Establishment and application of a TaqMan-based one-step real-time RT-PCR for the detection of novel goose-origin astrovirus

The Development of a Sensitive Droplet Digital Polymerase Chain Reaction Test for Quantitative Detection of Goose Astrovirus

(1) Goose astrovirus (GAstV) is a novel emerging pathogen that causes significant economic losses in waterfowl farming. A convenient, sensitive, and specific detection method for GAstV in field samples is important in order to effectively control GAstV. Droplet digital polymerase chain reaction (ddPCR) is a novel, sensitive, good-precision, and absolute quantitation PCR technology which does not require calibration curves. (2) In this study, we developed a ddPCR system for the sensitive and accurate quantification of GAstV using the conserved region of the ORF2 gene. (3) The detection limit of ddPCR was 10 copies/µL, ~28 times greater sensitivity than quantitative real-time PCR (qPCR). The specificity of the test was determined by the failure of amplification of other avian viruses. Both ddPCR and qPCR tests showed good repeatability and linearity, and the established ddPCR method had high sensitivity and good specificity to GAstV. Clinical sample test results showed that the positive rate of ddPCR (88.89%) was higher than that of qPCR (58.33%). (4) As a result, our results suggest that the newly developed ddPCR method might offer improved analytical sensitivity and specificity in its GAstV measurements. The ddPCR could be widely applied in clinical tests for GAstV infections.

Molecular epidemiology and virulence of goose astroviruses genotype-2 with different internal gene sequences

Goose astrovirus (GAstV) is a small, non-enveloped, single-stranded, positive-sense RNA virus. GAstV has rapidly spread across various regions in China since 2016. In Sichuan, out of 113 samples were collected from goose diseases between 2019 and 2022, 97 were positive for GAstV through PCR testing. Remarkably, over the past three years, GAstV outbreak in Sichuan has accounted for an astonishing 85.8% of all goose-origin viruses. Among these cases, 63.9% had single GAstV infections, 29.9% had dual infections, and 6.2% had quadruple infections. To comprehend the variations in virulence among distinct strains of GAstV. 12 representative strains of single GAstV infections were isolated. These strains exhibited distinct characteristics, such as prominent white urate depositions in organs and joints, as well as extensive tissues phagocytosis in major target organs' tissues. The conserved ORF1b genes and the variable ORF2 genes of these representative GAstV strains were sequenced, enabling the establishment of phylogenetic trees for GAstV. All GAstV strains were identified as belonging to genotype-2 with varying internal gene sequences. Experiments were conducted on GAstV genotype-2, both in vivo and in vitro, revealed significant variations in pathogenicity and virulence across susceptible cells, embryos, and goslings. This comprehensive study enhances researchers' understanding of the transmission characteristics and virulence of GAstV genotype-2, aiding in a better comprehension of their molecular epidemiology and pathogenic mechanism.

Rapid detection of goose astrovirus genotypes 2 using real-time reverse transcription recombinase polymerase amplification

BACKGROUND

Goose astrovirus (GoAstV) is an important pathogen that causes joint and visceral gout in goslings. It has been circulating in many provinces of China since 2017. Goose astrovirus genotypes 2 (GoAstV-2) is the main epidemic strain, and its high morbidity and mortality have caused huge economic losses to the goose industry. An accurate point-of-care detection for GoAstV-2 is of great significance. In this study, we developed a real-time reverse transcription recombinase polymerase amplification (RT-RPA) method for the on-site detection of GoAstV-2 infection.

RESULTS

The real-time RT-RPA reaction was carried out at a constant temperature of 39 °C, and the entire detection time from nucleic acid preparation to the end of amplification was only 25 min using the portable device. The results of a specificity analysis showed that no cross-reaction was observed with other related pathogens. The detection limit of the assay was 100 RNA copies/μL. The low coefficient of variation value indicated excellent repeatability. We used 270 clinical samples to evaluate the performance of our established method, the positive concordance rates with RT-qPCR were 99.6%, and the linear regression analysis revealed a strong correlation.

