The first whole genome sequence and pathogenicity characterization of a fowl adenovirus 4 isolated from ducks associated with inclusion body hepatitis and hydropericardium syndrome

Biological features of fowl adenovirus serotype-4

Fowl adenovirus serotype 4 (FAdV-4) is highly pathogenic to broilers aged 3 to 5 weeks and has caused considerable economic loss in the poultry industry worldwide. FAdV-4 is the causative agent of hydropericardium-hepatitis syndrome (HHS) or hydropericardium syndrome (HPS). The virus targets mainly the liver, and HPS symptoms are observed in infected chickens. This disease was first reported in Pakistan but has now spread worldwide, and over time, various deletions in the FAdV genome and mutations in its major structural proteins have been detected. This review provides detailed information about FAdV-4 genome organization, physiological features, epidemiology, coinfection with other viruses, and host immune suppression. Moreover, we investigated the role and functions of important structural proteins in FAdV-4 pathogenesis. Finally, the potential regulatory effects of FAdV-4 infection on ncRNAs are also discussed.

Identification, pathogenicity and molecular characterization of a novel fowl adenovirus 8b strain

Since 2012, there has been a noticeable upward trend in the global incidence of inclusion body hepatitis (IBH) cases, leading to substantial economic losses in the poultry industry. In response to this trend, the current study aimed to investigate the phylogenetic information, genetic mutations, and pathogenicity of the highly pathogenic fowl adenovirus (FAdV) strain HN1472, which was isolated from liver samples obtained from a laying flock affected by IBH. This investigation was carried out using 1-day-old specific pathogen-free (SPF) chickens. Recombination and phylogenetic analyses confirmed that HN1472 is a recombinant strain derived from FAdV-8a and FAdV-8b, and exhibited significant genetic divergence in the hexon, fiber, and ORF19 genes. Notably, the phylogenetic analysis identified recombination events in these regions. Furthermore, animal experiments revealed that HN1472 is a highly pathogenic isolate, causing 80% mortality and manifesting clinical signs of IBH in SPF chickens. Furthermore, the recombinant FAdV serotype 8b (FAdV-8b) was found to be widely distributed in various tissues, with a higher concentration in the livers and gizzard tissue at 3 d postchallenge (dpc). Collectively, these findings contribute to our current understanding of the factors influencing the pathogenicity and genetic diversity of FAdV serotype 8b (FAdV-8b) in China.

Isolation, characterization, evaluation of pathogenicity, and immunomodulation through interferon production of duck adenovirus type-3 (DAdV-3)

Duck adenovirus type-3 (DAdV-3) is a poorly characterized duck virus. A comprehensive analysis of the DAdV-3 pathogenicity and host immune response could be a valuable addition. Herein, DAdV-3 was isolated from Muscovy duck and virus-specific genes were confirmed by polymerase chain reaction (PCR). The obtained gene fragments were sequenced and compared with the reference sequence. Results confirmed that the clinically isolated virus was DAdV-3, named as HF-AN-2020. To evaluate DAdV-3 host immune response, the expression levels of MDA5, STING, IRF7, MAVS, and NF-κB, and inflammatory cytokines (IFN-β, IFN-γ, and IL-1β) were determined by quantitative reverse transcriptase PCR (qRT-PCR). The expression levels of IFN-β and IFN-γ were 32.6- and 28.6-fold, respectively, higher (P < 0.01) than the control group. It was found that the upregulation of STING and NF-κB pathways was directly involved in the regulation of inflammatory cytokines (IFN-β, IFN-γ, and IL-1β). Furthermore, the gene regulation pathways consecutively upregulated the expression levels of MDA5, STING, IRF7, MAVS, and NF-κB up to 31.6, 10.5, 31.4, 2.2, and 2.6-fold, respectively, higher (P < 0.01) than the control group. The TCID50 of DAdV-3 for Muscovy duck and chicken was 10-3.24/0.1 mL with 0% mortality, indicating low pathogenicity in both Muscovy ducks and chickens, but DAdV-3 can induce higher expression of interferons. Genome analysis showed mutations in 4 amino acids located in ORF19B (Ser to Thr), ORF66 (Leu to Phe, Ile to Leu), and ORF67 (Gly to stop codon). This study provides essential and basic information for further research on the mechanism of the cellular immune responses against adenoviruses.

Genomic and Pathologic Characterization of the First FAdV-C Serotype 4 Isolate from Black-Necked Crane

Fowl adenoviruses (FAdVs) are distributed worldwide in poultry and incriminated as the etiological agents for several health problems in fowls, and are capable of crossing species barriers between domestic and wild fowls. An FAdV strain was, for the first time, isolated from black-necked crane in this study, and was designated as serotype 4 Fowl aviadenovirus C (abbreviated as BNC2021) according to the phylogenetic analysis of its DNA polymerase and hexon gene. The viral genomic sequence analysis demonstrated that the isolate possessed the ORF deletions that are present in FAdV4 strains circulating in poultry fowls in China and the amino acid mutations associated with viral pathogenicity in the hexon and fiber 2 proteins. A viral challenge experiment with mallard ducks demonstrated systemic viral infection and horizontal transmission. BNC2021 induced the typical clinical signs of hepatitis-hydropericardium syndrome (HHS) with swelling and inflammation in multiple organs and showed significant viral replication in all eight organs tested in the virus-inoculated ducks and their contactees at 6 dpi. The findings highlight the importance of surveillance of FAdVs in wild birds.

Epidemiological investigation of infectious diseases in geese on mainland China during 2018-2021

Geese play an important role in agricultural economics, with China producing the vast majority of goose meat consumed worldwide annually. The variations in the avian viruses and co-infections result in substantial economic losses to the goose industry in China. To understand the evolutionary characteristics and co-infections of viruses, a broad epidemiological investigation of epizootic viruses of goose was carried out in nine provinces of China during 2018-2021. Here, the results indicated that, among the 1970 clinical samples, 50.81% (1001/1970) were positive for goose astrovirus (GAstV), 18.22% (359/1970) for avian orthoreovirus, 12.74% (251/1970) for goose parvovirus, 11.02% (217/1970) for H9N2 subtype avian influenza virus, 4.01% (79/1970) for Newcastle disease virus, and 2.08% (41/1970) for fowl adenovirus. Among the six viruses, co-infections comprised a large proportion (66.37%) in the field, of which the dual infection was more common. Additionally, phylogenetic analysis of GAstVs indicated that Chinese GAstVs had formed two distinct groups, that is, GAstV-1 and GAstV-2. GAstV-2 sub-genotype II-c had arisen as the dominant genotype in the whole country. Notably, all the H9N2-AIV isolated strains harboured the mammalian adaptation markers I155T, H183N, and Q226L (H3 numbering) in the HA gene, which promotes preferential binding to human-like α2-6-linked sialic acid receptors. And beyond that, we determined that the goose-origin Muscovy Duck Reovirus isolates, showing 51.7%-96% similarities to that of other waterfowl-origin orthoreovirus isolates in sequence homology analysis of the representative part of σC, are a new variant of waterfowl-origin orthoreovirus. These data provide valuable information about the prevalence of infectious diseases in geese on mainland China.

