Pathogenesis of Thai duck Tembusu virus in Cherry Valley ducks: The effect of age on susceptibility to infection

Dynamics of immune responses following duck Tembusu virus infection in adult laying ducks reveal the effect of age-related immune variation on disease severity

Duck Tembusu virus (DTMUV), an emerging avian pathogenic flavivirus, is notably associated with neurological disorders and acute egg drop syndrome in ducks. We previously demonstrated that the susceptibility of ducks to DTMUV infection varies significantly with age, with younger ducks (4-week-old) exhibiting more severe disease than older ducks (27-week-old). However, the immunological mechanisms underlying these age-related differences in disease severity remain unclear. In this study, we investigated the dynamics of immune responses following DTMUV infection in adult laying ducks (27-week-old) and compared them to our previous findings on young ducks (4 weeks old). The numbers of T helper, cytotoxic T, B, and non-T and B lymphocytes, as well as neutralizing antibody levels, were measured in parallel with DTMUV loads in the blood and target organs. Our results revealed that the number of non-T and B lymphocytes/myeloid cells in 27-week-old adult laying ducks infected with DTMUV remained consistently stable throughout the observation period, in contrast to findings in 4-week-old younger ducks, where myeloid cell responses were implicated in disease progression. Regarding lymphocyte responses, unlike in 4-week-old younger ducks, only cytotoxic T lymphocyte responses in 27-week-old older ducks showed a significant negative correlation with the reduction of viremia and viral loads in target organs, indicating their role in controlling viral replication in older ducks. Additionally, 27-week-old adult laying ducks infected with DTMUV exhibited high levels of neutralizing antibodies, which were significantly correlated with reduced viral loads in blood and target organs. Overall, the presence of robust DTMUV-specific neutralizing antibody and CTL responses, along with a finely tuned myeloid cell response likely plays a significant role in controlling severe neurological outcomes in 27-week-old adult laying ducks. This study highlights the age-related differences in immune responses following DTMUV infection, which potentially contribute to the varying disease severity among ducks of different ages. Understanding the interplay between the host and DTMUV provides significant implications for disease management strategies and vaccine development.

Lack of Serological and Molecular Evidence of Duck Tembusu Virus Infection in Ducks from South Korea

The duck Tembusu virus (DTMUV), an emerging flavivirus, has led to severe neurological disorders and substantial economic losses in the duck industry throughout Asia. Considering South Korea's increasing duck production and its strategic location along the East Asian-Australasian Flyway, this study aimed to assess the presence of DTMUV in South Korea to evaluate potential risks to the poultry industry. We performed a comprehensive serological survey of 1796 serum samples from broiler and breeder ducks collected between 2011 and 2023, alongside molecular detection tests on 51 duck flocks exhibiting suspected clinical signs of DTMUV infection. The absence of serological and molecular evidence for DTMUV or other flavivirus infections suggests that these viruses have not yet affected South Korean duck populations. These findings underscore the critical need for ongoing surveillance, given the virus's potential to disrupt agriculture and pose public health risks. The study also emphasizes the importance of maintaining stringent biosecurity measures and conducting further research to monitor and prevent DTMUV transmission, particularly due to the possible role of migratory birds and other vectors in spreading zoonotic diseases.

Molecular genotyping and subgenotyping of duck circovirus at duck farms in Thailand

Background and Aim

Ducks worldwide are infected with duck circovirus (DuCV), which causes feather abnormality, emaciation, and poor growth performance. DuCV is similar to other circoviruses that induce immunosuppression due to the occurrence of the bursae of Fabricius (BF) and spleen atrophies. In Thailand, retarded ducks with feather losses were submitted for disease investigation. The ducks presented low body weight gain, had small BF and spleens, and were consistent with duck-infected DuCV. Our study investigated the possibility of DuCV infection in duck flocks in Thailand. We also analyzed the genetic characteristics of the virus.

Materials and Methods

BF and spleen samples were collected from affected meat and layer ducks from six farms thought to have been infected with DuCV. These tissues were then subjected to histopathological examination and molecular identification using conventional polymerase chain reaction and nucleotide sequencing. To identify DuCV, phylogenetic trees were generated using MEGA version X software. Samples of tissues or swabs were collected to determine whether coinfections with bacteria and viruses existed.

Results

Phylogenetic analysis using the entire genome (1995-1996 bp) and cap gene (762 bp) revealed that the DuCV isolates circulating in Thailand belonged to DuCV genotype I, which was further subdivided into two sub-genotypes: sub-genotype I b and an unclassified sub-genotype based on reference sub-genotypes. Thai isolates have variations in 10 amino acid residues in the capsid protein. Ducks infected with Thai DuCV were also coinfected with Riemerella anatipestifer, Escherichia coli, Pasteurella multocida, duck viral enteritis, and duck Tembusu virus, which is consistent with previous DuCV infection studies.

