Department of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurerstrasse 190, 8057, Zurich, Switzerland.
Centre for Ecology and Conservation, University of Exeter, Penryn, TR10 9FE, UK.
BMC Ecol Evol. 2022 Aug 26;22(1):104. doi: 10.1186/s12862-022-02058-x.
Parasites can alter host and vector phenotype and thereby affect ecological processes in natural populations. Laboratory studies have suggested that Borrelia burgdorferi sensu lato, the causative agent of human Lyme borreliosis, may induce physiological and behavioural alterations in its main tick vector in Europe, Ixodes ricinus, which increase the tick's mobility and survival under challenging conditions. These phenotypic alterations may allow I. ricinus to colonise marginal habitats ('facilitation hypothesis'), thereby fuelling the ongoing range expansion of I. ricinus towards higher elevations and latitudes induced by climate change. To explore the potential for such an effect under natural conditions, we studied the prevalence of B. burgdorferi s.l. in questing I. ricinus and its variation with elevation in the Swiss Alps.
We screened for B. burgdorferi s.l. infection in questing nymphs of I. ricinus (N = 411) from 15 sites between 528 and 1774 m.a.s.l to test if B. burgdorferi s.l. prevalence is higher at high elevations (i.e. in marginal habitats). Opposite of what is predicted under the facilitation hypothesis, we found that B. burgdorferi s.l. prevalence in I. ricinus nymphs decreased with increasing elevation and that Borrelia prevalence was 12.6% lower in I. ricinus nymphs collected at the range margin compared to nymphs in the core range. But there was no association between Borrelia prevalence and elevation within the core range of I. ricinus. Therefore the observed pattern was more consistent with a sudden decrease in Borrelia prevalence above a certain elevation, rather than a gradual decline with increasing elevation across the entire tick range.
In conclusion, we found no evidence that B. burgdorferi s.l.-induced alterations of I. ricinus phenotype observed in laboratory studies facilitate the colonisation of marginal habitats in the wild. Rather, ticks in marginal habitats are substantially less likely to harbour the pathogen. These findings have implications for a better understanding of eco-evolutionary processes in natural host-parasite systems, as well as the assessment of Lyme borreliosis risk in regions where I. ricinus is newly emerging.
寄生虫可以改变宿主和媒介的表型，从而影响自然种群中的生态过程。实验室研究表明，伯氏疏螺旋体（Borrelia burgdorferi），即人类莱姆病的病原体，可能会导致其在欧洲的主要蜱媒介——蓖子硬蜱（Ixodes ricinus）产生生理和行为上的改变，从而增加蜱在挑战性条件下的移动性和存活率。这些表型改变可能使蓖子硬蜱能够在边缘栖息地定居（“促进假说”），从而助长因气候变化而导致的蓖子硬蜱向更高海拔和纬度的持续扩张。为了在自然条件下探索这种影响的潜力，我们研究了伯氏疏螺旋体在瑞士阿尔卑斯山的求偶硬蜱中的流行程度及其随海拔高度的变化。
我们从海拔 528 至 1774 米的 15 个地点筛选了 411 只求偶硬蜱若虫，以检测伯氏疏螺旋体是否在高海拔（即边缘栖息地）的感染率更高（即，是否符合促进假说）。结果与促进假说的预测相反，我们发现硬蜱若虫中伯氏疏螺旋体的感染率随着海拔的升高而降低，与核心区域的若虫相比，在分布范围边缘采集的若虫中伯氏疏螺旋体的感染率低 12.6%。但在硬蜱的核心分布范围内，伯氏疏螺旋体的感染率与海拔之间没有关联。因此，观察到的模式更符合在一定海拔以上伯氏疏螺旋体感染率突然下降，而不是整个蜱分布范围内随着海拔升高而逐渐下降的模式。
综上所述，我们没有发现证据表明实验室研究中观察到的伯氏疏螺旋体诱导的硬蜱表型改变有助于在野外边缘栖息地的定居。相反，在边缘栖息地的蜱虫不太可能携带病原体。这些发现对于更好地理解自然宿主-寄生虫系统中的生态进化过程，以及在新出现蓖子硬蜱的地区评估莱姆病风险具有重要意义。