Complete Genome Analysis and Animal Model Development of Fowl Adenovirus 8b

Inclusion body hepatitis (IBH), hydropericardium syndrome, and gizzard erosion associated with fowl adenovirus (FAdV) infection have caused notable economic losses worldwide. In 2020, severe IBH was observed in a layer chicken farm in Hebei Province, China. Liver samples were collected from layer chickens with severe IBH and virus isolation was performed in LMH cells. DNA sequence and bioinformatics analyses were conducted to determine the phylogenetic relationship and the pathogenicity assay was conducted in specific-pathogen-free (SPF) chickens. HeB20 strain was isolated and identified as FAdV-8b, and the complete genome was successfully sequenced (GenBank No. OK188966). Although widespread recombination in clinical strains has been reported within FAdVs, HeB20 showed some novel characteristics, and did not show any recombination, highlighting that recombinant and non-recombinant FAdV-8b coexist in the clinic poultry industry. Finally, pathogenicity animal model of HeB20 was developed and showed severe IBH and 10% mortality. Collectively, a new FAdV-8b strain (HeB20) was isolated and responsible for the severe IBH in layer chickens. Complete genome of HeB20 was sequenced and valuable for future epidemiological investigations. HeB20 was capable of inducing severe IBH and 10% mortality in SPF chickens; this animal model provides a powerful tool for the future vaccine development.

Advances in Vaccine Development of the Emerging Novel Genotype Fowl Adenovirus 4

Fowl adenovirus (FAdV) was first reported in Angara Goth, Pakistan, in 1987. For this reason, it is also known as “Angara disease.” It was later reported in China, Japan, South Korea, India, the United States, Canada, and other countries and regions, causing huge economic losses in the poultry industry worldwide. Notably, since June 2015, a natural outbreak of severe hydropericardium hepatitis syndrome (HHS), associated with a hypervirulent novel genotype FAdV-4 infection, has emerged in most provinces of China. The novel virus FAdV-4 spread rapidly and induced a 30-100% mortality rate, causing huge economic losses and threatening the green and healthy poultry breeding industry. Vaccines against FAdV-4, especially the emerging novel genotype, play a critical role and will be the most efficient tool for preventing and controlling HHS. Various types of FAdV-4 vaccines have been developed and evaluated, such as inactivated, live-attenuated, subunit, and combined vaccines. They have made great contributions to the control of HHS, but the details of cross-protection within FAdVs and the immunogenicity of different vaccines require further investigation. This review highlights the recent advances in developing the FAdV-4 vaccine and promising new vaccines for future research.

Molecular Characterization of a Novel Budgerigar Fledgling Disease Virus Strain From Budgerigars in China

Budgerigar fledgling disease virus (BFDV) is the causative polyomavirus of budgerigar fledgling disease, an important avian immunosuppressive disease in budgerigars (Melopsittacus undulatus). In the current study, we explored the etiological role and molecular characteristics of BFDV. We identified a novel BFDV strain, designated as SC-YB19, belonging to a unique cluster with three other domestic strains (WF-GM01, SD18, and APV-P) and closely related to Polish isolates based on complete sequences. Sequence analysis showed that SC-YB19 had an 18-nucleotide (nt) deletion in the enhancer region, corresponding to the sequence position 164–181 nt, which differed significantly from all other BFDV strains. Based on sequence alignment, three unique nucleotide substitutions were found in VP4 (position 821), VP1 (position 2,383), and T-antigen (position 3,517) of SC-YB19, compared with SD18, WF-GM01, QDJM01, HBYM02, APV7, and BFDV1. Phylogenetic analyses based on complete sequences suggested that SC-YB19, along with the domestic WF-GM01, SD18, and APV-P strains, formed a single branch and were closely related to Polish, Japanese, and American isolates. These results demonstrate that BFDV genotype variations are co-circulating in China, thus providing important insight into BFDV evolution.

Epidemiological investigation of duck adenovirus 3 in southern China, during 2018-2020

Duck adenovirus 3 (DAdV-3) has been identified as the causative agent of a disease characterized by swelling and hemorrhages in liver and kidney of Muscovy ducks, causing huge economic losses to the waterfowl industry in China. In this study, a total of 54 DAdV-3 outbreaks from 2018 to 2020 in China were monitored with samples being collected and analyzed. The hexon amino acid sequences of the 54 DAdV-3 outbreaks and the DAdV-3 reference strains from GenBank were 98.7% to 100% identical. Epidemiological analysis showed that DAdV-3 circulated in meat-type and egg-laying Muscovy ducks, and co-infections with other viral and bacterial pathogens, such as Muscovy duck parvovirus (MDPV), Muscovy duck-origin goose parvovirus (MDGPV), Adeno-associated virus (AAV), Duck hepatitis B virus (DHBV), R. anatipestifer, and E. coli were identified. In addition, 12 out of the 23 (52.2%) DAdV-3 strains were isolated by LMH cells and identified by DAdV-3 real-time PCR. The whole-genome sequence analysis demonstrated that 12 new DAdV-3 isolates share 99.8-100% identity with the DAdV-3 reference strains, and 9 new DAdV-3 isolates exhibit a truncated ORF67 gene. Our results enhance the understanding of the epidemiology and molecular characterization associated with DAdV-3 infection in China.Highlights First report of the epidemiology of duck adenovirus 3 infection in China.Mutant DAdV-3 strains (truncated ORF67) became the predominant virus.