Conclusion

Six DuCVs from ducks who were previously found to have feather loss, were underweight, had growth retardation, and had poor body condition were identified in this study as belonging to genotype I and constituting at least two sub-genotypes. Due to the immunosuppressive effects of DuCV, coinfection of bacterial and viral pathogens was typically observed in Thai DuCV-infected ducks.

Advancements in Research on Duck Tembusu Virus Infections

Duck Tembusu Virus (DTMUV) is a pathogen of the Flaviviridae family that causes infections in poultry, leading to significant economic losses in the duck farming industry in recent years. Ducks infected with this virus exhibit clinical symptoms such as decreased egg production and neurological disorders, along with serious consequences such as ovarian hemorrhage, organ enlargement, and necrosis. Variations in morbidity and mortality rates exist across different age groups of ducks. It is worth noting that DTMUV is not limited to ducks alone; it can also spread to other poultry such as chickens and geese, and antibodies related to DTMUV have even been found in duck farm workers, suggesting a potential risk of zoonotic transmission. This article provides a detailed overview of DTMUV research, delving into its genomic characteristics, vaccines, and the interplay with host immune responses. These in-depth research findings contribute to a more comprehensive understanding of the virus's transmission mechanism and pathogenic process, offering crucial scientific support for epidemic prevention and control.

Multiplex digital PCR: a superior technique to qPCR for the simultaneous detection of duck Tembusu virus, duck circovirus, and new duck reovirus

Duck Tembusu virus (DTMUV), duck circovirus (DuCV), and new duck reovirus (NDRV) have seriously hindered the development of the poultry industry in China. To detect the three pathogens simultaneously, a multiplex digital PCR (dPCR) was developed and compared with multiplex qPCR in this study. The multiplex dPCR was able to specifically detect DTMUV, DuCV, and NDRV but not amplify Muscovy duck reovirus (MDRV), Muscovy duck parvovirus (MDPV), goose parvovirus (GPV), H4 avian influenza virus (H4 AIV), H6 avian influenza virus (H6 AIV), and Newcastle disease virus (NDV). The standard curves showed excellent linearity in multiplex dPCR and qPCR and were positively correlated. The sensitivity results showed that the lowest detection limit of multiplex dPCR was 1.3 copies/μL, which was 10 times higher than that of multiplex qPCR. The reproducibility results showed that the intra- and interassay coefficients of variation were 0.06-1.94%. A total of 173 clinical samples were tested to assess the usefulness of the method; the positive detection rates for DTMUV, DuCV, and NDRV were 18.5, 29.5, and 14.5%, respectively, which were approximately 4% higher than those of multiplex qPCR, and the kappa values for the clinical detection results of multiplex dPCR and qPCR were 0.85, 0.89, and 0.86, indicating that the two methods were in excellent agreement.

Identification of the Tembusu Virus in Mosquitoes in Northern Thailand

Among emerging zoonotic pathogens, mosquito-borne viruses (MBVs) circulate between vertebrate animals and mosquitoes and represent a serious threat to humans via spillover from enzootic cycles to the human community. Active surveillance of MBVs in their vectors is therefore essential to better understand and prevent spillover and emergence, especially at the human-animal interface. In this study, we assessed the presence of MBVs using molecular and phylogenetic methods in mosquitoes collected along an ecological gradient ranging from rural urbanized areas to highland forest areas in northern Thailand. We have detected the presence of insect specific flaviviruses in our samples, and the presence of the emerging zoonotic Tembusu virus (TMUV). Reported for the first time in 1955 in Malaysia, TMUV remained for a long time in the shadow of other flaviviruses such as dengue virus or the Japanese encephalitis virus. In this study, we identified two new TMUV strains belonging to cluster 3, which seems to be endemic in rural areas of Thailand and highlighted the genetic specificities of this Thai cluster. Our results show the active circulation of this emerging flavivirus in Thailand and the need for continuous investigation on this poorly known but threatening virus in Asia.

Promiscuous feeding on multiple adult honey bee hosts amplifies the vectorial capacity of Varroa destructor

Varroa destructor is a cosmopolitan pest and leading cause of colony loss of the European honey bee. Historically described as a competent vector of honey bee viruses, this arthropod vector is the cause of a global pandemic of Deformed wing virus, now endemic in honeybee populations in all Varroa-infested regions. Our work shows that viral spread is driven by Varroa actively switching from one adult bee to another as they feed. Assays using fluorescent microspheres were used to indicate the movement of fluids in both directions between host and vector when Varroa feed. Therefore, Varroa could be in either an infectious or naïve state dependent upon the disease status of their host. We tested this and confirmed that the relative risk of a Varroa feeding depended on their previous host's infectiousness. Varroa exhibit remarkable heterogeneity in their host-switching behavior, with some Varroa infrequently switching while others switch at least daily. As a result, relatively few of the most active Varroa parasitize the majority of bees. This multiple-feeding behavior has analogs in vectorial capacity models of other systems, where promiscuous feeding by individual vectors is a leading driver of vectorial capacity. We propose that the honeybee-Varroa relationship offers a unique opportunity to apply principles of vectorial capacity to a social organism, as virus transmission is both vectored and occurs through multiple host-to-host routes common to a crowded society.

