| Summary: | <p>Abstract</p> <p>Background</p> <p>Recent studies have revealed expansion by an array of Filarioid nematodes' into the northern boreal region of Finland. The vector-borne nematode, <it>Setaria tundra</it>, caused a serious disease outbreak in the Finnish reindeer population in 2003–05. The main aim of this study was to understand the outbreak dynamics and the rapid expansion of <it>S. tundra </it>in the sub arctic. We describe the vectors of <it>S. tundra</it>, and its development in vectors, for the first time. Finally we discuss the results in the context of the host-parasite ecology of <it>S. tundra </it>in Finland</p> <p>Results</p> <p>Development of <it>S. tundra </it>to the infective stage occurs in mosquitoes, (genera <it>Aedes </it>and <it>Anopheles</it>). We consider <it>Aedes </it>spp. the most important vectors. The prevalence of <it>S. tundra </it>naturally infected mosquitoes from Finland varied from 0.5 to 2.5%. The rate of development in mosquitoes was temperature-dependent. Infective larvae were present approximately 14 days after a blood meal in mosquitoes maintained at room temperature (mean 21 C), but did not develop in mosquitoes maintained outside for 22 days at a mean temperature of 14.1 C. The third-stage (infective) larvae were elongated (mean length 1411 μm (SD 207), and width 28 μm (SD 2)). The anterior end was blunt, and bore two liplike structures, the posterior end slight tapering with a prominent terminal papilla. Infective larvae were distributed anteriorly in the insect's body, the highest abundance being 70 larvae in one mosquito. A questionnaire survey revealed that the peak activity of Culicidae in the reindeer herding areas of Finland was from the middle of June to the end of July and that warm summer weather was associated with reindeer flocking behaviour on mosquito-rich wetlands.</p> <p>Conclusion</p> <p>In the present work, <it>S. tundra </it>vectors and larval development were identified and described for the first time. <it>Aedes </it>spp. mosquitoes likely serve as the most important and competent vectors for <it>S. tundra </it>in Finland. Warm summers apparently promote transmission and genesis of disease outbreaks by favouring the development of <it>S. tundra </it>in its mosquito vectors, by improving the development and longevity of mosquitoes, and finally by forcing the reindeer to flock on mosquito rich wetlands. Thus we predict that global climate change has the potential to promote the further emergence of Filarioid nematodes and the disease caused by them in subarctic regions.</p>
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