Hypervirulent FAdV-4 infection induces activation of the NLRP3 inflammasome in chicken macrophages

Fowl adenovirus serotype 4 (FAdV-4) is the primary causative agent of hepatitis-hydropericardium syndrome (HHS) causing great economic losses to the world poultry industry. The exact factors responsible for the pathogenesis of hypervirulent FAdV-4 have not been completely elucidated. Hypervirulent FAdV-4 infection induces inflammatory damages in accompany with a high level of proinflammatory interleukin-1 beta (IL-1β) secretion in a variety of organs. Investigation of the mechanisms underlying hypervirulent FAdV-4-induced IL-1β secretion would contribute to understanding of the pathogenesis of FAdV-4. Here, we investigated whether FAdV-4 infection activates NLRP3 inflammasome in chicken macrophage cell line HD11. The results showed that stimulation of HD11 with hypervirulent FAdV-4 induced NLRP3- and Caspase-1-dependent secretion of IL-1β. Genetic knockdown of NLRP3 or Caspase-1 expression, a critical component of inflammasome, significantly downregulated IL-1β expression, indicating that activation of the NLRP3 inflammasome contributed to the FAdV-4-induced IL-1β secretion. Moreover, ATP signaling and potassium efflux were involved in the process of NLRP3 inflammasome activation. Our data indicated that hypervirulent FAdV-4 infection induces the activation of NLRP3 inflammasome and followed by massive secretion of IL-1β of macrophages, which thereby contribute to the inflamed lesion of tissues.

Pathogenicity and virus shedding ability of fowl adenovirus serotype 4 to ducks

Fowl adenovirus serotype 4 (FAdV-4) is the pathogen causing hepatitis-hydropericardium syndrome (HHS) in broilers. Since June 2015, it has emerged as one of the leading causes of economic losses in the poultry industry in China. Although most studies on FAdV-4 have focused on its pathogenicity to broilers, limited studies have been performed on other natural hosts such as ducks and geese. In this study, we assessed the pathogenicity of FAdV-4 to ducks of different ages through intramuscular injection and found that infected ducks showed severe growth depression. The infected ducks also suffered from extensive organ damage and had histopathological changes in the liver, spleen, and kidney. Although the virus infection caused lymphocyte necrosis of immune organs and the development of the bursa of Fabricius (bursa) was inhibited, the humoral immune response of infected ducks to FAdV-4 remained strong. The infected ducks also had high viral load in tissues and shed virus after the challenge. Overall, our research demonstrates that FAdV-4 can infect ducks and adversely affect the productivity of animals. And the viruses shed by infected ducks can pose a potential risk to the same or other poultry flocks.

Isolation and characterization of a novel strain of duck aviadenovirus B from Muscovy ducklings with acute hepatitis in China

A new disease designated as Pale liver disease (PLD) has been circulating in Chinese Muscovy duck flocks since 2014, which is characterized by fatigue, diarrhoea, sudden death and acute hepatitis with pale and haemorrhagic liver. In this study, the etiological agents of PLD were isolated, causing a significant cytopathic effect (CPE) by cell rounding. Virus particles were observed by transmission electron microscopic (TEM) observation. The same disease was reproduced by experimental infection with the isolate BG61. The whole genomes of isolates were 43,842 nt in length with a GC content of 47.11%, similar to French Muscovy duck adenovirus strain GR with a GC content of 46.08%. The isolates shared 99.71-99.95% and 93.31-93.33% identity with Chinese Muscovy duck adenovirus isolates and GR strain, respectively. The DNA polymerase gene of all Muscovy duck adenovirus strains formed a separate genetic lineage with 99.55-100% amino acid sequence identity. All Chinese Muscovy duck adenovirus isolates contained two fibre genes. In contrast, only one fibre gene was found in GR, the only representative strain in species Duck aviadenovirus B. Anti-DAdV-2 serum antibodies had a weak neutralizing activity against Chinese Muscovy duck adenovirus isolates. The phylogenetic trees of the complete genome, hexon and fibre proteins revealed that all Muscovy duck adenovirus strains formed a major genetic lineage consisting of two clades. Thus, both GR and Chinese Muscovy duck adenovirus strains were proposed to be included in the same species of Duck aviadenovirus B belonging to the genus Aviadenovirus. The species Duck aviadenovirus B included two serotypes or genotypes, such as GR, which represents the strain of serotype 1 or genotype 1 (DAdV B1) and Chinese Muscovy duck adenovirus strains, which belong to serotype 2 or genotype 2 (DAdV B2).

An inactivated vaccine based on artificial non-pathogenic fowl adenovirus 4 protects chickens against hepatitis-hydropericardium syndrome

Hepatitis-hydropericardium syndrome (HHS) in birds is mainly caused by virulent fowl adenovirus 4 (FAdV-4). A novel genotype, hypervirulent FAdV-4, emerged in 2015 with a high mortality rate ranging from 30 % to 100 % in chickens. Vaccination is an economically feasible method to control HHS. Although there have been various reports of inactivated vaccines from virulent wild-type FAdV-4 against HHS, biosafety threats of inactivated vaccines from potential pathogenic components have been presented to the poultry industry, and safer vaccines are urgently needed. A non-pathogenic recombinant FAdV-4 strain, designated as rHN20, was generated based on the hypervirulent strain in our previous study. Here, we developed a novel inactivated oil-adjuvanted vaccine derived from rHN20 strain and evaluated its immunogenicity in specific-pathogen-free chickens. Chickens subcutaneously or intramuscularly immunized with the inactivated vaccine produced high titers of neutralizing antibodies and were protected from a lethal dose of virulent wild-type FAdV-4 challenge. Collectively, an inactivated vaccine was developed, which was capable of providing full protection for chickens against HHS, and significantly reduced the potential biosafety threats.

Isolation, identification and genetic characterization analysis of a fowl aviadenovirus serotype 4 strain from Tianjin, China