Chicken-origin Cluster 3.2 Tembusu virus exhibits higher infectivity than duck-origin Cluster 2 Tembusu virus in chicks

In 2020, a chicken-origin Cluster 3 Tembusu virus (TMUV) caused outbreaks of a disease characterized by egg-drop syndrome in laying hens in China. In the present study, a TMUV strain, TMUV-GX, was isolated from tissue samples of laying hens with egg drop syndrome in south China. Phylogenetic analysis grouped TMUV-GX into TMUV Cluster 3.2, which was distinct from the prevalent TMUV Cluster 2 in duck flocks. To study the infectivity and pathogenicity of TMUV-GX in chickens and ducks, 7 day-old specific pathogen-free (SPF) chicks and SPF ducklings were infected with the same dose of the TMUV-GX. As a comparison, the duck-origin Cluster 2 strain, TMUV-JM, infection groups were set up in chicks and ducklings. Compared with the low infectivity and pathogenicity of TMUV-JM in chicks, the chicken-origin TMUV-GX displayed high replication competence in multiple tissues and caused tissues histopathological damage. In addition, the replication competence of TMUV-GX in ducklings was comparable to that of TMUV-JM. Our study revealed chicken-origin Cluster 3.2 TMUV exhibits high infectivity in chicks and ducklings, and suggested that chicken-origin Cluster 3.2 TMUV possesses a biological basis for widespread infection of chickens and ducks.

An Antibody Neutralization Determinant on Domain III and the First α-Helical Domain in the Stem-Anchor Region of Tembusu Virus Envelope Protein

Previous studies identified three neutralizing epitopes on domains I, II, and III of the Tembusu virus (TMUV) envelope (E). More evidence is needed to understand the molecular basis of Ab-mediated neutralization and protection against TMUV. In this study, we observed a neutralizing mAb, 6C8, that neutralized TMUV infection primarily by inhibiting cell attachment. In immunofluorescence assays, 6C8 recognized the premembrane and E proteins coexpressed in HEK-293T cells, but failed to react with premembrane or E expressed individually. Epitope mapping identified nine E protein residues positioned on BC/EF loops and F/G strands in domain III and the first α-helical domain in the stem region. Further investigation with mutant viruses showed that 6C8 pressure resulted in mutations at residues 330 of BC loop and 409 of the first α-helical domain, although 6C8 only exhibited a moderate neutralizing activity in BHK-21 cells and a weak protective activity in BALB/c mice and Shaoxing duck models. Mutations A330S and T409M conferred high- and low-level 6C8 resistance, respectively, whereas the combination of A330S and T409M mutations conferred moderate-level 6C8 resistance. As a result, a quasispecies comprising three groups of antigenic variants appeared in BHK-21 cell–derived viral stocks after repeated passages of TMUV strain Y in the presence of 6C8 treatment. Taken together, these findings have raised a concern about Ab-induced antigenic variations in vivo, and they have revealed information concerning the conformational structure of the 6C8 epitope and its role in constraint on antigenic variations. The present work contributes to a better understanding of the complexity of the TMUV immunogen.

The difference in CD4+ T cell immunity between high- and low-virulence Tembusu viruses is mainly related to residues 151 and 304 in the envelope protein

Tembusu virus (TMUV) can result in a severe disease affecting domestic ducks. The role of T cells in protection from TMUV infection and the molecular basis of T cell-mediated protection against TMUV remain largely uncharacterized. Here, we used the high-virulence TMUV strain Y and the low-virulence TMUV strain PS to investigate the protective role for TMUV-specific CD4⁺ and CD8⁺ T cells. When tested in a 5-day-old Pekin duck model, Y and PS induced comparable levels of neutralizing antibody, whereas Y elicited significantly stronger cellular immune response relative to PS. Using a duck adoptive transfer model, we showed that both CD4⁺ and CD8⁺ T cells provided significant protection from TMUV-related disease, with CD8⁺ T cell conferring more robust protection to recipient ducklings. For TMUV, CD4⁺ T cells mainly provided help for neutralizing antibody response, whereas CD8⁺ T cells mainly mediated viral clearance from infected tissues. The difference in T cell immunity between Y and PS was primarily attributed to CD4⁺ T cells; adoptive transfer of Y-specific CD4⁺ T cells resulted in significantly enhanced protective ability, neutralizing antibody response, and viral clearance from the brain relative to PS-specific CD4⁺ T cells. Further investigations with chimeric viruses, mutant viruses, and their parental viruses identified two mutations (T151A and R304M) in the envelope (E) protein that contributed significantly to TMUV-specific CD4⁺ T cell-mediated protective ability and neutralizing antibody response, with more beneficial effects being conferred by R304M. These data indicate T cell-mediated immunity is important for protection from disease, for viral clearance from tissues, and for the production of neutralizing antibodies, and that the difference in CD4⁺T cell immunity between high- and low-virulence TMUV strains is primarily related to residues 151 and 304 in the E protein.

