Tick Surveillance for Relapsing Fever Spirochete <i>Borrelia miyamotoi</i> in Hokkaido, Japan

During 2012–2013, a total of 4325 host-seeking adult ticks belonging to the genus Ixodes were collected from various localities of Hokkaido, the northernmost island of Japan. Tick lysates were subjected to real-time PCR assay to detect borrelial infection. The assay was designed for specific detection of the Relapsing fever spirochete Borrelia miyamotoi and for unspecific detection of Lyme disease-related spirochetes. Overall prevalence of B. miyamotoi was 2% (71/3532) in Ixodes persulcatus, 4.3% (5/117) in Ixodes pavlovskyi and 0.1% (1/676) in Ixodes ovatus. The prevalence in I. persulcatus and I. pavlovskyi ticks were significantly higher than in I. ovatus. Co-infections with Lyme disease-related spirochetes were found in all of the tick species. During this investigation, we obtained 6 isolates of B. miyamotoi from I. persulcatus and I. pavlovskyi by culture in BSK-M medium. Phylogenetic trees of B. miyamotoi inferred from each of 3 housekeeping genes (glpQ, 16S rDNA, and flaB) demonstrated that the Hokkaido isolates were clustered with Russian B. miyamotoi, but were distinguishable from North American and European B. miyamotoi. A multilocus sequence analysis using 8 genes (clpA, clpX, nifS, pepX, pyrG, recG, rplB, and uvrA) suggested that all Japanese B. miyamotoi isolates, including past isolates, were genetically clonal, although these were isolated from different tick and vertebrate sources. From these results, B. miyamotoi-infected ticks are widely distributed throughout Hokkaido. Female I. persulcatus are responsible for most human tick-bites, thereby I. persulcatus is likely the most important vector of indigenous relapsing fever from tick bites in Hokkaido.2012—2013年间，研究人员从日本最北端的岛屿北海道的多个采样点位，共采集到4325只寻找宿主的成年硬蜱属（Ixodes）蜱虫。随后对蜱虫裂解液开展实时PCR（real-time PCR）检测，以甄别疏螺旋体感染情况。该检测方法可特异性检出回归热螺旋体宫本疏螺旋体（Borrelia miyamotoi），同时可非特异性检测莱姆病相关螺旋体。宫本疏螺旋体在三种受试蜱种中的总体感染率分别为：全沟硬蜱（Ixodes persulcatus）2%（71/3532）、帕氏硬蜱（Ixodes pavlovskyi）4.3%（5/117）以及卵形硬蜱（Ixodes ovatus）0.1%（1/676）。全沟硬蜱与帕氏硬蜱的感染率显著高于卵形硬蜱。所有受试蜱种均检出与莱姆病相关螺旋体的共感染现象。本研究通过BSK-M培养基培养，从全沟硬蜱与帕氏硬蜱中分离获得6株宫本疏螺旋体。基于3个持家基因（glpQ、16S rDNA及flaB）分别构建的系统发育树显示，北海道分离株与俄罗斯宫本疏螺旋体聚类为同一分支，且可与北美及欧洲的宫本疏螺旋体相区分。利用8个基因（clpA、clpX、nifS、pepX、pyrG、recG、rplB及uvrA）开展的多位点序列分析表明，尽管所有日本宫本疏螺旋体分离株（含既往分离株）的宿主来源涵盖不同蜱种与脊椎动物类群，但均呈现遗传克隆性特征。综合上述结果可知，携带宫本疏螺旋体的蜱类广泛分布于北海道全域。由于全沟硬蜱雌蜱是引发绝大多数人类蜱叮咬事件的主要媒介，因此该蜱种极有可能是北海道本土蜱传回归热的最重要传播媒介。