ORIGINAL PAPER

Abstract Harmful parasites of the wild northern boreal mammals are still surprisingly poorly studied. In 2003– 2006, a peritonitis outbreak caused by the filarioid nematode, Setaria tundra, emerged in Finland’s reindeer population. In order to gain knowledge about the basic biology, epidemiology, and transmission dynamics of this parasite, samples for S. tundra were collected from reindeer and other cervids during the follow-up period 2004–2006. Using morphology and molecular biology methods, we describe here S. tundra’s first larval stage, microfilaria (smf), for the first time scientifically. The prevalence and densities of smf were higher in reindeer calves than in adults, overall prevalence being 42%. The overall smf prevalences for moose, wild forest reindeer and roe deer were 1.4–1.8%, 23%, and 39%, respectively. The focus of microfilaremia moved north and settled down in the south simultaneously with the peritonitis outbreak. The peak microfilaremia occurred in the first summer after the infection, and smf disappeared from the blood after 2 years. Captive reindeer were smf positive over the year. The prepatent period of S. tundra was estimated to be about 4 months, and the life span at least 14 months. This parasite likely has an important impact on boreal ecosystems