Symptomatic human norovirus infection in zebrafish embryos uncovers neural infection and extracellular vesicle-mediated transmission dynamics
Human norovirus (hNoV) is the leading global cause of acute gastroenteritis, imposing a substantial health and economic burden worldwide. Progress in understanding hNoV pathogenesis has been hindered by the lack of tractable small-animal models that recapitulate symptomatic infection. Although zebrafish larvae support hNoV replication, infection remains asymptomatic, limiting their utility for studying disease mechanisms and host-pathogen interactions. In this study, we report that the zebrafish embryo infection model, in which microinjection of hNoV at the early cell stage, resulted in robust systemic viral replication accompanied by overt pathological manifestations, including pericardial and renal edema, yolk and cranial opacity, and mortality by 3 days post-infection. Disease severity displayed marked individual variability and correlated closely with viral burden. Integrated multi-omics analyses, including bulk transcriptomics, untargeted metabolomics, and single-cell RNA sequencing, demonstrated that embryonic infection elicits a stronger and more coordinated antiviral response than larval-stage infection, while enabling widespread viral dissemination across diverse cell lineages. Approximately two-thirds of infected cells were derived from the nervous system or neural crest lineages, providing a potential mechanistic basis for the neurological complications occasionally reported in hNoV-infected patients. Furthermore, we identified a developmental stage-dependent role for extracellular vesicle (EV)-associated hNoV transmission: free virions mediated more efficient infection and higher symptomatic incidence in immunologically immature embryos, whereas EV-associated virions exhibited enhanced infectivity in more immunocompetent larvae. Together, these findings establish the zebrafish embryo as a versatile and accessible in vivo platform for studying symptomatic hNoV infection, reveal host maturity-dependent viral transmission strategies, and provide new opportunities for mechanistic studies and high-throughput evaluation of antiviral and vaccine candidates. Author summaryHuman norovirus is the leading cause of stomach flu worldwide but studying it has been difficult because the lack of a simple, small-animal model that actually gets sick from the virus. While older zebrafish larvae can harbor the virus, they do not show symptoms. In this study, we successfully created a new model by injecting human norovirus into zebrafish embryos at their early cell stage. Unlike the older larvae, these embryos developed clear symptoms, including fluid buildup around the heart and kidneys, tissue cloudiness, and death within three days. Using advanced genetic and metabolic tracking, we discovered that the virus spreads widely throughout the embryos body, particularly targeting cells in the nervous system. This link might explain why human patients occasionally suffer from neurological symptoms. Additionally, our study revealed that the virus changes its transmission strategy based on the animals age: it travels freely to infect vulnerable embryos but hides inside lipid vesicles to infect older, more immune-developed larvae. Ultimately, these findings provide a practical, efficient animal model to better understand norovirus sickness and rapidly test new vaccines and treatments.