Examining the Ecological Drivers of Bluetongue Virus Transmission

Vector-borne pathogens cause many diseases of medical and veterinary concern, and are largely expanding with changing environments. Bluetongue virus (BTV), a Culicoides-borne pathogen of wild and domestic ruminants, has been spreading in recent decades. BTV has a complex transmission ecology, infecting several species of both hosts and vectors and circulating in highly variable environments. In this dissertation, I focus on factors influencing BTV transmission, specifically host communities, development habitats, and temperature variability.
First, I study the effect of host abundance on vector abundance, trapping midges across livestock operations of varying sizes, assuming that if vector abundance increases with hosts, there is support for density-dependent transmission. I use hierarchical Bayesian models, estimating unobserved trapping efficacy, to separate this from the ecologically relevant midge abundance. Results indicate a positive effect of host abundance on vector-to-host ratio, suggesting that transmission is amplified in high-density environments.
Next, I conducted a survey of Culicoides development habitats, to classify what landscapes and microhabitats support development. I collected moist substrate from variable landscapes, and observed whether midges emerged, to characterize differential emergence across land use and substrate types. Diverse site types supported midge emergence, with natural spaces producing as many midges as more classically implicated livestock operations, though the latter showed higher proportions of Culicoides sonorensis, which are thought to be the most competent vectors of BTV in this region.
Lastly, I modeled the effect of seasonal temperature variability on Culicoides populations and BTV transmission, simulating across annual mean temperatures, seasonal variation, and pathogen introduction relative to seasonality. Epidemiological outcomes depended jointly on annual mean temperatures and seasonal variation, with larger outbreaks occurring when conditions were suitable for longer periods. This result yields different predictions of than traditional temperature-derived models, because our model incorporates cumulative effects of temperature, demonstrating the importance of considering seasonality when predicting transmission.

媒介传播病原体可引发诸多受医学与兽医学关注的疾病，并随环境变化呈现大范围扩散态势。蓝舌病毒（Bluetongue virus, BTV）是一种由蠓属（Culicoides）传播的病原体，可感染野生与家养反刍动物，近数十年来其传播范围持续扩大。BTV的传播生态学极为复杂，可侵染多种宿主与媒介物种，并在高度多变的环境中循环传播。本学位论文聚焦于影响BTV传播的相关因素，具体涵盖宿主群落、蠓的发育生境以及温度变异性。首先，本研究探讨宿主丰度对媒介丰度的影响：在不同规模的畜牧场布设蠓类诱捕装置，若媒介丰度随宿主丰度提升而增加，则可支持密度依赖型传播假说。本研究采用分层贝叶斯模型（hierarchical Bayesian models），通过估算未被观测到的诱捕效率，将其与生态学意义上的真实蠓丰度相分离。研究结果显示，宿主丰度对媒介-宿主比值存在正向影响，表明高密度环境下的BTV传播会被增强。其次，本研究开展了蠓属发育生境的调查，以明确哪些景观与微生境能够支持蠓的发育。研究人员从多样的景观中采集湿润基质，观测蠓是否成功羽化，以此表征不同土地利用类型与基质类型间的羽化差异。多种生境类型均可支持蠓羽化，其中自然生境产出的蠓数量与经典被认为与畜牧活动相关的生境相当，但后者的索诺拉库蠓（Culicoides sonorensis）占比更高——该物种被认为是本区域内BTV最具感染性的媒介。最后，本研究构建了季节温度变异性对蠓种群与BTV传播影响的模型，基于年均温、季节波动以及相对于季节节律的病原体引入情况开展模拟。流行病学结局同时受年均温与季节波动的共同调控，当适宜传播的持续时间更长时，暴发规模会更大。该结果与传统基于温度的预测模型所得结论存在差异，原因在于本模型纳入了温度的累积效应，证实了在预测传播风险时考虑季节节律的重要性。