Medical Entomology Unit, Department of Systematic Biology, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18d, SE-752 36 Uppsala, Sweden.
Parasit Vectors. 2012 Jan 10;5:8. doi: 10.1186/1756-3305-5-8.
Ixodes ricinus is the main vector in Europe of human-pathogenic Lyme borreliosis (LB) spirochaetes, the tick-borne encephalitis virus (TBEV) and other pathogens of humans and domesticated mammals. The results of a previous 1994 questionnaire, directed at people living in Central and North Sweden (Svealand and Norrland) and aiming to gather information about tick exposure for humans and domestic animals, suggested that Ixodes ricinus ticks had become more widespread in Central Sweden and the southern part of North Sweden from the early 1980s to the early 1990s. To investigate whether the expansion of the tick's northern geographical range and the increasing abundance of ticks in Sweden were still occurring, in 2009 we performed a follow-up survey 16 years after the initial study.
A questionnaire similar to the one used in the 1994 study was published in Swedish magazines aimed at dog owners, home owners, and hunters. The questionnaire was published together with a popular science article about the tick's biology and role as a pathogen vector in Sweden. The magazines were selected to get information from people familiar with ticks and who spend time in areas where ticks might be present.
Analyses of data from both surveys revealed that during the near 30-year period from the early 1980s to 2008, I. ricinus has expanded its distribution range northwards. In the early 1990s ticks were found in new areas along the northern coastline of the Baltic Sea, while in the 2009 study, ticks were reported for the first time from many locations in North Sweden. This included locations as far north as 66°N and places in the interior part of North Sweden. During this 16-year period the tick's range in Sweden was estimated to have increased by 9.9%. Most of the range expansion occurred in North Sweden (north of 60°N) where the tick's coverage area doubled from 12.5% in the early 1990s to 26.8% in 2008. Moreover, according to the respondents, the abundance of ticks had increased markedly in LB- and TBE-endemic areas in South (Götaland) and Central Sweden.
The results suggest that I. ricinus has expanded its range in North Sweden and has become distinctly more abundant in Central and South Sweden during the last three decades. However, in the northern mountain region I. ricinus is still absent. The increased abundance of the tick can be explained by two main factors: First, the high availability of large numbers of important tick maintenance hosts, i.e., cervids, particularly roe deer (Capreolus capreolus) during the last three decades. Second, a warmer climate with milder winters and a prolonged growing season that permits greater survival and proliferation over a larger geographical area of both the tick itself and deer. High reproductive potential of roe deer, high tick infestation rate and the tendency of roe deer to disperse great distances may explain the range expansion of I. ricinus and particularly the appearance of new TBEV foci far away from old TBEV-endemic localities. The geographical presence of LB in Sweden corresponds to the distribution of I. ricinus. Thus, LB is now an emerging disease risk in many parts of North Sweden. Unless countermeasures are undertaken to keep the deer populations, particularly C. capreolus and Dama dama, at the relatively low levels that prevailed before the late 1970s--especially in and around urban areas where human population density is high--by e.g. reduced hunting of red fox (Vulpes vulpes) and lynx (Lynx lynx), the incidences of human LB and TBE are expected to continue to be high or even to increase in Sweden in coming decades.
在欧洲，硬蜱是人类莱姆病（LB）螺旋体、蜱传脑炎病毒（TBEV）和其他人类和家养哺乳动物病原体的主要载体。1994 年的一项问卷调查结果表明，在瑞典中部和北部（斯韦阿兰和北瑞典），硬蜱的分布范围已经从 20 世纪 80 年代初到 90 年代初扩大。为了调查瑞典硬蜱的地理分布范围的北移和数量的增加是否仍在继续，我们在首次研究 16 年后的 2009 年进行了后续调查。
我们使用与 1994 年研究相似的问卷，在针对狗主人、房主和猎人的瑞典杂志上发布。问卷与一篇关于蜱生物学及其在瑞典作为病原体载体作用的科普文章一起发布。选择这些杂志是为了从熟悉蜱并在可能存在蜱的地区活动的人群中获取信息。
对两次调查的数据进行分析表明，在近 30 年的时间里，从 20 世纪 80 年代初到 2008 年，硬蜱的分布范围向北扩展。在 20 世纪 90 年代，蜱在波罗的海北部的新地区出现，而在 2009 年的研究中，蜱首次在北瑞典的许多地方被报道。这包括北纬 66°的地区以及北瑞典内陆地区。在这 16 年期间，瑞典的蜱分布范围估计增加了 9.9%。大部分的分布范围扩展发生在北瑞典（北纬 60°以北），硬蜱的覆盖面积从 20 世纪 90 年代初的 12.5%增加到 2008 年的 26.8%。此外，根据受访者的说法，在 LB 和 TBE 流行地区，南（哥特兰）和瑞典中部的蜱的数量明显增加。
结果表明，在过去的三十年里，硬蜱在北瑞典的分布范围扩大了，在瑞典中部和南部的数量明显增加。然而，在北部山区，硬蜱仍然不存在。蜱的数量增加可以用两个主要因素来解释：首先，大量重要的蜱维持宿主的高度可用性，即在过去三十年中，特别是鹿，尤其是狍的数量增加。其次，气候变暖，冬季较温和，生长季节延长，使得蜱和鹿在更大的地理区域内的生存和繁殖能力更强。狍的高繁殖潜力、高蜱感染率以及狍的分散能力，可能解释了硬蜱的分布范围扩大，特别是在远离旧的 TBE 流行地区的地方出现新的 TBEV 焦点。瑞典 LB 的地理存在与硬蜱的分布相对应。因此，LB 在瑞典的许多地区现在是一种新兴的疾病风险。除非采取措施将鹿的数量，特别是狍和赤鹿，保持在 20 世纪 70 年代末以来的相对较低水平，特别是在城市地区及其周围地区，因为那里的人口密度较高，否则通过减少对赤狐和猞猁的狩猎等措施，瑞典的人类 LB 和 TBE 的发病率预计将继续保持高位，甚至在未来几十年内还会增加。
