Comparative study of the pathogenicity of the mosquito origin strain and duck origin strain of Tembusu virus in ducklings and three-week-old mice

The A487 residue in the E protein of duck Tembusu virus significantly enhances viral replication and increases its neurovirulence in Kunming mice

Tembusu virus (TMUV), an emerging avian orthoflavivirus, causes severe egg-drop syndrome and encephalitis in ducks. Although ducks are the natural host, mice serve as a valuable model for studying neuropathogenesis, as TMUV-infected mice recapitulate key neurological symptoms observed in ducks, such as paralysis and encephalitis. In the previous study, we observed that the TMUV strain CQW1 exhibited unexpectedly low neurovirulence in mice compared with earlier strains, highlighting potential genetic determinants of pathogenicity that may influence viral evolution and disease outcomes in natural hosts. In this study, we investigated the murine neurovirulence of TMUV strains from two major phylogenetic clusters (2.1 and 2.2). The Cluster 2.2 strain CHN-YC demonstrated markedly higher neurovirulence in Kunming mice than Cluster 2.1 strains (CQW1 and SCS01), with robust viral replication in the brain, pronounced histopathological damage, and elevated proinflammatory cytokine levels. Comparative genomic analysis identified seven amino acid substitutions in the E-NS1 region, with variations unique to Cluster 2.1 strains or specific to CQW1. By introducing these substitutions into CQW1 via reverse genetics, we restored high murine neurovirulence and identified the E protein substitution V487A as critical for this phenotype. Mechanistically, E-V487A enhances viral assembly, which boosts replication efficiency in vitro and in vivo. This substitution is located in the E protein transmembrane domain, a region implicated in flavivirus particle formation. Our data revealed that a naturally occurring amino acid substitution located in the transmembrane domain of the Tembusu virus E protein is responsible for its high neurovirulence in mice.

IMPORTANCE

Tembusu virus is a mosquito-borne avian orthoflavivirus, exhibiting airborne transmission. Although it primarily affects domestic fowl, TMUV demonstrates high neurovirulence in mice during laboratory studies and has been reported to spill over into humans. Recent years have seen increased genetic diversity and an expanded host range of the virus. Strains belonging to phylogenetic cluster 3 can cause severe neurological symptoms and death in mice via intranasal infection, further highlighting its risk of potential transmission to mammals. Understanding their pathogenicity and the underlying molecular basis is crucial for assessing and preventing health risks to mammals. We identified a single amino acid substitution in the TMUV E protein that critically enhances viral replication and neurovirulence in mice. The data provide insights into the molecular mechanisms of Tembusu virus pathogenesis in mammals and underscore the impact of specific genetic mutations on the viral phenotype.

A novel goose-origin Tembusu virus exhibits pathogenicity in day-old chicks with evidence of direct contact transmission

In late 2020, an outbreak of Tembusu virus (TMUV)-associated disease occurred in a 45-day-old white Roman geese flock in Taiwan. Here, we present the identification and isolation of a novel goose-origin TMUV strain designated as NTU/C225/2020. The virus was successfully isolated using minimal-pathogen-free duck embryos. Phylogenetic analysis of the polyprotein gene showed that NTU/C225/2020 clustered together with the earliest isolates from Malaysia and was most closely related to the first Taiwanese TMUV strain, TP1906. Genomic analysis revealed significant amino acid variations among TMUV isolates in NS1 and NS2A protein regions. In the present study, we characterized the NTU/C225/2020 culture in duck embryos, chicken embryos, primary duck embryonated fibroblasts, and DF-1 cells. All host systems were susceptible to NTU/C225/2020 infection, with observable lesions. In addition, animal experiments showed that the intramuscular inoculation of NTU/C225/2020 resulted in growth retardation and hyperthermia in day-old chicks. Gross lesions in the infected chicks included hepatomegaly, hyperemic thymus, and splenomegaly. Viral loads and histopathological damage were displayed in various tissues of both inoculated and naïve co-housed chicks, confirming the direct chick-to-chick contact transmission of TMUV. This is the first in vivo study of a local TMUV strain in Taiwan. Our findings provide essential information for TMUV propagation and suggest a potential risk of disease outbreak in chicken populations.