Detection of suitable habitat areas in Japan of the Lyme disease and tick-borne encephalitis vectors Ixodes ovatus and Ixodes persulcatus based on abiotic factors.

In Japan, Lyme disease and tick-borne encephalitis (TBE) are primarily in the northernmost prefecture Hokkaido, where their primary vectors Ixodes ovatus and Ixodes persulcatus are most abundant. Recently, tick surveillance activities have collected both species across Japan, indicating potential expansion of their tick-borne pathogens (TBPs). We built a machine-learning (ML) model using available tick surveillance and environmental data to predict suitable habitat areas for I. ovatus and I. persulcatus and identify potential higher-risk areas of Lyme disease and TBE across Japan. Data on the occurrence and abundance of 11 vector tick species between 1990 and 2023 were extracted from studies identified via systematic literature search in two online databases or provided by local experts for ML development. After multiple iterations and permutations, separate Random Forest ML algorithms for I. ovatus and I. persulcatus were trained via 26 abiotic variables of climate and topography based on the occurrence and abundance respective to each species. Data on 93,289 ticks from 57 sources were extracted, and the ML algorithms' area under the curves were high (> 0.89). Climate-related variables were the strongest predictors (> 90% cumulative model importance) for both I. ovatus and I. persulcatus. High suitability for both species was identified in Hokkaido and cooler, wetter regions in central Honshu, while I. ovatus had a broader ecological niche than I. persulcatus, with moderate suitability in mountainous regions of central Kyushu and surrounding the Tokyo Bay area. Our ML models suggest high suitability areas for Ixodes vectors may be widespread in Japan, indicating expanding potential risks of TBPs.