D gene of DHV-1, the developed RT-LAMP assay detected the viral RNA of DHV-1 extracted from both allantoic fluid and liver samples of infected ducks. The assay is as sensitive as RT-PCR, and shows no cross-reaction with other common avian viral and bacterial pathogens. In addition to detection via ethidium bromide staining following gel electrophoresis, naked-eye observation after staining with SYBR Green I dye can be used to detect RT-LAMP products; this enables field application of this assay. The findings demonstrate that RT-LAMP can serve as a helpful tool for the detection and surveillance of DHV-1 in the poultry industry.

Development and application of an indirect immunohistochemical method for the detection of duck plague virus vaccine antigens in paraffin sections and localization in the vaccinated duckling tissues

The objective of the present study was to develop and apply a streptavidin-alkaline phosphatase labeling system of indirect immunohistochemistry (SP-IHC) to detect antigenic distribution and localization regularity of duck plague virus (DPV) vaccine antigens in paraformaldehyde-fixed paraffin-embedded tissues of experimentally vaccinated ducklings. Male New Zealand rabbits were immunized with purified DPV antigens, which were engaged by a combination of differential centrifugation and sucrose-density gradient ultracentrifugation. The rabbit anti-DPV polyclonal antibodies were purified and used as the primary antibodies. Forty-eight 28-d-old DPV-free Pekin ducklings were subcutaneously inoculated with attenuated DPV vaccine in the immunization group and sterile PBS in the control group. The tissues were collected at sequential time points between 4 h and 18 wk postvaccination (PV) and were prepared for SP-IHC observation. The presence of DPV-specific antigens was first observed in the liver and spleen at 12 h PV; in the bursa of Fabricius, thymus, Harderian gland, esophagus, and intestinal tract at 1 d PV; and in the heart, lung, kidney, pancreas, and brain at 3 d PV. The positive staining reaction could be detected in the vaccinated duckling tissues until 18 wk PV, and no positive staining cells could be observed in the controls. The highest levels of positive staining reaction were found in the liver, spleen, bursa of Fabricius, thymus, and intestinal tract, whereas a few DPV vaccine antigens were distributed in the heart, pancreas, and esophagus. The target cells had a ubiquitous distribution, especially in the mucosal epithelial cells, lamina propria cells, macrophages, hepatocytes, and lymphocytes, which served as the principal sites for antigen localization. These findings demonstrated that SP-IHC was a reliable method for detecting antigenic distribution and localization regularity of DPV vaccine antigens in routine paraffin sections. The present study may be useful for describing proliferation and distribution regularity of DPV vaccine in the vaccinated duckling tissues and enhance further studies and clinical application of attenuated DPV vaccine.

Detection of anatid herpesvirus 1 gC gene by TaqMan fluorescent quantitative real-time PCR with specific primers and probe

BACKGROUND

Anatid herpesvirus 1 (AHV-1) is known for the difficulty of monitoring and controlling, because it has a long period of asymptomatic carrier state in waterfowls. Furthermore, as a significant essential agent for viral attachment, release, stability and virulence, gC (UL44) gene and its protein product (glycoprotein C) may play a key role in the epidemiological screening. The objectives of this study were to rapidly, sensitively, quantitatively detect gC gene of AHV-1 and provide the underlying basis for further investigating pcDNA3.1-gC DNA vaccine in infected ducks by TaqMan fluorescent quantitative real-time PCR assay (FQ-PCR) with pcDNA3.1-gC plasmid.

RESULTS

The repeatable and reproducible quantitative assay was established by the standard curve with a wide dynamic range (eight logarithmic units of concentration) and very good correlation values (1.000). This protocol was able to detect as little as 1.0 x 101 DNA copies per reaction and it was highly specific to AHV-1. The TaqMan FQ-PCR assay successfully detected the gC gene in tissue samples from pcDNA3.1-gC and AHV-1 attenuated vaccine (AHV-1 Cha) strain inoculated ducks respectively.

CONCLUSIONS

The assay offers an attractive method for the detection of AHV-1, the investigation of distribution pattern of AHV-1 in vivo and molecular epidemiological screening. Meanwhile, this method could expedite related AHV-1 and gC DNA vaccine research.

Development and evaluation of an antigen-capture ELISA for detection of the UL24 antigen of the duck enteritis virus, based on a polyclonal antibody against the UL24 expression protein

An antigen-capture enzyme-linked immunosorbent assay (AC-ELISA) method was developed for the efficient detection of the UL24 antigen of the duck enteritis virus (DEV) using polyclonal antibodies. Ducks and rabbits were immunized, respectively, with expressed UL24 recombinant protein. The IgG antibodies against UL24 from ducks and rabbits were purified and used as the capture antibodies. The specificity of the optimized AC-ELISA was evaluated by use of DEV, duck hepatitis virus (DHV), duck hepatitis B virus (DHBV), gosling plague virus (GPV), Riemerella anatipestifer (R.A.), Escherichia coli (E. coli), Pasteurella multocida (P.M.) and Salmonella Enteritidis (S.E.). Only DEV specimens yielded a specific and strong signal. The limit of the sensitivity of this method for the detection of DEV was 46 ng/100 microl. Compared with PCR and virus isolation, the rate of agreement for the detection of experimentally infected sera was 100%. A comparative test used on clinical specimens between the neutralization test and the AC-ELISA showed that the proportions of true positives and true negatives by the AC-ELISA were 0.90 and 0.67 respectively. These results indicated that the AC-ELISA approach is rapid, sensitive, and reliable for specific detection of DEV antigen.