Rickettsia infection rate along an altitudinal gradient as influenced by population genetic structure of Ixodid ticks

Ixodid ticks, such as Ixodes ovatus and Haemaphysalis flava, are important vectors of tick-borne diseases in Japan, such as Japanese spotted fever caused by Rickettsia japonica. This study describes the population genetic structure and gene flow of I. ovatus and H. flava and their Rickettsia infection rates along an altitudinal gradient. A total of 346 adult I. ovatus and 243 H. flava were analyzed for the presence of Rickettsia by nested PCR targeting the 17kDA, gltA, rOmpA, and rOmpB genes. Population genetic structure was performed using the mitochondrial cytochrome oxidase 1 (cox1) marker. The Rickettsia infection rates were 13.26% in I. ovatus and 6.17% in H. flava. For I. ovatus, the global FST value revealed significant genetic differentiation among the different populations, whereas H. flava showed non-significant genetic differentiation. The cox1 I. ovatus cluster dendrogram showed two cluster groups, while the haplotype network and phylogenetic tree showed three genetic groups. The two cluster groups and the three genetic groups found in I. ovatus showed a significant difference in their Rickettsia infection rates and mean altitude per group. No significant differences were found in the mean altitude or Rickettsia infection rates of H. flava. Our results suggest that low gene flow in the I. ovatus populations has caused spatially heterogenous Rickettsia infection rates along the altitudinal gradient. This information can be used in understanding the relationship between the tick vector, its pathogen, and environmental factors, such as altitude, and for the control of tick-borne diseases in Japan.