CONCLUSIONS

The established real-time RT-RPA assay showed high rapidity, specificity and sensitivity, which can be widely applied in the laboratory, field and especially in the resource-limited settings for GoAstV-2 point-of-care diagnosis.

Localization and distribution of goose astrovirus 2 antigens in different tissues at different times

Goose astrovirus 2 (GAstV-2) causes visceral gout in goslings and has resulted in significant economic losses in the goose industry of China since its outbreak in 2017. To further investigate the distribution and localization of GAstV-2 in different tissues at different times, a monoclonal antibody (mAb)-based immunohistochemical (IHC) assay was developed to detect GAstV-2. A total of 80 1-day-old healthy goslings were inoculated with GAstV-2 via the oral (n = 40) and intramuscular routes (n = 40). GAstV-2 in the tissues of interest was detected using the established IHC assay. The results showed that positive signals were detected in most tissues at 1 day post-infection (dpi). Viral antigens were mainly distributed in the cytoplasm, and the staining intensity was higher in the renal tubular epithelial cells than in other cells. Taken together, our data demonstrated that GAstV-2 has a broad tissue tropism and primarily targets the kidneys. These results are likely to provide a scientific basis for further elucidation of the pathogenesis of GAstV-2.

Pathogenicity of goose astrovirus genotype 2 in chickens

Goose astrovirus genotype 2 (GAstV-2) is the causative agent causing severe visceral gout and joint gout in goslings, with mortality rates of affected flocks up to 50%. To date, continuous GAstV-2 outbreaks still pose a great threat to goose industry in China. Although most researches on GAstV-2 have focused on its pathogenicity to geese and ducks, limited studies have been performed on chickens. Herein, we inoculated 1-day-old specific pathogen-free (SPF) White Leghorn line chickens with 0.6 mL of GAstV-2 culture supernatant (TCID50 10-5.14/0.1 mL) via orally, subcutaneously and intramuscularly, and then assessed the pathogenicity. The results revealed that the infected chickens presented depression, anorexia, diarrhea, and weight loss. The infected chickens also suffered from extensive organ damage and had histopathological changes in the heart, liver, spleen, kidney, and thymuses. The infected chickens also had high viral load in tissues and shed virus after the challenge. Overall, our research demonstrates that GAstV-2 can infect chickens and adversely affect the productivity of animals. And the viruses shed by infected chickens can pose a potential risk to the same or other domestic landfowls.

Rapid diagnosis of duck Tembusu virus and goose astrovirus with TaqMan-based duplex real-time PCR

The mixed infection of duck Tembusu virus (DTMUV) and goose astrovirus (GoAstV) is an important problem that endangers the goose industry. Although quantitative PCR has been widely used in monitoring these two viruses, there is no reliable method to detect them at the same time. In this study, by analyzing the published genomes of DTMUV and goose astrovirus genotype 2 (GoAstV-2) isolated in China, we found that both viruses have high conservation, showing 96.5 to 99.5% identities within different strains of DTMUV and GoAstV, respectively. Subsequently, PCR primers and TaqMan probes were designed to identify DTMUV and GoAstV-2, and different fluorescent reporters were given to two probes for differential diagnosis. Through the optimization and verification, this study finally developed a duplex TaqMan qPCR method that can simultaneously detect the above two viruses. The lower limits of detection were 100 copies/μL and 10 copies/μL for DTMUV and GoAstV-2 under optimal condition. The assay was also highly specific in detecting one or two viruses in various combinations in specimens, and provide tool for clinical diagnosis of mixed infections of viruses in goose.