A fowl aviadenovirus serotype 4 (FAdV-4), Y17215-1, was isolated from the liver of chickens with Hydropericardium-hepatitissyndrome(HHS) in a chicken farm of Tianjin, China. Obvious cytopathic effects were observed in the infected chicken liver hepatocellular carcinoma cell line (LMH cells) at 24 h post infection (hpi), which consisted of enlarger and rounder shape of cells. The typical and specific green fluorescence was observed by indirect immunofluorescence assay (IFA). Tissue Culture Infectious Dose50 (TCID₅₀) of it measured after five stable passage in LMH cells reached 10^6.5TCID₅₀/0.1 mL. The strain was inoculated through allantoic membrane of 10-day specific pathogen free(SPF) Chick embryos, the thicker allantoic membranes were observed at 120 hpi. 7-day-old SPF chickens were inoculated with the strain via intramuscular (i.m.) or intranasal (i.n.) injection which resulted in 100% mortality of test chickens. Additionally, the sickness and death of cohabitation chickens in the test group were observed which indicated that the virus can infect healthy chickens by horizontal transmission. The sick chickens showed depression, anorexia and diarrhea with green watery feces. Y17215-1-inoculated chickens mainly presented swollen liver with blood spot, and the enhancement of effusion or yellow gel like effusion that were observed in the pericardium through necropsy. Histopathological examination showed focal necrosis of hepatocytes and characteristic eosinophilic inclusion bodies in the cytoplasm. The results showed that the Y17215-1 isolate had high pathogenicity to SPF chickens. The phylogenetic analysis of the major structural proteins including hexon, fiber-1 and fiber-2 revealed that Y17215-1 strain belongs to C species of fowl aviadenovirus of aviadenovirus family, and has high homology with other Chinese strains isolated in recent years, but was distinct from ON1、MX-SHP95、KR5 and other foreign isolates. This study laid a foundation for further study of epidemiological investigation, pathogenic mechanism as well as the diagnosis and control technology of FAdV-4.

The fowl adenovirus serotype 4 (FAdV-4) induce cellular pathway in chickens to produce interferon and antigen-presented molecules (MHCI/II)

FAdV-4 is the major strain of adenovirus that responsible for hydro-pericardial syndrome (HPS) in poultry. In this study, the virus's specific gene fragments were isolated from clinically suspected cases and amplified by PCR. Finally, after a viral infection to investigate the immune response of the host, the gene expression of MHC (major histo-compatible) molecules (MHCIα, MHCIIβ), Ii (Invariant Chain) gene, inflammatory cytokines (IFN-β, IFN-γ, and IL-1β), and transcription factors (MDA5, STING, IRF7, and NF-kB) were detected by real-time PCR (fluorescence technology). The results of sequence comparison showed that the clinically isolated virus was 100% homologous to a virulent strain of avian adenovirus group C serotype 4 (FAdV-4), which were named AH-FAdV-4. The TCID₅₀ and pathogenicity of the virus were determined that was 10^6.52/0.1 mL with a mortality rate of 100% in chickens and 0% in ducks. Furthermore, results showed that the expression level of MHCIα, MHCIIβ, and Ii genes in chicken embryo kidney cells significantly (P < 0.01) upregulated (increased) after infection, which was 43, 5.2, and 2.5 times higher than the control group. With the addition of PDTC, an inhibitor of NF-kB, then the expression level of MHCIα, MHCIIβ, and Ii was decreased significantly (P < 0.01) than the control group. The transcription levels of these genes were decreased 0.64, 0.27, and 0.26 respectively. Simultaneously, the expression levels of IFN-β, IFN-γ, and IL-1β were also significantly (P < 0.01) up-regulated (increased) 7.8, 22.7, and 5 times higher than the control group. It was found that up-regulation of STING and NF-κB pathways are directly involved in the regulation of inflammatory cytokines (IFN-β, IFN-γ, and IL-1β), MHC molecules (MHCIα, MHCIIβ), and Ii gene. The results also showed that the gene regulation pathways consecutively increased the expression levels of MDA5, STING, IRF7, and NF-kB. It is conducted that the expression levels of cytokines, MHC molecules, and li gene were increased by STING and NF-kB pathways.

Transcriptome Analysis Reveals the Potential Role of Long Noncoding RNAs in Regulating Fowl Adenovirus Serotype 4-Induced Apoptosis in Leghorn Male Hepatocellular Cells

Hepatitis-hydropericardium syndrome (HHS) is caused by fowl adenovirus serotype 4 (FAdV-4) and has resulted in considerable economic losses to the poultry industry globally. FAdV-4 elicits apoptosis in host cells. Long noncoding RNAs (lncRNAs) have emerged as important regulatory RNAs with profound effects on various biological processes, including apoptosis. However, it remains unknown whether lncRNAs participate in FAdV-4-induced apoptosis. In this study, RNA sequencing was applied to determine the transcription of cellular lncRNA in leghorn male hepatocellular (LMH) cells infected with FAdV-4. Cellular RNA transcription analysis demonstrated that FAdV-4 infection elicited 1798 significantly differentially expressed (DE) lncRNAs in infected LMH cells at 24 h post-infection (hpi) compared to mock control infection. In addition, 2873 DE mRNAs were also found. Target prediction and analyses revealed that 775 DE lncRNAs whose 671 target mRNAs were among the DE mRNAs were involved in several signaling pathways, including the AMPK signaling pathway, p53 signaling pathway and insulin signaling pathway. From these 775 DE lncRNAs, we identified 71 DE lncRNAs related to apoptosis based on their target gene functions. Subsequently, lncRNA 54128 was selected from the 71 identified DE lncRNAs, and its role in FAdV-4-induced apoptosis was verified. LncRNA 54128 interference significantly suppressed the rate of apoptosis, which was accompanied by reduced BMP4 transcription levels. To the best of our knowledge, this is the first study to analyze host lncRNA transcription during FAdV-4 infection. Our findings provide a better understanding of host responses to FAdV-4 infection and provide new directions for understanding the potential association between lncRNAs and FAdV-4 pathogenesis.

A Single Amino Acid at Residue 188 of the Hexon Protein Is Responsible for the Pathogenicity of the Emerging Novel Virus Fowl Adenovirus 4

Since 2015, severe hydropericardium-hepatitis syndrome (HHS) associated with a novel fowl adenovirus 4 (FAdV-4) has emerged in China, representing a new challenge for the poultry industry. Although various highly pathogenic FAdV-4 strains have been isolated, the virulence factor and the pathogenesis of novel FAdV-4 are unclear. In our previous studies, we reported that a large genomic deletion (1,966 bp) is not related to increased virulence. Here, two recombinant chimeric viruses, rHN20 strain and rFB2 strain, were generated from a highly pathogenic FAdV-4 strain by replacing the hexon or fiber-2 gene of a nonpathogenic FAdV-4, respectively. Both chimeric strains showed similar titers to the wild-type strain in vitro. Notably, rFB2 and the wild-type strain induced 100% mortality, while no mortality or clinical signs appeared in chickens inoculated with rHN20, indicating that hexon, but not fiber-2, determines the novel FAdV-4 virulence. Furthermore, an R188I mutation in the hexon protein identified residue 188 as the key amino acid for the reduced pathogenicity. The rR188I mutant strain was significantly neutralized by chicken serum in vitro and in vivo, whereas the wild-type strain was able to replicate efficiently. Finally, the immunogenicity of the rescued rR188I was investigated. Nonpathogenic rR188I provided full protection against lethal FAdV-4 challenge. Collectively, these findings provide an in-depth understanding of the molecular basis of novel FAdV-4 pathogenicity and present rR188I as a potential live attenuated vaccine candidate or a novel vaccine vector for HHS vaccines. IMPORTANCE HHS associated with a novel FAdV-4 infection in chickens has caused huge economic losses to the poultry industry in China since 2015. The molecular basis for the increased virulence remains largely unknown. Here, we demonstrate that the hexon gene is vital for FAdV-4 pathogenicity. Furthermore, we show that the amino acid residue at position 188 of the hexon protein is responsible for pathogenicity. Importantly, the rR188I mutant strain was neutralized by chicken serum in vitro and in vivo, whereas the wild-type strain was not. Further, the rR188I mutant strain provided complete protection against FAdV-4 challenge. Our results provide a molecular basis of the increased virulence of novel FAdV-4. We propose that the rR188I mutant is a potential live attenuated vaccine against HHS and a new vaccine vector for HHS-combined vaccines.

