Around Kilifi (approximately 3.5S, 40E), the rainy season is longer and lasts from September to January. Modelled Aedes population and EIR peak in October, but, unlike Garissa, there are indications of transmission by Aedes through to January for this region. The simulated Culex population, while small for most of the year, exhibits a large increase in January (Figure 6D), and Culex EIR (Figure 6E) indicates some transmission by Culex in January and February with a corresponding secondary peak in immature livestock incidence (Figure 6F) in February. Again this seems consistent with the findings of Sang et al. who report that both infected Culex and infected Aedes were collected from the Kilifi sites in January 2007. Baringo (approximately 0.5N, 36E), in the Rift Valley, lies within the high rainfall western region for which the model simulates year-round Culex presence, with EIR peaking between September and January. This location is on the very edge of the area of modelled Aedes emergence (and corresponding transmission by Aedes), which occurs in October and November, and to a lesser extent, in February and April (not shown). Sang et al. report that mosquitoes collected in February 2007 around Baringo were predominately of the Mansonia spp., although Aedes, Culex and Anopheles spp. were also collected. From the model results we might have expected both infected Aedes and Culex to be present, but Sang et al. report infection was only found in the Mansonia and Culex mosquitoes, with only small numbers of these being Culex. Finally, Kirinyaga, a highland region located on the southern slope of Mount Kenya, lies within a distinct area where for 2006/7 there are high levels of simulated Culex and low levels of Aedes. For 2006/7, the EIR plots (Figure 6C and Figure 6E) indicate very little transmission of RVF by either vector, most likely because of the negative impact of cooler
Parameters and are humidity-dependent, as calculated according to the dekadal (10 day accumulated) rainfall, in comparison to the rainfall threshold . K K.d Hoshen and Morse (2004) model parameters (Detinova 1962) Culex: “2 to 3 days” (Elizondo-Quiroga et al. 2006) Aedes: “2.13 to 3.16 days” (Ndiaye et al. 2006) Culex: = 57.8 to 71.0, = 9.6 to 10 (Madder et al. 1983) 10 38 (37 degree days +1 for biting/laying) mm d Hoshen and Morse (2004) model parameters 2. Extrinsic incubation Fixed length incubation period modelled using stages. 2 d d Culex: 1-2; Aedes: 3 Turell et al. (1985) (longer for temperatures below 20 °C; temperature function recommended) 3. Mature vector mortality Daily mosquito survival probability as a function of daily temperature modelled according to two schemes: Scheme 1: Scheme 2: Scheme 1: Martens et al. (1995). Scheme 2: Craig et al. (1999) (Hoshen and Morse (2004) model). Gad et al. (1989) reported Culex pipiens daily survival = 0.43 to 0.93 for temperatures of 15 to 27 °C in the Nile Delta (no clear relationship). Aedes (Costello & Brust 1971) 4. Host to vector infection Daily infection probability, from hosts and : for vector inoculation efficiency Culex: = 0.9 Aedes: = 0.6 Culex: 0.4 to 0.97 for > 17°C. Aedes: 0.55 to 0.59 Turrell et al. (1985) 5. Ovipositioning Number of eggs laid per fertile vector on day : Rainfall – linear scheme (Culex) Rainfall-independent scheme (Aedes) Maximum of vectors laying per day. = 0.3 (Culex) = 10 (Aedes) Calibrated