Haemadipsa japonica is a Japanese land leech that feeds on the blood of host animals, mainly the sika deer (Cervus nippon). The distribution of H. japonica has expanded since the 1990s, possibly due to sika deer population increases.Haemadipsa japonica populations were found in the northern and southern regions of Tochigi Prefecture, central Japan. Previous investigations reported that these populations are genetically differentiated between the north and south, although the sika deer distribution extends throughout the northern and southern regions. We hypothesized that the migration of sika deer might be limited, thus maintaining the two H. japonica genetic groups and restricting its range expansion. We evaluated this hypothesis by investigating the genetic structure of the mitochondrial DNA (mtDNA) and nuclear microsatellites (nSSR) of sika deer samples collected from Tochigi Prefecture and from H. japonica bloodmeal samples (invertebrate-derived DNA; iDNA). The sika deer mtDNA presented two haplotypes (G-and A-haplotypes) without a clear genetic structure. Conversely, the sika deer nSSR analysis indicated clear genetic differentiation between deer in the northern and southern regions, corresponding to the two H. japonica genetic groups. This indicates that sika deer movement is restricted to narrow regions in Tochigi Prefecture. Therefore, we conclude that the current H. japonica expansion in the prefecture likely depends on the migration of sika deer, only occurring within the northern and southern regions, even though this is driven by sika deer migration.

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