The emergence of a disease caused by a mosquito origin Cluster 3.2 Tembusu virus in chickens in China

In 2021, a chicken Tembusu virus (TMUV) caused outbreaks of a disease characterized by retarded growth and egg production decline in chickens in China. Two TMUV strains SD2021 and GX2021 were isolated from the diseased chickens and phylogenetic analysis of the E gene nucleotide sequence revealed that the chicken TMUV SD2021 and GX2021 were most close to mosquito origin TMUV in Cluster 3.2, which was distinct from the prevalent duck TMUVs in Cluster 2. The TMUV SD2021 caused growth retardation and neurological symptoms in chickens through both intranasal and intramuscular infection routes, but has no direct-contact transmissibility among chickens. The findings of this study highlight the pathogenicity of a chicken adapted mosquito-origin TMUV in chickens in China.

Antibody prophylaxis against Tembusu virus-associated disease

Earlier studies have shown that Tembusu virus (TMUV) can elicit high levels of neutralizing antibodies, but the ability of antibodies to protect against TMUV-associated disease and to inhibit replication of TMUV in vivo remains to be investigated. Here, we tested the prophylactic efficacy of TMUV immune serum directly using a 2-day-old Pekin duck model. Passive administration of the immune serum prior to challenge protected ducklings against morbidity and mortality, substantially reduced TMUV-caused tissue injury, and significantly decreased TMUV levels in the periphery and central nervous system. These findings demonstrate that antibodies play a dominant protective role in controlling TMUV-associated disease.

Effects of Aedes aegypti salivary protein on duck Tembusu virus replication and transmission in salivary glands

Duck Tembusu virus (DTMUV) infection is an arthropod-borne viral disease that affects many poultry species, including ducks, chickens, and geese. Aedes aegypti mosquito is an important vector of DTMUV. This study sought to determine whether any individual Ae. aegypti salivary protein modulated DTMUV replication in the mosquito salivary gland. Ae. aegypti salivary gland protein of 34 kDa (AaSG34) was found to be expressed explicitly in mosquito salivary glands and was upregulated following DTMUV infection. Thus, AaSG34 was silenced in mosquitoes via RNA interference using double strand RNA (dsRNA), and the mosquitoes were then infected with DTMUV to elucidate their effects on DTMUV replication and transmission. Transcripts of the DTMUV genome in salivary glands and virus titer in saliva were significantly diminished when AaSG34 was silenced, indicating that its presence enhances DTMUV replication in the salivary glands and DTMUV dissemination to saliva. Furthermore, the expression of antimicrobial peptides (AMPs) was upregulated upon AaSG34 silenced. Our results demonstrate that AaSG34 may play a vital role in the suppression of antiviral immune responses to enhance DTMUV replication and transmission. We thus provide new information on the effect of the AaSG34 salivary protein on DTMUV replication in Ae. aegypti as the mechanism of blocking virus transmission to the host.

Dynamics of cellular and humoral immune responses following duck Tembusu virus infection in ducks

Duck Tembusu virus (DTMUV), an emerging avian pathogenic flavivirus, causes severe neurological disorders and acute egg drop syndrome in ducks. However, the effects of DTMUV on duck immunological components and functions remain largely unknown. In this study, the dynamics of cellular and humoral immune responses of DTMUV-infected ducks were investigated. The numbers of CD4⁺ and CD8⁺ T, B and non-T and B lymphocytes as well as the levels of neutralizing antibodies were quantified in parallel with DTMUV loads in blood and target organs. Our results demonstrated that DTMUV infection caused severe losses of non-T and B lymphocyte/myeloid cell subpopulation, and reduction in phagocytic activity during 3-5 days after infection. We also found that the numbers of T and B cells were increased during the first week of DTMUV infection. A significant negative correlation between the levels of CD4⁺ and CD8⁺ T, B and non-T and B lymphocytes and viral loads in blood and target organ (spleen) was observed during the early phase of infection. Additionally, DTMUV infection induced an early and robust neutralizing antibody response, which was associated with DTMUV-specific IgM and IgG responses. The presence of neutralizing antibody also correlated with reduction of viremia and viral load in spleen. Overall, DTMUV elicited both cellular and humoral immune responses upon infection, in which the magnitude of these responses was correlated with reduction of viremia and viral loads in the target organ (spleen). The results suggested the critical role of both cellular and humoral immunity against DTMUV infection. This study expands our understanding of the immunological events following DTMUV infection in ducks. This article is protected by copyright. All rights reserved.