A peptide-based ELISA for detection of antibodies against novel goose astrovirus type 1

Goose gout disease is a high morbidity and mortality disease caused by novel serotype 1 goose astrovirus (GAstV-1), which has resulted in huge economic loss to the goose industry of China. However, few diagnostic methods have been developed for serological surveillance of GAstV-1. In our previous study, several novel B cell epitopes were identified in the ORF2 protein of GAstV-1. In this study, one novel peptide of 627-646 aa in the ORF2 recognized by monoclonal antibody (mAb) 6C6 was used as an antigen to develop an efficient peptide-based ELISA (pELISA) for detection of antibodies against GAstV-1. Specificity analysis showed that the pELISA only reacted with sera against GAstV-1, but not with sera against other pathogens tested. The sensitivity of the pELISA in detecting positive sera was higher than that of the IFA (Indirect immunofluorescence assay). The coefficients of variation (CV) of the intra-assay and inter-assay were both < 10%, indicating that the reproducibility of pELISA was good. For detection of clinical samples, the pELISA had 87.5% concordance with the IFA. Our data demonstrate that the pELISA generated here provides an accurate, rapid, and economical method for the detection antibodies against GAstV-1 for serological surveillance.

Infection and innate immune mechanism of goose astrovirus

Goose astrovirus (GAstV, genus Avian Astrovirus, family Astrovirus) was first discovered in 2005, but was not considered as a pathogen of gosling gout until 2016. Since then, goose astrovirus has erupted in Chinese goslings, causing at most 50% of gosling deaths. By December 2022, the disease had become epidemic and prevailed in goose farms in Jiangsu, Shandong, Anhui, Henan, Guangdong, Liaoning, Sichuan and other places in China. The disease mainly affects goslings within 3 weeks old. The typical symptoms of goose astrovirus are large deposits of urate in the viscera, joint cavity and ureter surface of infected goslings. Goose astrovirus infection can trigger high levels of iNOS, limiting goose astrovirus replication. The ORF2 domain P2 of the goose astrovirus activates the OASL protein, limiting its replication. Goose astrovirus can also activate pattern recognition receptors (RIG-I, MDA-5, TLR-3), causing an increase in MHC-Ia, MHC-Ib and CD81 mRNA, activating humoral and cellular immunity, thereby hindering virus invasion. Goose astrovirus also regulates the activation of IFNs and other antiviral proteins (Mx1, IFITM3, and PKR) in the spleens and kidneys to inhibit viral replication. The innate immune response process in goslings also activates TGF-β, which may be closely related to the immune escape of goose astrovirus. Gaining insight into the infection and innate immune mechanism of goose astrovirus can help researchers study and prevent the severe disease in goslings better.

Development of an indirect competitive ELISA method based on ORF2 detecting the antibodies of novel goose astrovirus

Goose astrovirus (GAstV) characterized by articular and visceral gout, is an emerging pathogen with a wide distribution on mainland China, leading to serious economic losses in the goose-raising industry. Because vaccines to prevent GAstV infections are not available currently, early diagnosis is critical when treating symptomatic geese and in preventing GAstV transmission. In this context, a highly sensitive indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) based on a monoclonal antibody (mAb) to detect GAstV-specific antibodies from geese was developed, and the detections were optimized. A series of experiments proved that the ic-ELISA shows excellent diagnostic performance and discriminatory power with high sensitivity and specificity. The ic-ELISA for GAstV detection was applied on 67 field serum samples, and comparing the detection results with the virus neutralization test verified the accuracy of the ic-ELISA. The correlation coefficient between the ic-ELISA and the virus neutralization test was 80%, demonstrating the proposed ic-ELISA method could be a useful and effective tool for the diagnostic, serological epidemiological investigation and immune monitoring of the GAstV in goose-producing regions.

An IgY Effectively Prevents Goslings from Virulent GAstV Infection

Goose astrovirus (GAstV) leads to viscera and joints urate deposition in 1- to 20-day-old goslings, with a mortality rate of up to 50%, posing a severe threat to entire colonies; however, there is no efficient prevention and control method for GAstV infection. This study describes a prophylactic anti-GAstV strategy based on the specific immunoglobulin Y (IgY) from egg yolk. The specific IgY was produced by 22-week-old laying hens intramuscularly immunized with the inactivated GAstV three consecutive times, with 2-week intervals. The egg yolk was collected weekly after the immunization and the anti-GAstV IgY titer was monitored using an agar gel immune diffusion assay (AGID). The results revealed that the AGID titer began to increase on day 7, reached a peak on day 49, and remained at a high level until day 77 after the first immunization. The specific IgY was prepared from the combinations of egg yolk from day 49 to day 77 through PEG-6000 precipitation. Animal experiments were conducted to evaluate the effects of prevention and treatment. The result of the minimum prophylactic dose of the IgY showed that the protection rate was 90.9% when 2.5 mg was administrated. Results of the prevention and the treatment experiments showed prevention and cure rates of over 80% when yolk antibody was administered in the early stages of the GAstV infection. These results suggested that the specific IgY obtained from immunized hens with the inactivated GAstV could be a novel strategy for preventing and treating GAstV infection.

