The impact of ENSO 2015-16 on vector-borne diseases: from Zika virus transmission in Latin America to malaria vectors in Tanzania.

The El Niño southern oscillation (ENSO) is the main climatic mode at interannual time scale. The warm phase of ENSO, El Niño is associated with warmer than average temperatures and the disturbance of Walker cells leading to changes in rainfall patterns over the Tropics. ENSO severely impacts the health of human and animal population. This study will present two recent research examples of ENSO impact on vector-borne diseases. Using a climate-driven R₀ (basic reproduction number) mathematical model for the transmission risk of Zika virus (ZIKV) that explicitly includes two key mosquito vector species Aedes aegypti and Aedes albopictus, we showed that ZIKV transmission risk in South America in 2015 was the highest since 1950. The R₀ maximum is related to favouring temperature conditions which caused simulated biting rates to be largest, mortality rates and extrinsic incubation periods to be smallest in 2015. In other words, the ZIKV outbreak in Latin America was fuelled by the 2015-2016 El Niño climate phenomenon affecting the region, in conjunction with other important socio-economic stressors. This work was a stepping stone for the development of an early warning system for arboviruses based on climate seasonal forecasts by another US team. We also investigated the impact of La Niña 2016-2017 on Anopheles malaria vectors in Tanzania. We monitored mosquito abundance and climatic conditions from June 2016 to Sep 2017 in five villages surrounding Ifakara in the Kilombero valley of Tanzania. Results show that both An. gambiae and An. funestus were negatively affected by a drought related to La Niña conditions. An. gambiae was predominantly found outdoors while An. funestus was mostly found indoors. An. gambiae was found to be more resilient to the drought. Our observation of higher outdoor biting during warmer conditions, in particular during late evenings, indicates that indoor vector control strategies may become proportionally less effective during this time.