Serological evidence of duck Tembusu virus infection in free-grazing ducks, Thailand

Development of a reverse transcription loop-mediated isothermal amplification based clustered regularly interspaced short palindromic repeats Cas12a assay for duck Tembusu virus

Duck Tembusu virus (DTMUV) is an emerging pathogen that poses a serious threat to the duck industry in China. Currently, polymerase chain reaction (PCR), quantitative PCR (qPCR) and reverse transcription loop-mediated isothermal amplification (RT-LAMP) are commonly used for DTMUV detection. However, these methods require complex steps and special equipment and easily cause false-positive results. Therefore, we urgently need to establish a simple, sensitive and specific method for the clinical field detection of DTMUV. In this study, we developed an RT-LAMP-based CRISPR-Cas12a assay targeting the C gene to detect DTMUV with a limited detection of 3 copies/μL. This assay was specific for DTMUV without cross-reaction with other common avian viruses and only required some simple pieces of equipment, such as a thermostat water bath and blue/UV light transilluminator. Furthermore, this assay showed 100% positive predictive agreement (PPA) and negative predictive agreement (NPA) relative to SYBR Green qPCR for DTMUV detection in 32 cloacal swabs and 22 tissue samples, supporting its application for clinical field detection.

Duck IL-7 as a novel adjuvant improves the humoral immune response to an inactivated duck tembusu virus vaccine

Duck tembusu virus (DTMUV), belonging to the Flavivirus genus, Flaviviridae family, has caused huge economic losses in the duck industry. However, the inactivated DTMUV vaccine requires multiple immunizations and has incomplete effectiveness. The humoral immune response is a key factor in the control of DTMUV infection. IL-7 derived from mammals has the ability to enhance antibody production. Whether duck IL-7 (duIL-7) possesses the ability to improve the humoral immunity of inactivated DTMUV vaccine has not yet been declared. Here, a beta-propiolactone (BPL)-inactivated DTMUV vaccine was employed to characterize the adjuvant property of duIL-7 in humoral immune responses. Intramuscular injection of DTMUV inactivated vaccine with or without duIL-7 was administered twice to the ducks. The results showed that duIL-7 was able to promote rapid antibody responses and enhance DTMUV-specific IgG and neutralizing antibody production to the vaccine. T follicular helper (Tfh) cells play a key role in assisting long humoral immunity. It was found that duIL-7 upregulated duIl-6 and duIl-21 gene expression at 3 w post first vaccination, which encode Tfh cell differentiation-related cytokines duIL-6 and duIL-21, respectively. This may be the reason that duIL-7 could prolong the humoral immune response to the inactivated DTMUV vaccine. Next, the ability of duIL-7 to simplify the immunization procedure of the inactivated DTMUV vaccine was tested. When ducks were immunized once, the titers of neutralizing antibodies in ducks from the inactivated DTMUV vaccine supplemented with duIL-7 group were significantly higher than those of ducks from the inactivated DTMUV vaccine group (P < 0.05). In addition, duIL-7 could assist the inactivated DTMUV vaccine in maintaining neutralizing antibodies at high levels during the whole experimental period. The viral titers in the ducks immunized with the inactivated DTMUV vaccine and duIL-7 were lower than those in the ducks immunized with the inactivated DTMUV vaccine alone at 3 days post infection (3 dpi, P < 0.05). Overall, duIL-7 possessed the ability to promote and prolong humoral immune responses to the inactivated DTMUV vaccine, even at one dose. This study provides a new efficient adjuvant for inactivated DTMUV vaccine development.

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.

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.

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.

Vector competence of Culex tritaeniorhynchus and Culex quinquefasciatus (Diptera: Culicidae) for duck Tembusu virus transmission

Duck Tembusu virus (DTMUV), an emerging infectious disease in ducks, was detected in Culex (Cx.) tritaeniorhynchus mosquitoes collected from a duck farm; however, the exact role of mosquitoes in the ecology of DTMUV in Thailand remains unclear. Vector competence of Cx. tritaeniorhynchus and Cx. quinquefasciatus was examined for DTMUV. Cx. tritaeniorhynchus mosquitoes were allowed to feed on four levels (10², 10³, 10⁴, and 10⁵ TCID₅₀/mL) of DTMUV, while Cx. quinquefasciatus were allowed to feed on two levels (10⁴ and 10⁵ TCID₅₀/mL) of DTMUV. Infection rates in Cx. tritaeniorhynchus were 1.6, 10.2, 35.8, and 59.3% after feeding on 10², 10³, 10⁴, and 10⁵ TCID₅₀/mL of DTMUV, respectively, while dissemination and transmission were 20.3 and 16.9% after feeding on 10⁵ TCID₅₀/mL of DTMUV. Infection rates in Cx. quinquefasciatus were 2.5 and 2.3% after feeding on 10⁴ and 10⁵ TCID₅₀/mL of DTMUV, respectively, with no virus dissemination and transmission found in all tested mosquitoes. Another study was conducted to examine the transovarial transmission of DTMUV in Cx. tritaeniorhynchus. Mosquitoes were allowed to feed on blood meal infected with 10⁵ TCID₅₀/mL of DTMUV. Each blood-fed mosquito was isolated and allowed to lay eggs. After oviparity, the mosquitoes were tested for DTMUV infection; 43 DTMUV infected and 37 non-infected female mosquitoes with eggs were included. A total of 182 F1 progeny from DTMUV infected mosquitoes and 145 F1 progeny from non-infected mosquitoes were tested for DTMUV but all were negative. Findings indicated the potential role of Cx. tritaeniorhynchus in the DTMUV transmission cycle in duck farms in Thailand. No transovarial transmission of DTMUV was found in Cx. tritaeniorhynchus.