A Review of Emerging Goose Astrovirus Causing Gout

In recent years, an infection in geese caused by goose astrovirus (GAstV) has repeatedly occurred in coastal areas of China and rapidly spread to inland provinces. The infection is characterized by joint and visceral gout and is fatal. The disease has caused huge economic losses to China's goose industry. GAstV is a nonenveloped, single-stranded, positive-sense RNA virus. As it is a novel virus, there is no specific classification. Here, we review the current understanding of GAstV. The virus structure, isolation, diagnosis and detection, innate immune regulation, and transmission route are discussed. In addition, since GAstV can cause gout in goslings, the possible role of GAstV in gout formation and uric acid metabolism is discussed. We hope that this review will inform researchers to rapidly develop effective methods to prevent and treat this disease.

Core antigenic advantage domain-based ELISA to detect antibody against novel goose astrovirus in breeding geese

Goose astrovirus (GAstV), the major causative agent of visceral and joint gout in goslings, is a novel pathogen greatly threatening waterfowl industry. Importantly, the horizontal and vertical transmissibility of GAstV posed a great challenge for disease prevention and control. Given the absence of commercial vaccine, restricting vertical transmission and protecting susceptible goslings must be a priority. Although many detection methods have been established, there is no serological method to detect GAstV-specific antibody, greatly limiting inspection and elimination of infected breeding bird. In this study, the B-cell epitopes of GAstV capsid protein were predicted, and its core antigenic advantage domain (shCAP) was expressed and purified. After authenticating the antigenicity, the recombinant shCAP protein was taken as the coating antigen, and an easily accessible indirect enzyme-linked immunosorbent assay (ELISA) was established to detect GAstV-specific antibody. The working conditions, including antigen concentration, serum dilution and incubation time, blocking buffer concentration, and color developing time, were gradually optimized by checkerboard titration. The cut-off OD₄₅₀ value of the indirect ELISA for positive sample was 0.379, and the analytical sensitivity was 1:800. There was no cross-reaction with sera against goose parvovirus (GPV), Tembusu virus (TUMV), H5 and H7 subtype avian influenza virus (AIV H5 + H7), and Newcastle disease virus (NDV). The assay was further applied to examine 73 breeding goose serum samples and shared excellent agreement of 93.5% (68/73) with western blot, which also suggested that GAstV is circulating in the goose population in China. In conclusion, the developed indirect ELISA is simple, specific, and sensitive, which will be greatly useful to screen GAstV infection and block vertical transmission. KEY POINTS: • B-cell epitopes of GAstV capsid protein were predicted and expressed as immunogen • A core antigenic advantage domain-based ELISA was established to detect GAstV-specific antibody • The established ELISA will contribute to inspection and elimination of infected breeding geese and provide a useful tool for large scale serological testing of GAstV in geese.

Establishment of TaqMan-based real-time PCR assay for rapid detection of duck circovirus

Duck circovirus (DuCV) is widespread across the world and causes feather disorders in young ducks. It was identified as the causative pathogen of duck beak atrophy and dwarfism syndrome and primary sclerosing cholangitis. In this study, we aimed to establish a TaqMan-based real-time PCR assay to detect DuCV. The primers and probe were designed based on the conserved region of the DuCV Rep gene. After optimizing the reaction conditions, the minimum virus detection limit of the designed PCR technique was 39.4 copies/μL, 100 times that of conventional PCR (cPCR). No cross-reaction with six other common duck viruses was observed. The intra- and inter-assay variations were less than 1%. The detection rate of DuCV-positive clinical samples using TaqMan-based real-time PCR was higher than that using SYBR Green-based real-time PCR and cPCR. Collectively, these results showed that the established TaqMan-based real-time PCR detected DuCV with high sensitivity and specificity, and significant repeatability, making it suitable for clinical use. Hence, it may be used as a novel tool for the diagnosis and epidemiological investigation of DuCV.