Development of Microsatellite Marker System to Determine the Genetic Diversity of Experimental Chicken, Duck, Goose, and Pigeon Populations

Poultries including chickens, ducks, geese, and pigeons are widely used in the biological and medical research in many aspects. The genetic quality of experimental poultries directly affects the results of the research. In this study, following electrophoresis analysis and short tandem repeat (STR) scanning, we screened out the microsatellite loci for determining the genetic characteristics of Chinese experimental chickens, ducks, geese, and pigeons. The panels of loci selected in our research provide a good choice for genetic monitoring of the population genetic diversity of Chinese native experimental chickens, ducks, geese, and ducks.

Genetic Characterization and Pathogenicity Analysis of Recently Isolated Fowl Adenovirus 8b in Korea

A Korean field strain of fowl adenovirus (FAdV) 8b was isolated from chickens showing high mortality. Isolated FAdV-8b strains with the hexon and fiber genes were genetically analyzed. The Korean FAdV-8b (K194/19) strain isolated in 2019 showed higher sequence identity with the FAdV-8b strain isolated in China but lower sequence identity with the Korean FAdV-8b (K187/08) strain isolated in 2008. The K194/19 strain formed a distinct subcluster within the FAdV-8b cluster in a phylogenetic tree based on hexon and fiber genes. FAdV can infect day-old chicks through vertical transmission, and so blood samples were obtained from 54-, 60-, and 63-wk-old parent chickens. FAdV-specific antibody levels were investigated with ELISA and virus neutralization (VN) tests with the K194/19 and K187/08 strains as antigens. In VN tests, all sera neutralized the K187/08 strain. However, the K194/19 strain was neutralized by sera collected from 60- and 63-wk-old chickens but not sera obtained from 54-wk-old chickens, indicating natural infection. Finally, to determine the pathogenicity of the K194/19 strain, 1-day-old and 4-wk-old specific-pathogen-free birds were infected with the K194/19 and K187/08 strains. No significant difference in pathogenicity was observed between the two strains. Although the K194/19 strain showed similar pathogenicity with the K187/08 strain, differences in nucleotide and amino acid sequences of the hexon and fiber genes may determine the evasion ability of the K187/08 neutralizing antibody, indicating the need for development of a novel FAdV vaccine.

A novel recombinase polymerase amplification assay for rapid detection of epidemic fowl adenovirus

Fowl adenovirus (FAdV) has posed a grave threat to the health of poultry, and the sudden outbreak highlights the importance of the new rapid diagnostic method for the control and prevention of transmission. Hence, in the present study, a novel recombinase polymerase amplification (RPA) assay, which was suitable for all 12 serotypes (FAdV-1 to 8a and 8b to 11) had been successfully launched to detect FAdV. Also, the entire amplification process could be completed in the isothermal condition when temperature ranged from 26 to 42°C within no more than 14 min, which was remarkably superior to endpoint polymerase chain reaction (98 min) with the same detecting sensitivity (as low as 0.1 fg viral DNA), avoiding sophisticated thermal cyclers with simple operation. Additionally, the same primers did not produce positive reactions with other viruses tested, demonstrating that the specificity of the RPA assay was acceptable. Moreover, this developed method could be efficiently used in the diagnosis of FAdV references and epidemic strains from different avian origins, thus making it a rapid, reliable, and point-of-care FAdV diagnostics tool, as well as an alternative to endpoint PCR.

Development of a rapid technique for extraction of viral DNA/RNA for whole genome sequencing directly from clinical liver tissues

Characterisation of the entire genome of Fowl aviadenoviruses (FAdV) requires isolation and propagation of the virus in chicken embryo liver or kidney cells, a process which is not only time consuming but may occasionally fail to result in viral growth. Furthermore, in a mixed infection, isolation in cell culture may result in the loss of viral strains. In this study, we optimised a FAdV DNA extraction technique directly from affected liver tissues using kaolin hydrated aluminium silicate treatment. The whole genome of FAdV was sequenced directly from extracted DNA without any targetted PCR based enrichment. The extraction method was also tested on avian liver tissues affected with the RNA virus Avian hepatitis E virus and demonstrated to yield sequencing grade RNA. Therefore, the method described here is a simple technique which is potentially useful for the extraction of sequencing grade DNA/RNA from tissues with high fat content.

The first complete genome sequence and pathogenicity characterization of fowl adenovirus 11 from chickens with inclusion body hepatitis in Pakistan

Inclusion body hepatitis (IBH), hydropericardium syndrome, and gizzard erosion associated with fowl adenovirus (FAdV) infections are reported globally and resulted in significant poultry industry economic losses. In 2018, severe IBH appeared in Pakistan in a 17-week-old layer flock. Subsequently, a FAdV-11 strain (designated as PKFAd18) was isolated from liver samples and identified based on phylogenetic analyses of the serotype-specific L1 region of the capsid hexon gene. There is no complete genome sequence of the Pakistani FAdV-11. This study successfully sequenced the complete genome of PKFAd18. The full genome of PKFAd18 contains 43 840 base pairs (bp) with a G + C content of 53.9 %, which is comparable to other FAdV serotypes. Similar to other FAdV-11 strains, PKFAd18 has only one fiber, while FAdV-1 and FAdV-4 have two fibers. Notably, PKFAd18 showed unique characteristics compared to other FAdV-11 strains. A natural large genomic deletion (1215 bp) appeared in tandem repeat region two, relative to the ON-NP2 strain. Phylogenetic analyses of the PKFAd18 penton gene showed higher homology with FAdV-9, highlighting potential natural recombination between FAdV-11 and FAdV-9. Moreover, the pathogenicity of PKFAd18 studied in specific-pathogen-free chickens showed that PKFAd18 is capable of inducing severe IBH and could be responsible for IBH in Pakistan. Thus, the first complete genome of FAdV-11 in Pakistan was sequenced in this study, which enriches the diversity of knowledge about FAdV-11 and is useful for developing diagnostics and vaccines for IBH induced by FAdV-11 in Pakistan.

