Assessing the relative roles of systemic, non-systemic and transovarial transmission pathways for severe fever with thrombocytopaenia syndrome virus and its implications for future research and intervention strategies

Severe fever with thrombocytopaenia syndrome virus (SFTSV) was identified by the World Health Organization as a priority pathogen due to its high case-fatality rate in humans and rapid spread. It is maintained in nature through three transmission pathways: systemic, non-systemic and transovarial. Understanding the relative contributions of these transmission pathways is crucial for developing evidence-informed public health interventions to reduce its spillover risks to humans. Using next-generation matrices, sensitivity analyses, elasticity analyses and random forest models, we estimated the basic reproduction number R0, relative contribution of each pathway, and identified the most sensitive model parameters across 27 scenarios. Results showed that R0 ranged from 0.72 to 2.08 across scenarios, increasing with higher tick abundance and longer viraemia. Transovarial transmission dominated in 26 scenarios, while the importance of the other two varied, with non-systemic transmission more important under high tick abundance, short viraemia or aggregated tick distribution. R0 dropped below 1 in all scenarios when transovarial transmission was excluded. These findings emphasize the need for interventions targeting transovarial transmission, such as reducing female adult tick survival and limiting large vertebrate host movement, and underscore the importance of laboratory studies measuring sensitive parameters including transovarial transmission efficiency, tick survival probabilities and the duration of viraemia and potential for non-systemic transmission for key animal host species with high seroprevalence rates.