Goose Nephritic Astrovirus Infection of Goslings Induces Lymphocyte Apoptosis, Reticular Fiber Destruction, and CD8 T-Cell Depletion in Spleen Tissue

The emergence of a novel goose nephritic astrovirus (GNAstV) has caused economic losses to the Chinese goose industry. High viral load is found in the spleen of goslings infected with GNAstV, but pathological injuries to the spleen due to GNAstV are largely unknown. In this study, 50 two-day-old goslings were infected orally with GNAstV, and 50 goslings were treated with PBS as control. Spleens were collected at different times following infection to assess damage. GNAstV infection caused visceral gout and urate deposition in joints, and resulted in 16% mortality. GNAstV was found in the lymphocytes and macrophages within the spleen. Lymphocyte loss, especially around the white pulp, and destruction and decline in the number of reticular fibers was observed in GNAstV-infected goslings. Moreover, in GNAstV-infected goslings, ultrahistopathological examination found that splenic lymphocytes exhibited condensed chromatin and apoptotic bodies, and reticular cells displayed damage to plasma membrane integrity and swollen mitochondria. Furthermore, TUNEL staining confirmed apoptosis of lymphocytes, and the mRNA levels of Fas and FasL were significantly increased in the GNAstV-infected goslings. In addition, GNAstV infection reduced the number and protein expression of CD8. In conclusion, GNAstV infection causes lymphocyte depletion, reticular cell necrosis, reticular fiber destruction, lymphocyte apoptosis, and reduction in CD8 levels, which contribute to spleen injury.

Isolation and characterization of a duck-origin goose astrovirus in China

In 2019, a new type of infectious disease characterized with haemorrhage and swellings of kidneys, occurred on commercial duck farms in Shandong province, China. Our systematic investigation led to the isolation of an astrovirus, designated AstV-SDTA strain and was isolated from a diseased duckling using LMH cells. Similar clinical symptoms were reproduced by experimental infection using the AstV-SDTA strain. The complete genome sequencing characterization of AstV-SDTA was conducted using next-generation sequencing (NGS) technique on Illumina HiSeq platform, and used polymerase chain reaction method to verify the NGS results for the obtained whole sequences. Phylogenetic analysis revealed that AstV-SDTA strain belongs to a novel goose astrovirus (GoAstV) branch of avian astroviruses, and the nucleotide homology based on the complete genome sequences among AstV-SDTA and other GoAstV strains deposited in Genbank was 97.2–98.8%. Taken together, these results suggest that the cross-species transmission of novel GoAstV between domestic waterfowl is possible. Further surveillance of novel GoAstV in poultry are needed in order to gain a better understanding of both the molecular and evolutionary characteristics of novel GoAstV.

Immune-related gene expression in the kidneys and spleens of goslings infected with goose nephritic astrovirus

Goose nephritic astrovirus (GNAstV) was first isolated in 2018, causing great economic losses to the goose industry. However, little is known about host immune response to GNAstV infection. In this study, forty 2-day-old goslings were randomly divided into 2 groups: infection and negative control groups. Each gosling in the infection group was challenged with 0.5 mL GNAstV-JSHA intramuscularly, whereas the gosling in the negative control group was inoculated with the same amount of PBS. Histopathological changes and virus location in the spleen and kidney were examined, and the expression of immune-related genes was determined by qPCR at 7 and 14 d after infection. Our results showed that GNAstV infection induced degeneration and necrosis of splenic lymphocytes and renal epithelial cells, and these cells were positive for the virus. In addition, GNAstV infection induced the activation of pattern recognition receptors (RIG-I, MDA-5, and TLR3) and key adaptor molecules (MyD88, MAVS, and IRF7) in the spleen and kidney, and upregulated the gene expression of interferon-α in the spleen and antiviral proteins (MX1, OASL, and IFITM3) in the spleen and kidney. Moreover, high expression levels of interleukin (IL)-1β and IL-8 in the spleen and iNOS in the spleen and kidney were found. These results indicated that GNAstV infection activated host innate immune response. Furthermore, GNAstV infection increased the expression levels of CD8⁺, MHCI, and MHCII, indicating that adaptive immune response was activated. Besides, TGF-β was highly expressed in the spleen and kidney, which may be an immune evasion strategy of GNAstV to cause infection. Interestingly, both IL-1β and IL-6 mRNA levels were decreased in the kidney, which may help reduce kidney lesions. This is the first study to report changes in immune-related gene expression in response to GNAstV infection, and our results provide insights into viral pathogenesis.

