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    • Vectorial capacity VC describes the threshold condition

    2018-10-23

    Vectorial capacity (VC) describes the threshold condition for a vector\'s ability to spread disease among humans (Patz et al., 1998; Massad and Coutinho, 2012), representing the average daily number of secondary cases generated by one primary case introduced into a susceptible population (Liu-Helmersson et al., 2014). It depends on six vector parameters (Patz et al., 1998), which are highly influenced by ambient temperature, both its mean value and diurnal temperature range (DTR) (Liu-Helmersson et al., 2014; Lambrechts et al., 2011; Carrington et al., 2013). VC has been used to model DEP globally for both Aedes vectors (Patz et al., 1998; Brady et al., 2014). Very few models incorporated DTR (Patz et al., 1998) and the temperature dependent transmission probabilities per bite to both humans and vectors (Lambrechts et al., 2011) when describing DEP (Liu-Helmersson et al., 2014) or vector competence. Including these factors would change the projected estimates of the impacts of climate on DEP, given strong temperature dependence of transmission probabilities per bite in humans and vectors and strong association of DTR with VC and vector competence (Liu-Helmersson et al., 2014; Lambrechts et al., 2011). In this study, we modeled VC to project DEP in Europe given changes in climate. Throughout this study, we have included DTR in all of our VC calculations. We expanded our previous relative VC model to VC, by including temperature dependent dynamics in the female vector-to-human population ratio (Liu-Helmersson et al., 2014) for both Ae. aegypti and Ae. albopictus under four projected emission scenarios with higher temporally and spatially resolution over two centuries. We estimated DEP for local dengue transmission, in terms of seasonality, intensity and duration, for Europe and examined ten European metropolitan cities ranging from North to South for the Epinephrine Bitartrate manufacturer 1901–2099 (for more details see Table S3 in the Supplementary information).
    Methods Vectorial capacity (VC) was used to estimate dengue epidemic potential (DEP). As shown in Equation (Hales et al., 2002; Patz et al., 1998; Massad and Coutinho, 2012), VC depends on six vector parameters: The six vector parameters used were 1) the average vector biting rate (a), 2) the probability of vector to human transmission per bite (b), 3) the probability of human to vector infection per bite (b), 4) the duration of the extrinsic incubation period – EIP (n), 5) the vector mortality rate (μ), and 6) the female vector-to-human population ratio (m). The time unit is one day. Each of the vector parameters depends on temperature (Liu-Helmersson et al., 2014). The temperature relationships for the first five vector parameters, 1)–5), were obtained from the peer-reviewed literature for Ae. aegypti; details are described elsewhere (Liu-Helmersson et al., 2014). For Ae. albopictus, only two vector parameters, 1) and 5), were found in the literature with temperature dependent relationships: the mortality rate (μ) and the total biting rate (a), which was taken as an inverse of the duration of gonotrophic cycle (Delatte et al., 2009). The remaining three parameters, 2)–4), in the VC were assumed to have the same temperature relation as those for Ae. aegypti (Liu-Helmersson et al., 2014). For Ae. Albopictus, the human biting rate is assumed to be 0.88 of the total biting rate based on the human and dogs experiment performed by Delatte et al. (2010). The probability of transmission per bite to human is assumed to be 0.7 of that for Ae. aegypti, based partially on the literature review conducted by Lambrechts et al. (2010). Due to a lack of reliable data, the female vector-to-human population ratio, m, is assumed to depend on temperature the same way as the life expectancy or inverse of the mortality rate, as used in a previous study (Brady et al., 2014). The maximum value of m (mmax) is assumed to be 1.5. The threshold cut-off for DEP was defined as VC*=0.2 (day). Here we assume that an epidemic potential is realized when VC reaches a level such that one infected person will infect at least one more person after dengue is introduced into a naïve population during his/her five-day infectious period (Liu-Helmersson et al., 2014; Nishiura and Halstead, 2007) (Supplementary information, Section S4). Sensitivity analysis was performed for the effect of the range of the infectious period (4–10days) (World Health Organization (WHO), 2012; Centers for Disease Control and Prevention (CDC), 2015; Chan and Johansson, 2012) on the dengue transmission windows (Supplementary information Section S5.3, Fig. S5A for Ae. aegypti and Fig. S5B for Ae. albopictus). This corresponds to a range of thresholds for DEP from 0.1 to 0.25 (day). We have chosen the threshold value of 0.2 (day), which is closer to the higher end of this range, to be conservative in our results presented.