Division of Vector-Borne Diseases, National Center for Emerging and Zoonotic Infectious Diseases, Centers for Disease Control and Prevention, 3156 Rampart Road, Fort Collins, CO 80521, United States.
Vector-Borne Disease Section, California Department of Public Health, 1616 Capitol Ave, Sacramento, CA 95814, United States.
Ticks Tick Borne Dis. 2024 May;15(3):102325. doi: 10.1016/j.ttbdis.2024.102325. Epub 2024 Feb 21.
Ixodes pacificus (the western blacklegged tick) occurs in the far western United States (US), where it commonly bites humans. This tick was not considered a species of medical concern until it was implicated in the 1980s as a vector of Lyme disease spirochetes. Later, it was discovered to also be the primary vector to humans in the far western US of agents causing anaplasmosis and hard tick relapsing fever. The core distribution of I. pacificus in the US includes California, western Oregon, and western Washington, with outlier populations reported in Utah and Arizona. In this review, we provide a history of the documented occurrence of I. pacificus in the US from the 1890s to present, and discuss associations of its geographic range with landscape, hosts, and climate. In contrast to Ixodes scapularis (the blacklegged tick) in the eastern US, there is no evidence for a dramatic change in the geographic distribution of I. pacificus over the last half-century. Field surveys in the 1930s and 1940s documented I. pacificus along the Pacific Coast from southern California to northern Washington, in the Sierra Nevada foothills, and in western Utah. County level collection records often included both immatures and adults of I. pacificus, recovered by drag sampling or from humans, domestic animals, and wildlife. The estimated geographic distribution presented for I. pacificus in 1945 by Bishopp and Trembley is similar to that presented in 2022 by the Centers for Disease Control and Prevention. There is no clear evidence of range expansion for I. pacificus, separate from tick records in new areas that could have resulted from newly initiated or intensified surveillance efforts. Moreover, there is no evidence from long-term studies that the density of questing I. pacificus ticks has increased over time in specific areas. It therefore is not surprising that the incidence of Lyme disease has remained stable in the Pacific Coast states from the early 1990s, when it became a notifiable condition, to present. We note that deforestation and deer depredation were less severe in the far western US during the 1800s and early 1900s compared to the eastern US. This likely contributed to I. pacificus maintaining stable, widespread populations across its geographic range in the far western US in the early 1900s, while I. scapularis during the same time period appears to have been restricted to a small number of geographically isolated refugia sites within its present range in the eastern US. The impact that a warming climate may have had on the geographic distribution and local abundance of I. pacificus in recent decades remains unclear.
太平洋硬蜱（西部黑腿蜱）分布于美国远西部，常叮咬人类。直到 20 世纪 80 年代，这种蜱被认为是莱姆病螺旋体的传播媒介，才开始引起人们的关注。后来，人们发现它也是美国远西部引起无形体病和硬蜱复发性发热的主要媒介。太平洋硬蜱在美国的核心分布区包括加利福尼亚州、俄勒冈州西部和华盛顿州西部，犹他州和亚利桑那州也有报道。在本文中，我们回顾了自 19 世纪 90 年代至今美国太平洋硬蜱的文献记录，并讨论了其地理分布与景观、宿主和气候的关系。与美国东部的黑腿蜱（Ixodes scapularis）不同，在过去的半个世纪里，太平洋硬蜱的地理分布没有明显变化的证据。20 世纪 30 年代和 40 年代的实地调查记录表明，太平洋硬蜱分布于南加州至北华盛顿的太平洋沿岸、内华达山脉山麓以及犹他州西部。县级采集记录通常包括通过拖拽采样或从人类、家畜和野生动物身上采集到的太平洋硬蜱的幼蜱和成虫。1945 年，比肖普和特伦布莱（Bishopp and Trembley）对太平洋硬蜱的地理分布估计与 2022 年疾病控制与预防中心（Centers for Disease Control and Prevention）的报告相似。没有明显证据表明太平洋硬蜱的分布范围在扩大，除了在新地区的蜱类记录可能是由于新的或强化的监测工作造成的。此外，长期研究也没有表明在特定地区，太平洋硬蜱的数量密度随时间增加。因此，自 20 世纪 90 年代初，当莱姆病成为法定报告疾病以来，太平洋沿岸各州的莱姆病发病率一直保持稳定，这并不奇怪。我们注意到，与美国东部相比，19 世纪和 20 世纪初，美国远西部的森林砍伐和鹿群破坏程度较轻。这可能有助于太平洋硬蜱在 20 世纪初保持其地理分布范围的稳定和广泛种群，而在同一时期，黑腿蜱似乎局限于美国东部目前分布范围内少数几个地理上孤立的避难所。气候变暖对太平洋硬蜱在最近几十年的地理分布和局部丰度的影响尚不清楚。