Tembusu virus infection in laying chickens: Evidence for a distinct genetic cluster with significant antigenic variation

Tembusu virus (TMUV) associated disease is a growing cause of egg production decrease and encephalitis in domestic waterfowl, with expanding distribution. In previous studies, TMUV isolates were phylogenetically classified into two genetic lineages and different clusters with varied pathogenicity. However, little is known about the phenotypic and virulence characteristics of cluster 3 isolates within the duck TMUV lineage. In this study, the etiological agent causing egg drop in a laying chicken farm in southern China was investigated and a TMUV was isolated from pooled tissue samples. Genome sequencing and phylogenetic analysis grouped the isolate into TMUV cluster 3 with closest relation to the mosquito-origin TMUV YN12193. Cross-neutralization testing using convalescent sera revealed significant antigenic variation between the isolate and a representative strain of cluster 2.2. The experimental infection of SPF hens confirmed the ability of the isolate to replicate in multiple tissues and led to ovary damage. Additionally, high seroconversion rates (95.83%-100%) were detected in the three flocks following retrospective investigation. Our study demonstrates the occurrence of cluster 3 TMUV infection in laying chickens and that the virus exhibits significant antigenic variation compared with cluster 2 TMUV.

Pathogenesis of Thai duck Tembusu virus in BALB/c mice: Descending infection and neuroinvasive virulence

Duck Tembusu virus (DTMUV) is an emerging flavivirus that causes systemic disease in an avian host. The predominant cluster of DTMUV circulating in Thailand was recently classified as cluster 2.1. The pathogenesis of this virus has been extensively studied in avian hosts but not in mammalian hosts. Six-week-old BALB/c mice were intracerebrally or subcutaneously inoculated with Thai DTMUV to examine clinical signs, pathological changes, viral load and virus distribution. Results demonstrated that the virus caused disease in BALB/c mice by the intracerebral inoculation route. Infected mice demonstrated both systemic and neurological symptoms. Pathological changes and virus distribution were observed in all tested organs. Viral load in the brain was significantly higher than in other organs (p < .05), and the virus caused acute death in BALB/c mice. The virus was disseminated in all parts of the body, but no virus shedding was recorded in saliva and faeces. Findings highlighted the potential of Thai DTMUV to transmit disease in mammalian hosts.

A quasispecies in a BHK-21 cell-derived virulent Tembusu virus strain contains three groups of variants with distinct virulence phenotypes

Previous studies resulted in the isolation of a low-virulence plaque-purified variant from the third passage (P3) in BHK-21 cells of a Tembusu virus (TMUV) isolate, suggesting the presence of viral quasispecies in the P3 culture. To confirm this notion, the fourth passage virus (P4) was prepared by infecting BHK-21 cells with P3 for isolation of more variants. We isolated 10 plaque-purified viruses. Comparative genome sequence analysis identified six of the 10 viruses as genetically different variants, which harbored a total of eight amino acid differences in the envelope, NS1, NS3, and NS5 proteins. When tested in a 2-day-old Pekin duck model, P4 caused 80 % mortality, belonging to a high-virulence TMUV strain. Out of the six genetically different variants, two presented high-virulence, one exhibited moderate-virulence, and three displayed low-virulence, causing 60 %-70 %, 40 %, and 10 % mortalities, respectively. These results demonstrate that P4 contains at least three groups of variants with distinct virulence phenotypes. Analysis of links between the eight residues and virulence of the six variants identified NS1 protein residue 183 and NS5 protein residues 275 and/or 287 as novel determinants of TMUV virulence. The analysis also provided a new clue for future studies on the molecular basis of TMUV virulence in terms of genetic interaction of different proteins. Overall, our study provides direct evidence to suggest that TMUV exists in in vitro culture of a virulent isolate as a quasispecies, which may enhance our understanding of molecular mechanism of TMUV virulence.

Mapping of a unique epitope on domain III of the envelope protein of Tembusu virus

We recently developed a Tembusu virus (TMUV)-specific monoclonal antibody (MAb) 12F11, which was found to recognize a long amino acid sequence between residues 8 and 77 of domain III of the envelope protein (EDIII). Here, the epitope recognized by MAb 12F11 was mapped using alanine substitutions combined with dissociation constant analysis. The findings, and prediction of tertiary structure of TMUV EDIII, showed that the MAb 12F11 epitope contained one critical residue and 13 peripheral residues. Moreover, the antigenic site was shown to span four loops (N-terminal region, AB, BC, and CD) and three β-strands (A, B, and D). The present work contributes to the understanding of antigenic structure of TMUV envelope protein.

