Importance of Considering Seasonality in Tick Activity When Assessing Spatial Expansion Potential: A Case Study on Haemaphysalis longicornis.

Haemaphysalis longicornis, a tick vector of the severe fever with thrombocytopenia syndrome (SFTS) virus, has long been established in parts of East Asia and Oceania but is now rapidly expanding in the eastern US, raising significant concerns about its further domestic and international spread. In this study, we mapped the spatial expansion risk of H. longicornis in North America and Europe by training a habitat suitability model with its occurrence data from East Asia and Oceania. Our model incorporated ecologically relevant predictors for tick survival, which have been neglected in previous habitat suitability modeling for this species. Specifically, we employed temperature, relative humidity, saturation deficit, precipitation, and vegetation level as predictors, with the first three distinguishing between tick questing and non-questing seasons to account for their varying impacts on tick survival during periods of activity and dormancy. The best-fitting model included seasonal saturation deficit and temperature as predictors along with vegetation, with the threshold values of saturation deficit and temperature for suitability closely aligning with existing literature. Spatial projections based on the best-fitting model most closely delineated the current boundaries of H. longicornis occurrences in the eastern US. In contrast, for the western US, where no H. longicornis has been detected yet, the model predicted only a few isolated coastal areas as suitable. This finding contrasts with earlier studies, which projected extensive suitable areas along the coast and extending inland. In Europe, the model also identified limited suitability, mainly confined to coastal areas of southern Europe. In conclusion, by incorporating meteorological predictors that account for seasonal tick activity, our findings reshape the understanding of H. longicornis spatial expansion risk in western North America and Europe, underscoring the need for refined and targeted surveillance strategies.