Background: Recent studies have revealed expansion by an array of Filarioid nematodes' into the
northern boreal region of Finland. The vector-borne nematode, Setaria tundra, 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 S. tundra in the sub arctic. We
describe the vectors of S. tundra, and its development in vectors, for the first time. Finally we
discuss the results in the context of the host-parasite ecology of S. tundra in Finland
Results: Development of S. tundra to the infective stage occurs in mosquitoes, (genera Aedes and
Anopheles). We consider Aedes spp. the most important vectors. The prevalence of S. tundra
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 m (SD 207), and width 28 m (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.
Conclusion: In the present work, S. tundra vectors and larval development were identified and
described for the first time. Aedes spp. mosquitoes likely serve as the most important and
competent vectors for S. tundra in Finland. Warm summers apparently promote transmission and
genesis of disease outbreaks by favouring the development of S. tundra 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