Interactions of duck Tembusu virus with Aedes aegypti and Aedes albopictus mosquitoes: Vector competence and viral mutation

Duck Tembusu virus (DTMUV) is an emerging flavivirus that causes severe disease in avian hosts, while also affecting mammalian hosts; however, information on viral interaction with mosquito vectors for mammalian hosts is limited. Vector competence of Aedes (Ae.) aegypti and Aedes albopictus mosquitoes for DTMUV were investigated. Both Aedes mosquito species were orally infected with DK/TH/CU-1 strain of Thai DTMUV and isolated DTMUV from BALB/c mouse. Genomes of the viruses isolated from hosts and vectors were analyzed and compared with the positive virus. Findings showed that both Aedes mosquito species could serve as vectors for DTMUV with minimum viral titer in blood meal of 10⁶ TCID₅₀/mL. After taking blood meal with viral titer at 10⁷ TCID₅₀/mL, vector competence of the mosquitoes was significantly different from the lower titer in both species. Both Aedes species did not support development of the isolated viruses from mouse. A point mutation of nucleotide and amino acid was found in all isolated DTMUV from Ae. aegypti saliva, while other viruses were similar to the positive virus. Our findings demonstrated that both Ae. aegypti and Ae. albopictus had potential to transmit the virus and play important roles in the viral transmission cycle in mammalian hosts, while viral mutation occurred in Ae. aegypti mosquitoes.

New Insights into the Biology of the Emerging Tembusu Virus

Reported for the first time in 1955 in Malaysia, Tembusu virus (TMUV) remained, for a long time, in the shadow of flaviviruses with human health importance such as dengue virus or Japanese encephalitis virus. However, since 2010 and the first large epidemic in duck farms in China, the threat of its emergence on a large scale in Asia or even its spillover into the human population is becoming more and more significant. This review aims to report current knowledge on TMUV from viral particle organization to the development of specific vaccines and therapeutics, with a particular focus on host-virus interactions.

Duck Tembusu virus induces stronger cellular responses than Japanese encephalitis virus in primary duck neurons and fibroblasts

Duck Tembusu virus (DTMUV) and Japanese encephalitis virus (JEV) are mosquito-borne flaviviruses. These two viruses infect ducks; however, they show different neurological outcomes. The mechanism of DTMUV- and JEV-induced neuronal death has not been well investigated. In the present study, we examined the differences in the mechanisms involved in virus-induced cell death and innate immune responses between DTMUV KPS54A61 strain and JEV JaGAr-01 strain using primary duck neurons (DN) and duck fibroblasts (CCL-141). DN and CCL-141 were permissive for the infection and replication of these two viruses, which upregulated the expression of innate immunity genes. Both DTMUV and JEV induced cell death via a caspase-3-dependent manner; however, DTMUV triggered more cell death than JEV did in both CCL-141 and DN. These findings suggest that DTMUV infection causes apoptosis in duck neurons and fibroblasts more strongly than JEV. Levels of the mRNA expression of innate immunity-related genes after DTMUV infection were generally higher than levels after JEV infection, suggesting that DTMUV-induced immune response in duck cells may exhibit toxic effect rather than protective effects. This article is protected by copyright. All rights reserved.

Development and Validation of a Universal One-Step RT-PCR Assay for Broad Detection of Duck Tembusu Virus

Duck Tembusu virus (DTMUV), a mosquito-borne flavivirus, has been identified as a causative agent of an emerging disease in ducks. Since its first report in 2010, several clusters of DTMUV have increasingly been identified and caused outbreaks in many Asian countries. This highlights the need for improved and novel broad detection assays in order to detect all circulating clusters of DTMUV. In this study, a universal one-step reverse-transcription PCR (RT-PCR) assay targeting a highly conserved region of the NS5 gene was developed and validated for broad detection of all DTMUV clusters. The newly developed universal RT-PCR assay could specifically detect all clusters of DTMUV without cross-reactions with common duck viruses and other related flaviviruses. The assay was able to detect DTMUV as low as a 0.001 50% embryo lethal dose/milliliter. The performance of the assay was evaluated by using experimental and field clinical samples. The assay could successfully detect DTMUV in all experimentally DTMUV-infected samples and gave a higher DTMUV detection rate (36%) than the previously reported envelope-specific RT-PCR assay (30%) in field clinical samples. All the positive samples were confirmed DTMUV-positive by DNA sequencing. In conclusion, the newly developed universal RT-PCR assay exhibited high accuracy, specificity, and sensitivity in broad DTMUV detection, thus providing an improved screening assay for routine detection and epidemiologic surveillance of DTMUV.

Minocycline prevents primary duck neurons from duck Tembusu virus-induced death

Duck Tembusu virus (DTMUV), a neurotropic flavivirus, is a causative agent of severe neurological diseases in different birds. No approved vaccines or antiviral therapeutic treatments are available to date. The poultry industry experiences significant economic losses due to DTMUV infections. Minocycline is a second-generation semi-synthetic tetracycline analogue that is commonly used as an antimicrobial treatment. Experimental studies have indicated the successful protective effects of minocycline against neuronal cell death from neurodegenerative diseases and viral encephalitis. The aim of this study was to investigate the effects of minocycline on DTMUV infection in neurons. Primary duck neurons were treated with minocycline, which exhibited neuroprotective effects via anti-apoptotic function rather than through viral replication inhibition. Minocycline might serve as a potential effective drug in DTMUV infection.