Fowl adenoviruse-4 infection induces strong innate immune responses in chicken

Fowl adenovirus (FAdV), as the causative agent of hepatitis-hydropericardium syndrome (HHS), poses a significant threat to the poultry industry in China in recent years. In this study, we investigated the immunopathogenesis of a FAdV-4 strain HN/151025 in 60-day-old chickens. The virus was highly virulent in chickens, with a broader tissue tropism in chickens, causing 60 % mortality. Postmortem findings of dead chickens showed mild HHS and liver degeneration and necrosis. Importantly, FAdV-4 infection induced significant upregulation of genes encoding most toll-like receptors, some cytokines (interleukin-1β, 2, 6, 8, and 18, and interferon-γ), most of avian β-defensins, myeloid differentiation primary response protein 88, p38 mitogen-activated protein kinases, and inducible nitric oxide synthase, in tissues of infected chicken, especially in spleen and bursa of Fabricius. There was also a significant positive correlation between FAdV-4 genome load and the mRNA expression levels of most of these factors in specific infected tissues. The results indicated the potential role of these proteins in host immune response against FAdV-4 infection. However, overexpression of these proteins might contribute to tissue damage of FAdV-4 infected chickens, and eventually lead to chicken death.

Pathogenesis of Hypervirulent Fowl Adenovirus Serotype 4: The Contributions of Viral and Host Factors

Since 2015, severe outbreaks of hepatitis-hydropericardium syndrome (HHS), caused by hypervirulent fowl adenovirus serotype 4 (FAdV-4), have emerged in several provinces in China, posing a great threat to poultry industry. So far, factors contributing to the pathogenesis of hypervirulent FAdV-4 have not been fully uncovered. Elucidation of the pathogenesis of FAdV-4 will facilitate the development of effective FAdV-4 vaccine candidates for the control of HHS and vaccine vector. The interaction between pathogen and host defense system determines the pathogenicity of the pathogen. Therefore, the present review highlights the knowledge of both viral and host factors contributing to the pathogenesis of hypervirulent FAdV-4 strains to facilitate the related further studies.

Pathogenicity of fowl adenovirus (FAdV) serotype 4 strain SDJN in Taizhou geese

Hydropericardium hepatitis syndrome (HHS) is a fatal disease in chickens, mainly caused by fowl adenovirus serotype 4 (FAdV-4). Since June 2015, HHS has appeared in many provinces in China. The disease has spread from broilers to laying hens, breeders and Cherry Valley ducks, seriously endangering the health of the poultry industry in China. In July 2016, an infectious disease was noticed in a goose farm in Jinan, Shandong Province, China, and hydropericardium was the main finding in post mortem investigations. In the actual study, we isolated a FAdV-4 strain from the livers of naturally-infected goslings and designated it as SDJN. We first evaluated its pathogenicity by inoculating Taizhou geese at 10, 20, and 30 days of age with 10^-7.15EID₅₀/0.2 ml doses of the SDJN strain in 1 ml allantoic fluid via subcutaneous injection or oral infection. Clinical signs and pericardial effusion appeared in geese infected subcutaneously at 10 days of age, whereas 20- and 30-day-old geese were not susceptible to FAdV-4. The results of real-time PCR showed that the replication ability of FAdV-4 in geese correlated with the age. Furthermore, results from clinical chemistry showed that FAdV-4 damaged the liver and kidney in geese and the results paralleled viral load and gross lesions. Consequently, FAdV-4 was pathogenic in geese, and the pathogenicity was related to age and mode of infection. This study is the first experimental infection of FAdV-4 in geese, which will provide a basis for further understanding of the disease. RESEARCH HIGHLIGHTS Pathogenicity tests with a FAdV-4 were conducted in geese, which included data on clinical signs, gross pathology, histopathology, clinical chemistry and viral load. FAdV-4 could replicate in geese and HHS was successfully induced. Pathogenicity of FAdV-4 in geese was related to the age and routes of infection.

LAMP real-time turbidity detection for fowl adenovirus

Background

Fowl adenovirus (FAdV) is an infectious agent that can cause jaundice, severe anaemia, dyspnoea and reproductive disorders in fowls. Since 2015, FAdV disease has been rapidly spreading among broiler chickens in China, where it has caused significant economic losses. In this study, a loop-mediated isothermal amplification (LAMP) real-time turbidity method with strong specificity to FAdV was established.

Results

The established assay was specific to FAdV-4, and the lower limit of detection was 75 copies/μL of extracted DNA. The positive detection rate for the suspected tissue samples was 33.3% (14/42), which was consistent with that of the real-time PCR.

Conclusion

The proposed LAMP method can quickly and accurately detect prevalent FAdV via real-time turbidity assay, thereby providing a diagnostic platform for the prevention and control of the FAdV disease.

Genetic characterization of fowl adenovirus serotype 4 isolates in Southern China reveals potential cross-species transmission

Increasing numbers of hepatitis-hydropericardium syndrome (HHS) outbreaks associated with Fowl adenovirus 4 (FAdV-4) have been confirmed in several provinces of China since 2015, mainly affecting 3-5-week-old broiler chicks, resulting in significant losses to the poultry industry. However, little is currently known regarding the molecular epidemiology and host specificity of FAdV-4 associated with HHS in Southern China. In the present study, we isolated 37 FAdV-4 strains from 52 suspected cases of HHS (33 from broilers, one from a layer, two from ducks, and one from a mandarin duck) from Guangdong province during 2016 to 2017. All 37 FAdV-4 strains obtained showed 100% identity of hexon genes at the nucleotide level, and also showed 100% nucleotide sequence identities with strains obtained from other provinces such as Shandong, Zhejiang, and Anhui, which grouped into a FAdV-C cluster. To our knowledge, this represents the first report of an FAdV-4 strain (GZ1) from a mandarin duck with HHS. Experimental infection of the GZ1 strain via intramuscular injection led to a 100% mortality rate in 21-day-old specific pathogen-free chickens. These data indicate the possibility of the cross-species transmission of FAdV-4, highlighting the need for implementing strict biosecurity measures to avoid the mixing of different bird species.

