School of Biology and Ecology, University of Maine, 5722 Deering Hall, Orono, ME 04469.
Vector-Borne Disease Laboratory, Maine Medical Center Research Institute, 81 Research Drive, Scarborough, ME 04074.
Ticks Tick Borne Dis. 2022 Jan;13(1):101872. doi: 10.1016/j.ttbdis.2021.101872. Epub 2021 Nov 19.
The incidence and geographic range of vector-borne diseases have been expanding in recent decades, attributed in part to global climate change. Blacklegged ticks (Ixodes scapularis), the primary vector for multiple tick-borne pathogens in North America, are spreading rapidly beyond their historic post-colonial range and are thought to be constrained mainly by winter temperature at northern latitudes. Our research explored whether winter climate currently limits the distribution of blacklegged ticks and the pathogens they transmit in Maine, U.S.A., by contributing to overwinter mortality of nymphs. We experimentally tested tick overwinter survival across large-scale temperature and snowfall gradients and assessed factors contributing to winter mortality in locations where blacklegged tick populations are currently established and locations where the blacklegged tick has not yet been detected. We also tested the hypothesis that insulation in the tick microhabitat (i.e., by leaf litter and snowpack) can facilitate winter survival of blacklegged tick nymphs despite inhospitable ambient conditions. Overwinter survival was not significantly different in coastal southern compared to coastal and inland northern Maine, most likely due to sufficient snowpack that protected against low ambient temperatures at high latitudes. Snow cover and leaf litter contributed significantly to overwinter survival at sites in both southern and northern Maine. To further assess whether the current distribution of blacklegged ticks in Maine aligns with patterns of overwinter survival, we systematically searched for and collected ticks at seven sites along latitudinal and coastal-inland climate gradients across the state. We found higher densities of blacklegged ticks in coastal southern Maine (90.2 ticks/1000 m) than inland central Maine (17.8 ticks/1000 m) and no blacklegged ticks in inland northern Maine. Our results suggest that overwinter survival is not the sole constraint on the blacklegged tick distribution even under extremely cold ambient conditions and additional mechanisms may limit the continued northward expansion of ticks.
近年来，虫媒病的发病率和地理范围一直在扩大，部分原因是全球气候变化。黑腿蜱（Ixodes scapularis）是北美的多种蜱传病原体的主要媒介，其分布范围正在迅速超出其历史殖民后范围，并被认为主要受到北方纬度冬季温度的限制。我们的研究通过探讨冬季气候是否会通过促进若虫的越冬死亡率，从而限制美国缅因州黑腿蜱及其传播病原体的分布。我们通过在大规模的温度和降雪梯度上进行实验，测试了蜱的越冬存活率，并评估了在黑腿蜱种群目前存在的地点和尚未发现黑腿蜱的地点，导致冬季死亡率的因素。我们还测试了一个假设，即蜱微生境（即落叶和积雪）的隔热作用可以促进黑腿蜱若虫在冬季的生存，尽管环境条件恶劣。沿海南部与沿海和内陆北部缅因州相比，越冬存活率没有显著差异，这很可能是由于高纬度地区有足够的积雪保护，免受低温环境的影响。在缅因州南部和北部的地点，雪盖和落叶对越冬存活率有显著贡献。为了进一步评估黑腿蜱在缅因州的当前分布是否与越冬存活率模式一致，我们系统地在该州沿纬度和沿海-内陆气候梯度的七个地点进行了搜索和采集蜱。我们发现，沿海南部缅因州（90.2 只/1000m）的黑腿蜱密度高于内陆中部缅因州（17.8 只/1000m），而内陆北部缅因州没有黑腿蜱。我们的结果表明，即使在极端寒冷的环境条件下，越冬存活率并不是限制黑腿蜱分布的唯一因素，其他机制可能会限制蜱的进一步向北扩张。
