Natural fatal infection of Tembusu virus in bottlenose dolphins in Thailand.

Interspecies transmission of viruses poses significant risks to animal and human health. Tembusu virus (TMUV), an emerging flavivirus, is primarily associated with avian diseases. This study reports the first documented natural infection of TMUV in mammals, specifically zoo dolphins in Thailand, offering insights into its evolution, transmission dynamics, and zoonotic potential. In July 2023, three bottlenose dolphins developed neurological symptoms and died. Postmortem analyses, including histopathology, immunohistochemistry, high-throughput sequencing, and transmission electron microscopy, confirmed TMUV infection. Viral loads were high in brain and lung tissues, followed by kidney and spleen whereas the TMUV antigen was identified in only brain tissue. TMUV was localized in neurons and astroglia cells, and immunohistochemistry revealed CD3-positive T lymphocyte perivascular cuffing in the brain. Phylogenetic analysis placed the dolphin TMUV strains within cluster 3, related to strains found in mosquitoes in China. Retrospective analysis of dolphin samples from 2019 confirmed persistent TMUV circulation. Viral isolation on Vero cells showed characteristic cytopathic effects, and transmission electron microscopy revealed enveloped virions. This study highlights the virus's ability to infect diverse hosts, including mammals. The findings underscore the need for continuous surveillance and a One Health approach to mitigate emerging viral threats.