Figure 3. Variation in mean summer temperatures (lines) in Finnish Lapland with the known outbreaks of Setaria tundra in reindeer (1973 and 2003) and moose (1989) indicated (bars). Annual mean summer temperatures were recorded at three observation stations: Kevo, Kuusamo, and Sodankyla¨ (Fig. 1). during the origin of the recent disease outbreak (Laaksonen et al., 2007, 2008a). Emergence of disease conditions is not immediate and is associated with considerable lag time between initial transmission, infection, and development of adult parasites in the mammalian host. Summer tempera- tures greater than 14 C increase infection rates, but the morbidity manifests the following summer only if the weather conditions are still favorable. According to meat inspection data, the prevalence of S. tundra infection had already increased in slaughter-reindeer in 2002, one year before the outbreak (Laaksonen et al., 2007), and the degree of disease was correlated with the intensity of infection (Laaksonen et al., 2007). It is therefore reasonable to as- sume that the parasite must reach prevalence high enough in the population before the other conditions enabling the outbreak surpass a threshold. Such a relationship is clearly demonstrated empirically by the interaction between cumulative long-term trends in temperature, mean sum- mer temperatures exceeding 14 C, and by the fact that outbreak years with the subsequent emergence of diseases in cervids have been warm (mean 13.4 C; range 12.6–14.2; Fig. 3). In this system, temperature increases greater than 14 C may represent a tipping point for development and ampliﬁcation of parasite populations (Hoberg et al., 2008a). These interacting cumulative factors provide an explanation for the temporal duration over 2 years (Kummeneje, 1980; Rehbinder et al., 1975; Laaksonen et al., 2007) and geographic extent of emergent disease. In this regard, the outbreak reached upper Lapland (Kevo, the region of greatest reindeer density) in 1973 when Climate Change, a Driver for Filarioid Nematode Outbreaks 11 temperatures were elevated in 1972, but not in 2003–2005 after the summer of 2002 when the summer temperature did not exceed 14 C (Laaksonen et al., 2007, 2008a). As evidence of this cumulative interaction, the documented genesis of the epidemic in 2003 occurred in the area where the host density is lowest and subsequently spread into adjacent areas (Sodankyla¨) in 2004 and 2005 (Tt 12.9 and 14.3 C, respectively (Database of Finnish Meteorological Institute) where reindeer were considerably more abundant (Laaksonen et al., 2008b). The Arctic climate is under accelerating change resulting in both incremental (cumulative) warming and extreme (episodic) weather events that inﬂuence temperature and hydrological processes (Patz et al., 1996). Such processes serve as the physical drivers for changing dynamics in pat- terns of distribution (host and geographic), development, survival, and transmission for an array of pathogens (Kutz et al., 2004, 2005; Hoberg et al., 2008a). Cascading and cumulative effects of climate change include shifts in host– pathogen relationships. Thus, in the future in many areas, conditions for a diverse assemblage of disease vectors and intermediate hosts of parasites will likely improve leading to a modiﬁcation in the intricate balance that determines the distribution for emergent disease (Dobson and Foufopoulus, 2001; Hoberg et al., 2008a). For example, in certain latitu- dinal zones, coincidental with melting permafrost and increasing rainfall, the area and number of suitable habitats for mosquitoes may expand. Pathogen transmission and patterns of emerging disease may become further altered through spatial and temporal shifts in geographic range, patterns of reproduction, and longevity for vectors (Patz et al., 2000; Hoberg et al., 2008b). Higher temperatures also may increase the biting rates for vectors and shorten the incubation period for pathogens (Patz et al., 2000) and alter the behavior of hosts (Laaksonen et al., 2009). Factors asso- ciated with the life history of both arthropod intermediate and ungulate deﬁnitive hosts can be convergent in ampliﬁ- cation of parasite populations. For example, in the study area during periods of high temperatures, dense herds of reindeer aggregated in the mosquito-rich wetlands in response to available water, abundant forage, and habitat-related ther- moregulation (Laaksonen et al., 2009). Alternatively, accel- erated climate change may have the potential for negative or neutral effects, and not all environmental perturbation will lead to emergence, particularly where tolerances and resil- ience of developmental stages for parasites and vectors are exceeded by ambient conditions (Lafferty, 2009; Gubler et al., 2001).