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.

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

Several duck Tembusu virus (DTMUV) clusters have been identified since its first emergence in 2010. However, the pathogenesis evaluation of DTMUV has been restricted to cluster 2.2 Chinese DTMUVs. In this study, the pathogenesis of a cluster 2.1 Thai DTMUV was investigated in three ages of Cherry Valley ducks (1-, 4- and 27-week-old). In each age, 35 ducks were inoculated with a cluster 2.1 Thai DTMUV and evaluated for clinical signs, virus distribution and shedding, pathology and serological response. Our results demonstrated that all duck ages were susceptible to Thai DTMUV; however, Thai DTMUV induced greater disease severity in younger ducks (1- and 4-week-old) when compared to older ducks (27-week-old) reflected by higher morbidity and mortality rates, and higher degree of pathological severity. Corresponding to these results, longer-term viremia, higher levels of viral loads in tissues and lower neutralizing antibody titers were also observed in younger ducks compared to those in older ducks. However, it should be noted that a significant drop in egg production was found in older ducks, which also indicates the susceptibility to Thai DTMUV in older ducks. Interestingly, prolonged shedding period with high viral loads was observed in older ducks even without showing clinical signs, suggesting the potential role of the older ducks as the carriers of Thai DTMUV. This finding highlights the importance of monitoring DTMUV and preventing the transmission of DTMUV in adult ducks. Overall, this study provides insights into the pathogenesis and infection dynamics of a cluster 2.1 Thai DTMUV in ducks.

Duck Tembusu virus detection and characterization from mosquitoes in duck farms, Thailand

Duck Tembusu virus (DTMUV), an emerging infectious disease in ducks, belongs to the Flavivirus genus and Flaviviridae family. The transmission of DTUMV involves mosquito vectors; however, the exact role of mosquitoes in the ecology of DTMUV in Thailand remains unclear. This study was conducted to examine DTMUV detection and characterization from mosquitoes in duck farms in central Thailand. Mosquitoes were collected from two duck farms in Sing Buri Province and two duck farms in Ang Thong Province from September 2015 to July 2016 using four CDC-light traps. A total of 30,841 mosquitoes were collected and identified to seven species (Anopheles (An.) barbirostris, An. stephensi, Culex (Cx.) gelidus, Cx. quinquefasciatus, Cx. tritaeniorhynchus, Mansonia (Ma.) annulifera and Ma. uniformis). The most common collected species from each duck farm and each collection time was Cx. tritaeniorhynchus. Mosquitoes were pooled according to species, location, and collection time and then examined for DTMUV by RT-PCR. A total of 273 mosquito pools were examined, with only one pool of Cx. tritaeniorhynchus collected from Sing Buri Province in November 2015 testing positive for DTMUV. Phylogenetic analysis of the polyprotein genes demonstrated that a mosquito-derived Thai DTMUV was grouped into subcluster 2.1 and most closely related to the 2013 Thai DTMUVs. Thus, this study indicated that Cx. tritaeniorhynchus may play a role as a vector in the transmission of DTMUV in Thailand. However, additional studies concerning the vector competence of this mosquito for DTMUV are needed.

Genetic characterization of duck Tembusu virus in Thailand, 2015-2017: Identification of a novel cluster

Duck Tembusu virus (DTMUV) infected cases have increasingly been observed in several duck farms in Thailand since its first report in 2013. However, information on the genetic characteristic of DTMUVs recently circulating in ducks in Thailand is limited. In this study, we investigated the geographic distribution and genetic characteristic of DTMUVs recently circulating in ducks in Thailand during 2015-2017. Of the 288 clinical samples obtained from 89 ducks farms located in duck raising areas of Thailand, 65 samples (22.57%) of 34 duck farms (38.20%) were DTMUV positive. Our results demonstrated that DTMUV was extensively distributed in duck raising areas of Thailand. Phylogenetic analysis of the E and NS5 genes revealed that DTMUVs circulating in Thailand were divided into three distinct clusters, including cluster 1, subcluster 2.1 and a novel cluster 3. Among these three clusters, subcluster 2.1 was a predominant cluster of DTMUV circulating in duck populations in Thailand during 2015-2017. It is interesting to note that a novel cluster of DTMUV (cluster 3), which was genetically different from any of the previously reported DTMUV clusters, was first identified in this study. In conclusion, our data demonstrated the circulation of different clusters of DTMUV and the presence of a novel DTMUV cluster in ducks in Thailand. This study highlights the high genetic diversity of DTMUVs in Thailand and the necessity of the routine surveillance of DTMUV for early detection, prevention and control of newly emerging DTMUVs.