Simple and Visible Detection of Novel Astroviruses Causing Fatal Gout in Goslings Using One-Step Reverse Transcription Polymerase Spiral Reaction Method

In this study, a one-step isothermal method combining polymerase spiral reaction (PSR) with reverse transcription (RT-PSR) was established for rapid and specific detection of novel astroviruses causing fatal gout in goslings (N-GoAstV). The one-step RT-PSR was accomplished at the optimal temperature of 62°C and time of 40 min and used primers simply designed as conventional PCR primers, and the results of detection were visible to the naked eye. The detection limit of PSR was above 34.7 copies/μL at a 95% probability level according to probit regression analysis. The assay specifically detected N-GoAstV, and no other reference viruses were detected. These results suggest that the newly established RT-PSR assay could, in one step, accomplish reverse-transcription, amplification, and result determination providing a visible, convenient, rapid, and cost-effective test that can be carried out onsite, in order to ensure timely quarantine of N-GoAstV-infected birds, leading to effective disease control.

Rapid detection of H146-like goose calicivirus using a TaqMan-based real-time PCR assay

H146-like goose-origin calicivirus (H146-like GCV) is a novel Caliciviridae family member in the Sanovirus genus that was recently discovered and proposed to cause runting-stunting syndrome and urate deposition in geese. At present, however, there is a lack of epidemiological information pertaining to the dynamics and distribution of H146-like GCV. The development of novel molecular diagnostic approaches capable of rapidly and accurately detecting this virus would support the strengthening, the prevention, and control of H146-like GCV infection. In the present study, we therefore used a TaqMan probe and primers specific for the viral nonstructural (NS) gene to develop a highly sensitive and specific PCR assay capable of detecting this H146-like GCV. The assay reproducibly detected 5.07 × 10² copies of a recombinant DNA plasmid containing the NS gene, with a dynamic range of 8 orders of magnitude (10²-10⁹ copies). Importantly, no cross-reactivity was observed with common viruses that affected waterfowl, and when we used this assay to evaluate clinical samples, we found it to be more sensitive and faster than traditional PCR. In summary, herein, we developed a novel TaqMan-based real-time PCR approach that could reliably detect and diagnose H146-like GCV. This tool will allow for the real-time diagnosis of H146-like GCV infections, enabling researchers to better understand the epidemiology and clinical presentation of this disease.

Short RNA Universal Coding for Topological Transformation Nano-barcoding Application

With the advent of innovative genomic discovery toolkits such as RT-PCR, genetic information can be quickly decrypted, and this has resulted in significant progress in overcoming diseases. However, RT-PCR has the serious problem of frequent errors, and the demand for a new gene diagnostic system is emerging. Herein, we propose a universal coding system for the effective detection of short single-stranded DNA or RNA by using a topological transformation-based nano-barcoding technique (TNT). Our goal was to develop a dedicated diagnostic device that unifies the other gene groups, thus resulting in minimum testing. In a universal coding system consisting of two separate circulation structures, different gene groups become generalized into specific single genes with the same sequence by a strand-displacement reaction and are then amplified, eventually being quickly detected in one TNT system. Simple gene diagnostic systems like this make high-speed, point-of-care diagnostic technologies, and we are very confident that these will provide clinical gene detection in the near future.