Prevalence of Fowl Adenovirus Serotype 4 and Co-Infection by Immunosuppressive Viruses in Fowl with Hydropericardium Hepatitis Syndrome in Shandong Province, China

Fowl adenovirus serotype 4 (FAdV-4) is the pathogenic agent of hydropericardium hepatitis syndrome (HHS) in chickens and ducks, which has caused huge economic losses for the Chinese poultry industry since 2015. In order to objectively determine the prevalence and co-infection status of the virus in Shandong province in China, we analyzed a total of 679 clinical cases of chickens and ducks from 36 farms in the province. The results showed that the FAdV-4 infection rate was 65.2% (443/679), and the rate in breeder ducks was almost two-fold higher than that in breeder chickens (68.57% vs. 34.30%). Notably, co-infection by H9N2 avian influenza virus, infectious bursal disease virus, and/or chicken infectious anemia virus was very common in the 443 FAdV-4-positive cases. Furthermore, phylogenetic analysis of the hexon genes of four Shandong FAdV-4 isolates revealed that these strains clustered into Indian reference strains, indicating that the Shandong FAdV-4 strains might have originated in India. These findings provide the first data on the prevalence and co-infection status of FAdV-4 in Shandong province, which may serve as a foundation for the prevention of FAdV-4 in the field.

An emerging new fowl adenovirus genotype

In this work, we examined the diversity of fowl adenovirus (FAdV) types occurring in Hungary. From diseased chicken flocks in Eastern Hungary, 29 FAdV strains were isolated between 2011 and 2015. We performed molecular typing of the isolates based on their partial hexon sequences. The results showed that representatives from every FAdV species from A to E are present in Hungary, but compared to the findings from our previous survey, a lower number of different FAdV types were detected. Inclusion body hepatitis was always associated with FAdV-2 or -8b, gizzard erosion was caused in almost every case by FAdV-1. Numerous strains belonging to species FAdV-B were found. The complete genome sequence of a candidate new genotype strain, showing the highest divergence from the reference FAdV-5, was determined using next generation sequencing. In order to provide results compatible with the serology-based type classification, multiple genomic regions, including the major antigenic determinants, of the new isolate (strain 40440-M/2015) were compared to their counterparts in the prototype FAdV-5 (strain 340) from species FAdV-B, at both nucleotide and amino acid sequence levels. In different comparative analyses, the two strains were always found to have larger divergence between each other than any two of the most closely related FAdV serotypes. This new emerging FAdV genotype is already present in Hungary and Austria, though its exact pathological role requires further investigations. The introduction of a novel FAdV (geno)type for the classification of these strains is further supported.

Airborne Transmission of a Serotype 4 Fowl Adenovirus in Chickens

Serotype 4 fowl adenovirus (FAdV-4) is the main pathogen for hydropericardium syndrome (HPS) in chickens. It has caused major economic losses in the global poultry industry. Currently, FAdV-4's transmission routes in chickens remain unclear. Here we investigate the airborne transmission routes of FAdV-4 in chickens. A total of 45 ten-day-old chickens were equally divided into three groups (infected group/isolator A, airborne group/isolator B, and control group/isolator C). Of note, isolators A and B were connected by a leak-free pipe. The results showed that the virus could form a viral aerosol, detected in isolators two days post infection (dpi). The viral aerosol reached a peak at 4 dpi in the infected group. Healthy chickens in the airborne group were infected by the virus at 8 dpi. The chickens of the airborne group demonstrated subclinical symptoms capable of shedding the virus for some time. This finding suggests that FAdV-4 can be efficiently transmitted among chickens by aerosol transmission. These findings have significant implications for developing strategies to control this infectious disease epidemic.

Pathogenicity and molecular characterization of a fowl adenovirus 4 isolated from chicken associated with IBH and HPS in China

Background

Since July in 2015, an emerging infectious disease, Fowl adenovirus (FAdV) species C infection with Hepatitis-Hydropericardium syndrome was prevalent in chicken flocks in China. In our study, one FAdV strain was isolated from commercial broiler chickens and was designated as SDSX1.The phylogenetic information, genetic mutations and pathogenicity of SDSX1 were evaluated.

Results

The phylogenetic analysis indicated that SDSX1 is a strain of serotype 4, FAdV-C. The amino acid analysis of fiber-2 showed that there were more than 20 mutations compared with the non-virulent FAdV-C strains. The pathogenic evaluation of SDSX1 showed that the mortality of one-day-old chickens inoculated SDSX1 was 100%. The typical histopathological changes of SDSX1 were characterized by the presence of basophilic intranuclear inclusion bodies in hepatocytes. The virus copies in different tissues varied from10⁷ to 10¹¹ per 100 mg tissue and liver had the highest virus genome copies.

Conclusion

In conclusion, the isolate SDSX1, identified as FAdV-4, could cause one-day-old chicks’ typical inclusion body hepatitis (IBH) and hepatitis-hydropericardium syndrome (HHS) with 100% mortality. The virus genome loads were the highest in the liver. Molecular analysis indicated that substitutions in fiber-2 proteins may contribute to the pathogenicity of SDSX1.

Development and evaluation of a droplet digital PCR assay for the detection of fowl adenovirus serotypes 4 and 10 in attenuated vaccines

In recent years, there has been an increase in reported cases of fowl adenovirus serotype 4 (FAdV-4) in chickens in China. The use of live attenuated vaccines contaminated with FAdV-4 has been proved to be one of the important causes of massive outbreaks of hydropericardium syndrome. To detect the contamination with FAdV-4 in attenuated vaccines more promptly and accurately, a droplet digital PCR (ddPCR) assay was developed for the rapid detection of FAdV-4 and FAdV-10. The ability of this assay to detect FAdV-4 contamination in attenuated Newcastle disease virus vaccines was assessed in comparison to a quantitative real-time PCR (qPCR) and a conventional PCR assay. The findings indicated that the ddPCR assay could detect FAdV-4 contamination at 0.1 EID₅₀/1,000 feathers, while the qPCR could detect FAdV-4 contamination at 1 EID₅₀/1,000 feathers with identical genomic targets, which was 1,000-fold more sensitive than conventional PCR detection with a sensitivity of 10² EID₅₀/1,000 feathers. The ddPCR assay also showed high specificity for FAdV-4/10 and no positive signals were detected for other FAdVs. Consequently, the intuitive and rapid results were especially suitable for the detection of FAdV-4 contamination in vaccines. In this study, a ddPCR assay was developed to effectively detect and quantify low-dose FAdV-4 contamination, providing a new method for rapid detection of FAdV-4 contamination in various samples, especially vaccines.

