Johnson Tammi L, Fischer Robert J, Raffel Sandra J, Schwan Tom G
Laboratory of Zoonotic Pathogens, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT, USA.
Division of Vector-Borne Diseases, Centers for Disease Control and Prevention, Fort Collins, CO, USA.
Parasit Vectors. 2016 Nov 10;9(1):575. doi: 10.1186/s13071-016-1863-0.
An unrecognized focus of tick-borne relapsing fever caused by Borrelia hermsii was identified in 2002 when five people became infected on Wild Horse Island in Flathead Lake, Montana. The terrestrial small mammal community on the island is composed primarily of pine squirrels (Tamiasciurus hudsonicus) and deer mice (Peromyscus maniculatus), neither of which was known as a natural host for the spirochete. Thus a 3-year study was performed to identify small mammals as hosts for B. hermsii.
Small mammals were captured alive on two island and three mainland sites, blood samples were collected and examined for spirochetes, and serological tests performed to detect anti-B. hermsii antibodies. Ornithodoros hermsi ticks were collected and fed on laboratory mice to assess infection. Genomic DNA samples from spirochetes isolated from infected mammals and ticks were analyzed by multilocus sequence typing.
Eighteen pine squirrels and one deer mouse had detectable spirochetemias when captured, from which 12 isolates of B. hermsii were established. Most pine squirrels were seropositive, and the five species of sciurids combined had a significantly higher prevalence of seropositive animals than did the other six small mammal species captured. The greater diversity of small mammals on the mainland in contrast to the islands demonstrated that other species in addition to pine squirrels were also involved in the maintenance of B. hermsii at Flathead Lake. Ornithodoros hermsi ticks produced an additional 12 isolates of B. hermsii and multilocus sequence typing identified both genomic groups of B. hermsii described previously, and identified a new genomic subdivision. Experimental infections of deer mice with two strains of B. hermsii demonstrated that these animals were susceptible to infection with spirochetes belonging to Genomic Group II but not Genomic Group I.
Pine squirrels are the primary hosts for the maintenance of B. hermsii on the islands in Flathead Lake, however serological evidence showed that numerous additional species are also involved on the mainland. Future studies testing the susceptibility of several small mammal species to infection with different genetic types of B. hermsii will help define their role as hosts in this and other endemic foci.
2002年，在蒙大拿州弗拉特黑德湖的野马岛上，5人感染了由赫氏疏螺旋体引起的蜱传回归热，但此前该感染源未被发现。岛上的陆生小型哺乳动物群落主要由松鼠（美洲红松鼠）和鹿鼠组成，而这两种动物都不被认为是这种螺旋体的天然宿主。因此，开展了一项为期3年的研究，以确定作为赫氏疏螺旋体宿主的小型哺乳动物。
在两个岛屿和三个大陆地点活捉小型哺乳动物，采集血样并检查其中的螺旋体，同时进行血清学检测以检测抗赫氏疏螺旋体抗体。采集赫氏钝缘蜱并让其叮咬实验室小鼠以评估感染情况。对从受感染的哺乳动物和蜱中分离出的螺旋体的基因组DNA样本进行多位点序列分型分析。
捕获的18只松鼠和1只鹿鼠血液中可检测到螺旋体血症，从中分离出12株赫氏疏螺旋体。大多数松鼠血清呈阳性，与其他6种捕获的小型哺乳动物相比，5种松鼠的血清阳性动物患病率显著更高。与岛屿相比，大陆上小型哺乳动物种类更多，这表明除松鼠外，其他物种也参与了弗拉特黑德湖赫氏疏螺旋体的传播。赫氏钝缘蜱又分离出12株赫氏疏螺旋体，多位点序列分型鉴定出了先前描述的赫氏疏螺旋体的两个基因组群，并确定了一个新的基因组亚群。用两株赫氏疏螺旋体对鹿鼠进行实验性感染，结果表明这些动物对基因组群II的螺旋体感染敏感，但对基因组群I的螺旋体不敏感。
松鼠是弗拉特黑德湖岛屿上赫氏疏螺旋体传播的主要宿主，然而血清学证据表明，大陆上还有许多其他物种也参与其中。未来针对几种小型哺乳动物对不同基因类型赫氏疏螺旋体感染易感性的研究，将有助于明确它们在这个以及其他地方疫源地作为宿主的作用。