Effect of TMUV on immune organs of TMUV infected ducklings

Tembusu Virus (TMUV), a pathogenic member of Flavivirus family, acts as the causative agent of egg-laying and has severely threatened the duck industry over the past few years. Thus far, the pathogenicity of such virus has been extensively studied, whereas TMUV on immune system has been less comprehensively assessed, especially on ducklings that exhibit more susceptible to TMUV attack. Accordingly, in the present study, 5-day-old ducklings were infected with TMUV-TC2B (10⁴ TCID₅₀) via intravenous injection, and mock ones were inoculated with phosphate-buffered saline (PBS) in identical manner as control. At 1 day-post inoculation (dpi), the innate immunity was strongly activated, and reacted rapidly to TMUV invasion, which was reflected as the significantly up-regulated IFN-stimulated genes (ISGs), especially in immune organs (e.g., thymus, bursa of Fabricius (BF) and spleen). Subsequently, under the continuous monitoring, the levels of IgA, IgM and IgG acting as the representative immunoglobulins (Igs) were constantly higher than those of mock ducklings, demonstrating that humoral immunity also played a major role in anti-virus infection. Despite the immune system activated positively, TMUV still caused systemic infection, and in particular, the immune organs were subject to severe damage in the early infection. With our constant observation, the injury of spleen and BF turned out to be getting more serious, and at 6 dpi, TMUV antigen was widely detected in both of two immune organs by immunohistochemistry (IHC) and main histopathological lesion presented as lymphocytopenia. Moreover, the elevated apoptosis rate of splenic lymphocytes and the alteration of immune organ index also revealed the damage of lymphoid organs and similarly, it is worth noting that severe damages were detected in thymus of TMUV-infected ducklings as well. In brief, the present study systematically described the dynamic damage of immune system after being attacked by TMUV and presented insights into the research of pathogenicity.

Substantial Attenuation of Virulence of Tembusu Virus Strain PS Is Determined by an Arginine at Residue 304 of the Envelope Protein

The Tembusu virus (TMUV) PS strain, derived by several passages and plaque purifications in BHK-21 cells, displays markedly lower virulence in Pekin ducklings relative to a natural isolate of TMUV, but the potential virulence determinants and the in vivo mechanisms for substantial virulence attenuation of the passage variant remain unknown. Here, we constructed a series of chimeric and mutant viruses and assessed their virulence using a 2-day-old Pekin duckling model. We showed that residue 304 in the envelope (E) protein is the molecular determinant of TMUV virulence. Further investigations with mutant and parental viruses demonstrated that acquisition of positive charges at E protein residue 304 plays a critical role in substantial attenuation of neurovirulence and neuroinvasiveness, which is linked to enhanced binding affinity for glycosaminoglycans (GAGs). In Pekin ducklings infected by subcutaneous inoculation, an Arg at residue 304 in the E protein was shown to contribute to more rapid virus clearance from the circulation, markedly reduced viremia, and significantly decreased viral growth in the extraneural tissues and the central nervous system, relative to a Met at the corresponding residue. These findings suggest that the in vivo mechanism of virulence attenuation of the TMUV passage variant closely resembles that proposed previously for GAG-binding variants of other flaviviruses. Overall, our study provides insight into the molecular basis of TMUV virulence and the in vivo consequences of acquisition of a GAG-binding determinant at residue 304 in the E protein of TMUV.IMPORTANCE TMUV-related disease emerged in 2010 and has a significant economic impact on the duck industry. Although the disease was originally recognized to affect adult ducks, increasing evidence has shown that TMUV also causes severe disease of young ducklings. It is, therefore, essential to investigate the pathogenesis of TMUV infection in a young duckling model. The significance of our studies is in identifying E protein residue Arg304 as the molecular determinant for TMUV virulence and in clarifying the crucial role of positive charges at E protein residue 304 in virulence attenuation of a TMUV passage variant. These data will greatly enhance our understanding of the pathogenesis of TMUV infection in ducklings and have implications for development of a safe and efficient vaccine.

Evaluation of host systems for efficient isolation and propagation of duck Tembusu virus

Several phylogenetic clusters of duck Tembusu virus (DTMUV) that caused outbreaks in ducks in Asia have been identified since its emergence in 2010, highlighting the need for an efficient host system that can support isolation of all circulating phylogenetic clusters of DTMUV. In this study, various host systems, including different avian embryonated eggs (duck and chicken) and cell cultures (primary duck embryo fibroblast (DEF), primary chicken embryo fibroblast (CEF), baby hamster kidney (BHK-21), African green monkey kidney (Vero) and Aedes albopictus clone C6/36 (C6/36) cells), were evaluated and compared for their ability to support DTMUV isolation and propagation. Our results showed that all host systems were susceptible to DTMUV infection; however, BHK-21 and primary DEF cells supported more efficient replication of DTMUV compared to the other host systems. BHK-21 cells had the highest DTMUV isolation rate when tested with experimental and field clinical samples. All circulating phylogenetic clusters of DTMUV, including clusters 1, 2 and 3, were successfully isolated from duck clinical samples using BHK-21 cells. In conclusion, our findings supported the use of BHK-21 cells as a host system for primary isolation of all circulating phylogenetic clusters of DTMUV from duck clinical samples. This study highlights the importance of selecting the most appropriate host system for efficient isolation and propagation of DTMUV from duck clinical samples.RESEARCH HIGHLIGHTS DTMUV replicated more efficiently in BHK-21 and primary DEF cells than in other host systems tested.BHK-21 cells had the highest DTMUV isolation rate.All DTMUV phylogenetic clusters were successfully isolated from the samples using BHK-21 cells.BHK-21 cells were the most efficient host system for DTMUV isolation.

