8 Sauli Laaksonen et al. Finland, including those in reindeer (Rangifer tarandus) in 1973 and 2003–2005 (Laaksonen et al., 2007) and in moose (Alces alces) in 1989 (Nygren, 1990). The ﬁrst recognized outbreak of S. tundra in Fennoscandia was reported by Rehbinder et al. (1975) in 1972–1973 among Swedish reindeer originating from forest herds at relatively low altitudes; in contrast, the parasite was apparently absent in animals from mountain herds. In 1973, S. tundra was associated with ‘‘rather’’ poor body condition and perito- nitis. Later, in the winter of 1973–1974, tens of thousands of reindeer died in the northern part of Finnish reindeer herding area, where severe peritonitis and large numbers of Setaria sp. worms were commonly found in postmortem examination (see Laaksonen et al., 2007). During this outbreak, the Finnish reindeer population decreased from 140,000 to 98,000. The reduction was most intense in the northern (Sodankyla¨ level; Fig. 1) and eastern (Kuusamo) areas where the population decreased by 40 and 30%, respectively (Database of Finnish Reindeer Herders’ Asso- ciation). Further in 1973–1974, S. tundra was observed for the ﬁrst time in Norway where an explosive epidemic among northern herds resulted in widespread condemna- Figure 1. Distribution of herding areas for Finnish reindeer (black) and the location of the three weather stations representing the region. tion of meat, livers, and adjacent tissues due to the effects of peritonitis, hepatitis, and perihepatitis (Kummeneje, 1980). The most recent epidemic in reindeer rapidly emerged in 2003 across the southern parts of the Finnish reindeer herding area and was ﬁrst noticed in the Kuusamo region. As of 2005, expansion of the outbreak was exten- sive, and only the reindeer in the northernmost part of Finland (Kevo area) appeared to remain free of disease, although carriers of S. tundra also appeared in northern herds (Laaksonen et al., 2008a). An outbreak of Setaria sp. (later determined to be S. tundra (Laaksonen et al., 2008a)) in Finnish moose was reported in 1989 by Nygren (1990) based on an extensive examination of the genital tracts and adjacent tissues from 1,132 females originating from the Finnish reindeer herding area. The number of moose examined in this study repre- sented 83% of hunted females in the area. Setaria sp. or a marked inﬂammatory granulomatous reaction associated with the parasite was recorded in 29% of the adult hinds, whereas 46% of calves and 55% of yearlings were infected. The epidemic was most intense in the southern and middle parts of the reindeer herding area, whereas moose from the northern part appeared free of infection. Although this outbreak took place within the core of the reindeer hus- bandry area, and the reindeer population was in its peak (Database of Finnish Reindeer Herders’ Association), no meat inspection reports indicate increased morbidity associated with Setaria in reindeer. Setaria tundra nematodes have a complex life cycle where infective larvae are transmitted to ungulate hosts by mosquitoes (Culicidae) and in which many climate-related drivers play a major role (Laaksonen et al., 2009). Massive swarms of blood-feeding insects attacking herds of caribou/ reindeer are well known (Anderson and Nilssen, 1998). Some estimates suggest that reindeer can be exposed to attacks of approximately 8,000 mosquitoes per hour during the mass appearance of blood sucking insects (Kadnikov, 1989), which locally is known as a ‘‘ra¨kka¨’’ (see Laaksonen et al., 2009). Prevalence of S. tundra among different mosquito populations in 2004 across endemic regions in Finland was 0.5–2.5% (Laaksonen et al., 2009). In these conditions, reindeer could hourly be attacked by 40 to 200 mosquitoes carrying infective larvae of S. tundra. Habitat preferences by reindeer during periods of high tempera- tures during the summer also may contribute as a deter- minant of exposure to infection. During the height of warm summers, hundreds of reindeer congregate in dense herds in low elevation, mosquito-rich wetlands coinciding with