The Natural Large Genomic Deletion Is Unrelated to the Increased Virulence of the Novel Genotype Fowl Adenovirus 4 Recently Emerged in China

Since 2015, severe hydropericardium-hepatitis syndrome (HHS), caused by a highly pathogenic fowl adenovirus 4 (FAdV-4), emerged in China. In our previous study, the FAdV-4 has been identified as a novel genotype with a unique 1966-bp nucleotide deletion (1966Del) between open reading frame 42 and 43. In this study, the natural 1966Del was frequently identified among 17 clinical isolates and other reported Chinese clinical strains. To investigate the relationship between 1966Del and the increased virulence of the novel FAdV-4, a CRISPR/Cas9 operating platform for FAdV-4 was developed for the first time in this study. Based on this platform, a Re1966 strain was rescued, inserted the relative 1966Del sequence of a nonpathogenic strain KR5. In the pathogenicity study, the Re1966 strain retained high virulence for specific-pathogen-free chickens, similar to the parental wild-type HLJFAd15, although the survival time of chickens infected with Re1966 was much longer. Therefore, the natural 1966Del was identified as a non-essential site for the increased virulence of the emerged novel FAdV-4. Although further research on the virulence-determining region or point within the genome of the novel FAdV-4 is needed, the CRISPR/Cas9 operating platform for the novel FAdV-4 was developed and successfully applied to edit the genomic DNA for the first time, and it provides a novel powerful tool for both basic virology studies and vaccine vector development of FAdVs.

Identification of two novel fowl adenovirus C-specific B cell epitopes using monoclonal antibodies against the capsid hexon protein

The diseases associated with fowl adenovirus (FAdV) infection, such as inclusion body hepatitis (IBH), hepatitis-hydropericardium syndrome (HPS), and gizzard erosion (GE), were first reported in Pakistan in 1987, and subsequent outbreaks have been reported worldwide, especially in China, where severe outbreaks of HPS with high mortality from 30 to 100% were recently reported and resulted in significant economic losses to the poultry industry. The diagnosis methods of FAdVs were mostly limited to the nucleotide sequence of hexon by PCR and DNA sequencing. The aim of this study was to generate B cell epitope maps of the species- and serotype-specific hexon L1 using monoclonal antibodies (mAbs) and bioinformatics tools for the development of novel diagnostic methods. In this study, the hexon L1 (230 amino acids) was expressed and used to generate 10 mAb-expressing hybridoma cell lines against the relative protein peptide. Subsequently, we defined the linear peptide epitopes recognized by these mAbs using a series of partially overlapping peptides derived from the FAdV-C hexon protein amino acid sequence to map mAbs reactivity. Finally, a common B cell epitope (₃₁PLAPKESMFN₄₀) for all species FAdVs and two FAdV-C-specific epitopes (₇₉KISGVFPNP₈₇ and ₁₈₁DYDDYNIGTT₁₉₀) were identified. These mAbs and their defined epitopes may support the development of the universal or species-specific differential diagnostic methods of FAdVs.

Fowl Adenovirus 4 (FAdV-4)-Based Infectious Clone for Vaccine Vector Development and Viral Gene Function Studies

Fowl adenovirus 4 (FAdV-4) is associated with economically important poultry diseases. Recent studies of fully sequenced genomes of FAdV-4 isolates suggest potential genomic regions associated with virulence and amenable for manipulation and vector development. Direct manipulation of viral genomes is cumbersome, as opposed to that of infectious clones-viral genomes cloned into plasmid or cosmid vectors. In this work, we generated an infectious clone, pFAdV-4 ON1, containing the entire viral genome of a nonpathogenic FAdV-4 (ON1 isolate). pFAdV-4 ON1 was used for targeted deletion of open reading frames (ORFs) 16 and 17 and replacement with the enhanced green fluorescence protein (EGFP) expression cassette to generate recombinant viruses. These viruses were viable, and EGFP was expressed in infected cells. Their replication, however, was significantly reduced with respect to that of the wild-type virus. These observations suggest the potential utility of FAdV-4 as a vaccine vector and the importance of ORFs 16 and 17 for virus replication at wild-type levels. To our knowledge, this is the first report of an infectious clone based on the FAdV-4 genome, and our results demonstrate its utility for studies of virulence determinants and as a platform for either vaccine or gene delivery vectors.

Different Dynamic Distribution in Chickens and Ducks of the Hypervirulent, Novel Genotype Fowl Adenovirus Serotype 4 Recently Emerged in China

A hypervirulent fowl adenovirus serotype 4 (FAdV-4) has caused hepatitis-hydropericardium syndrome (HHS) with mortalities that range from 30 to 80% in outbreaks across China since 2015. The FAdV-4 strain was characterized as a novel genotype based on the specific genome characteristics. However, our understanding of the dynamic distribution, tissue tropism, and pathogenesis of the novel FAdV-4 is incomplete. In this study, a new, sensitive and FAdV-4-specific real-time PCR was developed and applied to detect the dynamic distribution of the duck origin, novel FAdV-4 strain HLJDAd15 in experimentally infected special-pathogen free (SPF) chickens and ducks. Notably, the pathogenicity and replication pattern of HLJDAd15 were completely different between chickens and ducks. Severe hydropericardium and 10% mortality were induced in chickens, whereas no clinical signs were observed in any duck. The virus replicated was detected throughout the study in both chickens and ducks. However, only one replication peak with a high virus concentration appeared in chickens at 5 days post infection (dpi), whereas two peaks with relatively low virus titres appeared in ducks at 7 and 21 dpi. Thus, ducks could be a natural reservoir of the novel FAdV-4 absent of clinical signs, and a new transmission route from ducks shedding FAdV-4 continually to chickens was revealed, which might aggravate the outbreak of HHS in chickens. This study provides the first accurate quantitative data for the replication kinetics of the novel FAdV-4 in different hosts. The different pathogenicity, dynamic distribution and replication pattern in chickens and ducks provide a foundation for further clarification of the pathogenesis of the novel FAdV-4.