Data_Sheet_1_Climate Conditions During a Rift Valley Fever Post-epizootic Period in Free State, South Africa, 2014–2019.pdf

Rift Valley fever virus (RVFV) activity in Southern Africa tends to occur during periods of sustained elevated rainfall, cooler than normal conditions, and abundant vegetation cover creating ideal conditions for the increase and propagation of populations of RVFV mosquito vectors. These climatic and ecological conditions are modulated by large-scale tropical-wide El Niño–Southern Oscillation (ENSO) phenomena. The aim of this 5-year study was to investigate climatic conditions during Rift Valley fever “post-epizootic” period in Free State province of the Republic of South Africa, which historically experienced the largest RVF outbreaks in this country. We collected satellite-derived rainfall, land surface temperature (LST), and normalized difference vegetation index (NDVI) data since 2014 to understand broad environmental conditions in the years following a period of sustained and widespread large RVF outbreaks (2008–2011) in the region. We found this post-epizootic/interepizootic period to be characterized by below-normal rainfall (~-500 mm), above LSTs (~+12°C), depressed NDVI (60% below normal), and severe drought as manifested particularly during the 2015–2016 growing season. Such conditions reduce the patchwork of appropriate habitats available for emergence of RVFV vectors and diminish chances of RVFV activity. However, the 2016–2017 growing season saw a marked return to somewhat wetter conditions without any reported RVFV transmission. In general, the aggregate vector collections during this 5-year period follow patterns observed in climate measurements. During the 2017–2018 growing season, late and seasonally above average rainfall resulted in a focal RVF outbreak in one location in the study region. This unanticipated event is an indicator of cryptic RVF activity during post-epizootic period and may be a harbinger of RVFV activity in the coming years.

南部非洲的裂谷热病毒（Rift Valley fever virus, RVFV）流行多暴发于持续降雨偏多、气温低于常态、植被覆盖充足的时期——此类环境可为RVFV蚊媒种群的增殖与扩散提供理想条件。此类气候与生态条件受全球热带尺度的厄尔尼诺-南方涛动（El Niño–Southern Oscillation, ENSO）现象调控。本项为期5年的研究旨在调查南非共和国自由邦省的裂谷热"post-epizootic"（流行后）时期的气候条件，该地区曾是该国历史上裂谷热（Rift Valley fever, RVF）暴发规模最大的区域。我们收集了2014年以来的卫星反演降雨、地表温度（land surface temperature, LST）以及归一化植被指数（normalized difference vegetation index, NDVI）数据，以解析该区域在2008至2011年持续大范围大规模RVF暴发后的数年整体环境状况。研究发现，该流行后/流行间歇期的典型特征为降雨量低于常态（约偏少500毫米）、地表温度偏高（约偏高12℃）、归一化植被指数较常态低60%，且伴随严重干旱，这一特征在2015至2016年生长季尤为显著。此类环境会压缩RVFV蚊媒适宜滋生的零散生境规模，降低RVFV流行的可能性。但2016至2017年生长季，当地气候显著转向相对湿润，并未出现RVFV传播的相关报道。总体而言，本研究5年期间收集的蚊媒总样本量变化趋势与气候监测数据的变化规律一致。2017至2018年生长季，降雨偏晚且季均降雨量高于平均值，导致研究区域内某一地点发生了局域性RVF暴发。这一未预期的事件表明流行后时期存在隐秘的RVFV流行活动，或可成为未来数年RVFV出现流行的前兆。