Patterns of duck Tembusu virus infection in ducks, Thailand: a serological study

Duck Tembusu virus (DTMUV), a mosquito-borne flavivirus, has been identified as a causative agent of an emerging viral disease in ducks, causing significant economic losses to the duck-producing industry. In Thailand, DTMUV has been detected sporadically in ducks since the first report in 2013. However, information on the patterns of DTMUV infection in ducks in Thailand is limited. In this study, a serological survey of DTMUV on ducks raised in farming and free-grazing systems was conducted during 2015-2016. Blood samples of farm ducks (n = 160) and free-grazing ducks (n = 240) were collected in the summer, rainy, and winter seasons during 2015-2016 and tested for DTMUV infection. Our results showed that DTMUV infection in ducks in Thailand occurred all year-round; however, the patterns of DTMUV infection varied between 2 duck-raising systems. Significant seasonal pattern was found in free-grazing ducks, whereas no seasonality was observed in farm ducks. Notably, DTMUV infection in ducks in Thailand was highest in the winter season. In conclusion, our data indicate distinct patterns of DTMUV infection between farm and free-grazing ducks, and the year-round circulation of DTMUV in ducks in Thailand, with peaks in the winter season. This information will help reduce the risk of DTMUV transmission through prevention and control strategies focusing on the peak period. Routine surveillance of DTMUV in ducks is essential for early detection of DTMUV allowing the implementation of control measures in a timely manner.

Pathogenicity of a Jinding duck-origin cluster 2.1 isolate of Tembusu virus in 3-week-old Pekin ducklings

Tembusu virus (TMUV) infection most commonly affects breeder and layer ducks during laying period, and can also affect young ducks below 7 weeks of age. Here, we report our investigation of a TMUV-caused fatal disease of Jingding ducklings (Anas platyrhynchos domesticus) in Northeast China. The disease resulted in mortalities of up to 40 % in 2 to 4-week-old ducks, up to 25 % in 5 to 6-week-old ducks, and less than 10 % in 7 to 8-week-old ducks. Using a TMUV-specific reverse transcription-PCR assay, all 44 ducks collected from 10 different farms were found positive for TMUV. Phylogenetic analysis of the E nucleotide sequence revealed that five of the six TMUV strains detected from three young ducks and three laying ducks were grouped within cluster 2.1. Inoculation of the liver sample of a 40-day-old sick duck in BHK-21 cells resulted in isolation of cluster 2.1 TMUV strain H. In experimental infections performed using 3-week-old Pekin ducklings (Anas platyrhynchos domesticus) (n = 30; 10 birds/group), high mortality (60 %) was caused by strain H, in sharp contrast with a very low mortality (10 %) caused by strain Y which was isolated during outbreaks of the TMUV-related disease of young Jinding ducks in 2014 in the same region. These findings clearly demonstrated that the cluster 2.1 TMUV strain H is more pathogenic for 3-week-old ducklings as compared to the cluster 2.2 TMUV strain Y. The present study may enhance our understanding of pathogenicity of TMUV in young ducks, and will stimulate further studies on the pathogenesis of TMUV infection.

Construction of the recombinant duck enteritis virus delivering capsid protein VP0 of the duck hepatitis A virus

Duck hepatitis A virus type 1 (DHAV-1) disease causes significant economic losses to the duck industry. Duck enteritis virus (DEV) is frequently used as a viral vector for aquatic poultry vaccination, but no recombinan DEV expressing DHAV-1 VP0 has been developed. In this study, we established a system for rescuing the DEV C-KCE vaccine strain by transfecting cells with six fosmid DNAs. We generated a recombinant virus (rDEV-ul41VP0) by inserting the VP0 gene of DHAV-1 into the ul41 region in the DEV C-KCE genome. DHAV-1 VP0 was stably expressed in the rDEV-ul41VP0 infected cells, but did not affect the replication properties of DEV C-KCE in cells. Duck experiments showed that rDEV-ul41VP0 could provided full protection against the lethal DEV Chinese standard challenge (DEV CSC) and conferred 70% protection against DHAV-1 161/79 at 3 days postvaccination. These results indicate that rDEV-ul41VP0 rapidly induces protection against DEV CSC and DHAV-1 in ducks, and can be served as a bivalent vaccine against DEV and DHAV-1.