Genetic structure and Rickettsia infection rates in Ixodes ovatus and Haemaphysalis flava ticks across different altitudes.

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 Rickettsia infection rates influenced by the population genetic structure of I.ovatus and H. flava 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. The population genetic structure was analyzed utilizing 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. A significant difference was observed in Rickettsia infection rates and mean altitude per group between the two cluster groups and the three genetic groups identified within I. ovatus. No significant differences were found in the mean altitude or Rickettsia infection rates of H. flava. Our results suggest a potential correlation between the low gene flow in I. ovatus populations and the spatially heterogeneous Rickettsia infection rates observed 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.

Comparative population genetic structure of two ixodid tick species (Acari:Ixodidae) (Ixodes ovatus and Haemaphysalis flava) in Niigata prefecture, Japan.

Ixodid ticks (Acari:Ixodidae) are essential vectors of tick-borne diseases in Japan. In this study, we characterized the population genetic structure and inferred genetic divergence in two widespread and abundant ixodid species, Ixodes ovatus and Haemaphysalis flava. Our hypothesis was that genetic divergence would be high in I. ovatus because of the low mobility of their small rodent hosts of immature I. ovatus would limit their gene flow compared to more mobile avian hosts of immature H. flava. We collected 320 adult I. ovatus from 29 locations and 223 adult H. flava from 17 locations across Niigata Prefecture, Japan, and investigated their genetic structure using DNA sequences from fragments of two mitochondrial gene regions, cox1 and the 16S rRNA gene. For I. ovatus, pairwise FST and analysis of molecular variance (AMOVA) analyses of cox1 and 16S sequences indicated significant genetic variation among populations, whereas both markers showed non-significant genetic variation among locations for H. flava. A cox1 gene tree and haplotype network revealed three genetic groups of I. ovatus. One of these groups consisted of haplotypes distributed at lower altitudes (251-471 m.a.s.l.). The cox1 sequences of I. ovatus from Japan clustered separately from I. ovatus sequences reported from China, suggesting the potential for cryptic species in Japan. Our results support our hypothesis and suggest that the host preference of ticks at the immature stage may influence the genetic structure of the ticks. This information may be important for understanding the tick-host interactions in the field to better understand the tick-borne disease transmission and in designing an effective tick control program.

The influence of roads on the fine-scale population genetic structure of the dengue vector Aedes aegypti (Linnaeus).

Dengue is endemic in tropical and subtropical countries and is transmitted mainly by Aedes aegypti. Mosquito movement can be affected by human-made structures such as roads that can act as a barrier. Roads can influence the population genetic structure of Ae. aegypti. We investigated the genetic structure and gene flow of Ae. aegypti as influenced by a primary road, España Boulevard (EB) with 2000-meter-long stretch and 24-meters-wide in a very fine spatial scale. We hypothesized that Ae. aegypti populations separated by EB will be different due to the limited gene flow as caused by the barrier effect of the road. A total of 359 adults and 17 larvae Ae. aegypti were collected from June to September 2017 in 13 sites across EB. North (N1-N8) and South (S1-S5) comprised of 211 and 165 individuals, respectively. All mosquitoes were genotyped at 11 microsatellite loci. AMOVA FST indicated significant genetic differentiation across the road. The constructed UPGMA dendrogram found 3 genetic groups revealing the clear separation between North and South sites across the road. On the other hand, Bayesian cluster analysis showed four genetic clusters (K = 4) wherein each individual samples have no distinct genetic cluster thus genetic admixture. Our results suggest that human-made landscape features such as primary roads are potential barriers to mosquito movement thereby limiting its gene flow across the road. This information is valuable in designing an effective mosquito control program in a very fine spatial scale.

Diversity and distribution of ticks in Niigata prefecture, Japan (2016-2018): Changes since 1950.

We performed tick surveys in all regions (Kaetsu, Chuetsu, Joetsu, and Sado) of the Niigata prefecture, Japan. A total of 105 field surveys were done from 2016 to 2018 in 41 sites, from north to south, in the prefecture. All 4806 ticks collected were identified and classified by species, sex, and developmental stage. Twelve species were recorded: Dermacentor taiwanensis, Haemaphysalis flava, Haemaphysalis hystricis, Haemaphysalis japonica, Haemaphysalis longicornis, Haemaphysalis megaspinosa, Ixodes ovatus, Ixodes nipponensis, Ixodes persulcatus, Ixodes monospinosus, Ixodes columnae, and Ixodes turdus. The major tick species in Niigata prefecture were H. flava, H. longicornis, and I. ovatus and they comprised 93.4% of all samples. These three species have one generation per year. Climatic and anthropogenic factors may be involved in the substantial change of the endemic species composition from a previous tick survey (1959) in the Niigata prefecture. These factors include increasing temperatures, introduction of new hosts such as the wild boar, highway construction, and a rural exodus facilitating animal migration and reproduction. Tick hosts suitable for the transmission of Japanese spotted fever, Lyme borreliosis, and SFTS occur in Niigata prefecture. Heightened awareness of these three tick-borne diseases is needed for preparation and disease prevention.

Spotted fever group rickettsiae (SFGR) detection in ticks following reported human case of Japanese spotted fever in Niigata Prefecture, Japan.

Japanese spotted fever, a tick-borne disease caused by Rickettsia japonica, was firstly described in southwestern Japan. There was a suspicion of Rickettsia japonica infected ticks reaching the non-endemic Niigata Prefecture after a confirmed case of Japanese spotted fever in July 2014. Therefore, from 2015 to 2017, 38 sites were surveyed and rickettsial pathogens were investigated in ticks from north to south of Niigata Prefecture including Sado island. A total of 3336 ticks were collected and identified revealing ticks of three genera and ten species: Dermacentor taiwanensis, Haemaphysalis flava, Haemaphysalis hystricis, Haemaphysalis longicornis, Haemaphysalis megaspinosa, Ixodes columnae, Ixodes monospinosus, Ixodes nipponensis, Ixodes ovatus, and Ixodes persulcatus. Investigation of rickettsial DNA showed no ticks infected by R. japonica. However, three species of spotted fever group rickettsiae (SFGR) were found in ticks, R. asiatica, R. helvetica, and R. monacensis, confirming Niigata Prefecture as a new endemic area to SFGR. These results highlight the need for public awareness of the occurrence of this tick-borne disease, which necessitates the establishment of public health initiatives to mitigate its spread.