10 Sauli Laaksonen et al. Figure 2. Predicted probabilities of Setaria tundra outbreaks as a function of mean summer temperature, Tt and Tt - 1 ( C), for an outbreak and the preceding year. The predictions were obtained from a generalized linear model (occurrence of epidemic regressed against Tt, Tt - 1 and their interaction). RESULTS The results of generalized linear modelling indicated that the occurrence of an epidemic increased with Tt (b = 6.60 ± 3.39 (s.e.); P = 0.0004). However, this effect was conditional on Tt - 1 since a signiﬁcant Tt 9 Tt - 1 interaction was also observed (b = -0.86 ± 0.23, P = 0.0002). On the basis of our model, high Tt was likely to cause an epidemic only if Tt - 1 was higher than 14 C (Fig. 2). The mean summer temperature for the region in study period was 12 C (range, 8.8–15.1). DISCUSSION Our data obtained in this study suggest that warm episodes or otherwise extreme events of temperature, embedded within long-term trends for climate warming, are a factor that promotes serious outbreaks and emergence of S. tun- dra. These factors may represent a generalized determinant for occurrence and outbreaks for other ﬁlarioid parasites across northern latitudes. There are many nonclimatological and climatological factors that enhance the life cycle of S. tundra, its trans- mission, and survival in the reindeer population. Many of these factors remain linked to climate at some level. The effect of these drivers on the patterns of distribution for parasites in both deﬁnitive hosts and vectors, overall host- pathogen relationships and emergence of an outbreak, may be both incremental (cumulative) and extreme (episodic). The synchrony between peak periods of S. tundra micro- ﬁlaremia, peak activity of vectors, shedding of adult rein- deer, and low immunity status of calves (Laaksonen et al., 2008a, b, 2009) probably promotes the transmission of S. tundra from adult carriers to calves. Contributing factors in determining patterns of transmission and outbreaks are seen in the high capacity of S. tundra to produce microﬁ- lariae (Nikander et al., 2007), high vector efﬁciency (La- aksonen et al., 2009), the stress and host behavior during warm periods (Helle and Aspi, 1984; Helle et al., 1992; Pollard et al. 1996; Mo¨rchel and Klein, 1997; Anderson and Nilssen, 1998; Weladj et al., 2002; Laaksonen et al., 2009), and the impact on host physiology (Laaksonen et al., 2008a) of insect harassment. The observations also suggest that the migrations of reindeer (Laaksonen et al., 2009) and the characteristics of reindeer pastures (Rehbinder et al., 1975; Dadaev, 1984; Laaksonen et al., 2009) are involved. On the population level, the abundance of sylvatic reser- voirs (Laaksonen et al., 2007, 2008a, b), the host density (Laaksonen et al., 2008b), and massive antiparasitic treat- ment in semidomesticated reindeer (Laaksonen et al., 2008b), as well as growing immunity in host populations may interact (Laaksonen et al., 2008a). The possibility for emergence of the outbreaks being linked to the introduction of a new S. tundra strain that could have been more pathogenic for hosts cannot be ex- cluded (Nikander et al., 2007; Laaksonen et al., 2008a; Hoberg, 2010). Interestingly, molecular sequence data indicate that discrete populations of S. tundra may be cir- culating in moose and reindeer, respectively, even in zones of sympatry (Laaksonen et al., 2008a, b). During the recent epidemic in reindeer during 2003–2005, the moose popu- lation peaked in northern Finland and no cases of acute peritonitis or living adult S. tundra worms were detected; only a few preadults were found encapsulated on the sur- face of livers (Laaksonen et al., 2007). Thus, parasites cir- culating in reindeer and roe deer, demonstrated as the causative agent of the 2003–2005 outbreak, appear to be discrete from those that may be circulating in moose or other cervids (Laaksonen et al., 2007, 2008a). Parasite species diversity and population structure for Setaria are factors that require further exploration in determining relationships to past and future outbreaks. The results obtained from model-development in the current study are congruent with the observations made