areas of considerable activity by vectors (Laaksonen et al., 2009). In these areas the microclimate is favorable for mosquitoes, and it is presumed that hyperendemic foci of infection enhance the potential for transmission; for example, in these wetland habitats, the infection rate of S. tundra in mosquitoes can reach and occasionally exceed 2.5% (Laaksonen et al., 2009). Vector breeding sites are not limited by humidity or water resources, because mires and bogs constitute approximately 30–40% of the total land area in the regions where outbreaks of S. tundra have been recognized. In contrast, only approximately 12% of upper Lapland is bog habitat (Kumpula et al., 1999), which may restrict transmission (Laaksonen et al., 2007, 2008a). Where favorable environmental and host factors coincide, the transmission of S. tundra is highly dependent on the life span of female mosquitoes and the speciﬁc demographics of the population (Laaksonen et al., 2009); survival of adult mosquitoes is in part dependent on both temperature and humidity (Barry and Juliano, 2001). For example, during the latest epidemic, older females com- prised a considerable component of the ‘‘wild’’ mosquito population but densities were low (Laaksonen et al., 2009). In contrast, the S. tundra outbreak in Sweden in 1973 was associated with unusually warm weather and with the appearance of especially large numbers of mosquitoes (Rehbinder et al., 1975). The presence and proportion of older female mosquitoes in a given population, however, is a more critical factor for transmission of S. tundra rather than the density alone. Older females may have fed several times and are thus more likely to have become infected with tens of S. tundra larvae (Laaksonen et al., 2009). Within limits, warmth also affects the development, reproduction, longevity, and feeding habits of mosquito vectors (Clements, 1963, Delatte et al., 2009), and the lowest temperature of the seasonal regime is a limiting factor for survival (e.g., in Aedes albopictus) (Lo¨wenberg- Neto and Navarro-Silva, 2004). Furthermore, the hemato- phagic activities and oviposition of mosquitoes, for exam- ple Ae. albopictus, may be signiﬁcantly inﬂuenced by temperature (Calado and Navarro-Silva, 2002). Finally, the development of S. tundra to the infective stage in mos- quitoes is highly temperature-dependent and inversely re- lated. Warmth decreases the time required for the larval development of S. tundra (Laaksonen et al., 2009). It is obvious that these factors of the biology of hosts (both deﬁnitive host and arthropod vectors) and parasites may act in synergy with the environmental conditions to en- Climate Change, a Driver for Filarioid Nematode Outbreaks 9 hance the potential for signiﬁcant transmission, infection, and disease outbreaks (Lafferty, 2009; Gubler et al., 2001). Mosquito-borne pathogens and diseases are predicted to be among the most climate-sensitive maladies (Patz et al., 1996); however, to date, little empirical evidence has been presented. Hoberg et al. (2008a, b) discussed some interactions among these drivers related to climate, and the present study provides strong empirical evidence of the relationship. The outbreaks in 1973 and 2003–2005 led to substantial losses to the herding economy and had a pro- nounced inﬂuence on the welfare of calves (Laaksonen et al., 2007, 2008b). The outbreaks have revealed that S. tundra can act as a signiﬁcant pathogen for reindeer, which was evident at both ante- and postmortem inspection of slaughter animals and in histological and experimental examinations (Laaksonen et al., 2007, 2008b). Thus, these parasites represent a considerable threat to biological diversity, ecological continuity, and cultural and socio- economic well-being in the region. METHODS We studied the effects of ambient summer temperatures on the known outbreaks of S. tundra in populations of Finnish cervids, based on historical weather data (1961– 2004) from three independent meteorological stations of the Finnish Meteorological Institute representing the reindeer herding area of Finland (Kuusamo, Sodankyla¨ and Kevo) (Fig. 1). The weather stations are the only ones in the study area. Kuusamo and Sodankyla¨ represent the outbreak areas in 1989 and 2003–2005, whereas all the stations represent the outbreak area in 1973. We tested a hypothesis that the probability of Setaria out- break in year t depends on the summer mean tempera- tures (summer period: June–August) during year t and year t - 1. We also assessed whether a high current year summer temperature, Tt, was sufﬁcient for an epidemic or whether the effect of current year temperature de- pended on summer temperature during year t - 1. To do this, we ﬁtted a generalized linear mixed model with binomial errors into the data. The occurrence of an epidemic was a dichotomous dependent variable; mean summer temperatures Tt, Tt - 1, and their interaction, as well as site (a random effect) were independent vari- ables in the model. The analysis was performed using PROC GENMOD (SAS Institute).