Impact of ENSO 2016-17 on regional climate and malaria vector dynamics in Tanzania

Large scale modes of climate variability, including the El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD), have been shown to significantly impact mosquito-borne diseases in the Tropics, including malaria. However, the mechanistic cascade from ENSO and the IOD, to induced changes in regional climate and ultimately mosquito abundance and behaviour is poorly understood. Mosquito population dynamics, behaviour and their potential to transmit disease are all sensitive to micro-climatic conditions. The warm phase of ENSO (El Nino) tends to be associated with increased precipitation and outbreaks of various vector-borne diseases, while the cold phase (La Nina) can cause drought during the short rains over East Africa. The sensitivity of Anopheles mosquito population dynamics and host-seeking behaviour to ENSO and to the resulting micro-climatic conditions, were investigated in the Kilombero Valley in Tanzania. From June 2016 to September 2017, changes in the timing and intensity of the rainy seasons and temperature due to the ENSO 2016-17 were observed. Mosquitoes were collected using Centres for Disease Control and Prevention (CDC) light traps indoors and mosquito electrocuting traps in-and outdoors. Changes in abundance and biting behaviour of Anopheles arabiensis and Anopheles funestus were correlated with climate and microclimate. The impacts of El Nino on climate and mosquito abundance were not clear. However, the study area experienced a drought due to La Nina during which both vector species declined significantly. An. arabiensis densities stayed more stable at higher temperatures and were found in higher numbers outdoors with respect to An. funestus. For both species, indoor temperature and season determined their host-seeking location, with higher temperatures and the wet season driving them outside. The study confirmed the influence of ENSO and micro-climate on malaria vector abundance and host-seeking behaviour, generating hypotheses for predicting the impact of future ENSO on malaria risk and vector control. Our observation of higher outdoor biting during warmer conditions indicates that indoor vector control strategies may become proportionally less effective during this time.