Evidence for continued transmission of parasitic nematodes in reindeer during the Arctic winter.

Living in the high Arctic, the Svalbard reindeer (Rangifer tarandus platyrhynchus) and its trichostrongyle nematodes experience a long cold winter from October to late May/early June. Over this period, transmission would be expected to be low. However, in culled reindeer the abundance of infection increased from autumn to late winter, providing evidence for continued transmission within this period. To our knowledge this is the first time this has been demonstrated in a climate with temperatures consistently below 0 degrees C. In one winter (1996-1997), the average fraction of nematodes found as larvae in the abomasal mucosa increased from around 10% to 50% between October and March. This suggests that arrested development took place throughout the winter. We found no evidence for an efficient acquired immune response towards the nematodes. The abundance of infection did not tend to decrease with increasing host age after an earlier peak, but levelled off instead, as predicted by a simple immigration-death model. In the late winter when the nutritional plane is low, both adult reindeer and calves had high worm burdens at intensities that may affect their condition and fitness.