Rapid detection of H146-like goose calicivirus using real-time RT-PCR with a Taqman minor groove binder probe

H146-like goose-origin calicivirus (H146-like GCV) is a novel Caliciviridae family member in the Sanovirus genus that was associated with gosling growth retardation syndrome growth retardation syndrome complicated by visceral urate deposition. However, there is no accurate and high throughput real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) available for the rapid and highly sensitive identification of H146-like GCV. In this study, a pair of specific primers and a TaqMan minor groove binder (MGB) probe were designed based on a conserved region in the nonstructural (NS) gene sequence. The TaqMan-MGB probe-based one-step qRT-PCR assay was capable of detecting quite low number of targeting nucleic acid as low as 5.07 copies/μL and had excellent intra-assay and inter-assay repeatability with the coefficient of variation (CV) value from 0.558% to 1.293%. The assay was highly specific for H146-like GCV, without cross-reactions with other non-targeted goose-origin viruses, and 62 suspicious tissue samples infected with H146-like GCV from different regions of Fujian Province were used in this study to verify the feasibility and effectiveness of this assay in clinical diagnosis. The results indicated that our assay for the diagnosis and quantification of H146-like GCV was highly sensitive and specific, and should provide a reliable real-time tool for epidemiological and pathogenetic study of H146-like GCV infection, enabling researchers to better understand the epidemiology and clinical presentation of this disease.

Novel goose-origin astrovirus infection in geese: the effect of age at infection

Since 2017, a serious infectious disease characterized by visceral gout has emerged in China's main goose-producing regions. The disease has caused huge economic losses to China's goose industry. In our previous study, we determined that the pathogen causing gout in goslings is a novel goose-origin astrovirus, designated as AStV/SDPY/Goose/1116/17 (AStV-SDPY) strain. To investigate the effect of host age on the outcome of novel goose-origin astrovirus infection, 200 1-day-old healthy goslings were selected to be experimentally infected at 1, 5, 15, 25, and 35 D of age. It was shown in experimental infection that the AStV-SDPY strain was highly pathogenic in goslings aged 1 to 15 D, causing growth repression, severe visceral urate deposition, and even death, whereas goslings infected at 25 and 35 D of age showed mild symptoms. Histopathologic examination indicated that lesions occurred mainly in the kidney and liver of infected goslings, which is correlated to the severity of clinical signs and gross lesions. Viral RNA was detected in all representative tissues, and virus shedding was detected continuously within 15 D after inoculation. Higher viral copy number, especially in vital organs such as the liver and kidney, was developed in the goslings infected at 1 to 15 D of age than older geese. In addition, clinical chemistry and inflammatory cytokines showed that younger geese are more sensitive to AStV infection. Overall, our study demonstrates that the pathogenicity of AStV-SDPY in goslings is partly associated with the age of infection, laying a foundation for further study of the pathogenic mechanism of this virus.

Rapid and visual detection of novel astroviruses causing fatal gout in goslings using one-step reverse transcription loop-mediated isothermal amplification

To visually and rapidly detect a novel goose astrovirus (N-GoAstV) causing fatal gout in goslings, an isothermal detection method based on one-step reverse transcription loop-mediated isothermal amplification (one-step RT-LAMP) was established. The one-step RT-LAMP assay for N-GoAstV detection, using Bst 3.0 DNA polymerase with strong reverse transcription activity and primer sets targeting the opening reading frame 1b (ORF1b) of N-GoAstV, could be completed in 30 min using a water bath at 61°C; the detection results could be visually observed by adding a pH-sensitive dye containing phenol red and cresol red. The detection limit of the one-step RT-LAMP assay was 57.8 copies, which was similar to that of reverse transcription–quantitative polymerase chain reaction. The assay specifically detected N-GoAstV without any cross-reaction with other reference viruses, and this was further confirmed using enzyme digestion. These results indicated that the newly established RT-LAMP assay could accomplish reverse transcription, amplification, and visual result determination in one step, and the results obtained via this rapid and cost-effective method could be used to support disease control on farms in terms of N-GoAstV infection.