19-EEID US-UK Collab: Disentangling transport and trophic effects of animal movement on infectious disease

移动的生物，如许多种类的鸟类、昆虫和有蹄类动物，长期以来一直吸引着科学家和公众，因为这些动物旅行的距离很远，它们对所遇到的生态系统的动态产生了巨大的影响。移动生物的高局部密度和远距离移动为致病病原体的出现和传播提供了看似理想的条件，然而，移动宿主如何改变流动性较低（即常驻）宿主的感染动态，人们知之甚少。在本提案中，我们利用胃肠道线虫在世界上最完整和最大的陆生动物迁徙系统之一——塞伦盖蒂角马中广泛存在的优势，来推进对动物运动对寄生虫动力学影响的理解。我们假设移动宿主至少在两个重要方面影响寄生宿主的动态。首先，移动宿主可以在其他宿主访问过的地方传播寄生虫，从而造成寄生虫密度高的地区。其次，移动寄主可以深刻地改变寄生虫成熟的环境，例如通过践踏和觅食缩短植被高度，这可以直接或间接地改变寄生虫传播给常驻寄主的风险。由于它们对植被的巨大影响，移动食草动物可能通过以下两种方式促进或减少寄生虫暴露风险：1)将寄生虫集中在剩余的植被上，或2)通过消耗去除寄生虫。或者，移动寄主可能通过对植被的影响间接改变其他食草动物的行为，通过iii)促进植被的再生从而吸引食草动物，或iv)消耗所有食物从而取代它们。我们的目标是确定哪些直接和间接，积极/消极的影响是最重要的，以及它们的机制基础。我们将通过研究高度移动的角马对环境中寄生虫丰度的影响，以及对四种野生动物物种的影响：托比、哈特贝、非洲角水牛和格兰特瞪羚。在野生动物物种中，我们将使用跟踪数据（GPS环、相机陷阱、空间分布模型）和动物寄生虫感染动态的分子分析相结合来研究这些不同的机制。这项工作将与操纵两种主要植物机制的实验相结合，这些机制会改变环境中寄生虫的可用性：粪便密度和草上的放牧强度。我们将利用这些信息来开发移动和常驻宿主感染动态的一般模型，这将使我们能够深入了解感染最有可能增加或减少的条件。发生在常驻野生动物宿主身上的相同动态也可能发生在常驻牲畜身上，然而，在世界各地的许多系统中，人们对野生动物和牲畜是否共享同一种寄生虫缺乏了解。因此，我们项目的一个更广泛的影响是检查流动野生动物在多大程度上改变了居住牲畜的感染强度，两个群体是否共享相同的寄生虫种群，以及流动野生动物是否通过作为对抗寄生虫药物治疗敏感的寄生虫的避难所（即对驱虫虫耐药性的天然缓冲）而使牲畜受益。鉴于胃肠道寄生虫在其生命周期中有一个环境阶段，生态学在其感染和传播动力学中起着核心作用，这为疾病生态学、动物健康和野生动物-牲畜相互作用等一系列令人兴奋的未来研究问题打开了大门，并为当地动物卫生专业学生和当局提供了培训机会。

Mobile organisms, such as many species of birds, insects and ungulates, have long fascinated scientists and the general public because of the vast distances over which these animals travel and the large impacts they have on the dynamics of ecosystems they encounter. The high local densities and long-distance movements of mobile organisms provide seemingly ideal conditions for the emergence and spread of disease-causing pathogens, yet the ways in which mobile hosts modify the infection dynamics of less-mobile (i.e. resident) hosts is poorly understood. In this proposal, we take advantage of the widespread occurrence of gastrointestinal nematodes in one of the most intact and largest terrestrial animal migration systems in the world - the Serengeti wildebeest - to advance understanding of the impact of animal movement on parasite dynamics. We hypothesize that mobile hosts can affect parasite dynamics in resident hosts in at least two important ways. First, mobile hosts can shed parasites in locations visited by other hosts, thereby creating areas of high parasite density. Second, mobile hosts can profoundly change the environment in which parasites mature, for example by shortening the height of vegetation both through trampling and foraging, which can directly and indirectly modify the risk of parasite transmission to resident hosts. Because of their large effects on vegetation, mobile herbivores may promote or reduce parasite exposure risk by either i) concentrating parasites on the remaining vegetation, or ii) removing parasites through consumption. Alternatively, mobile hosts may alter the behaviour of other herbivores indirectly through their effects on vegetation, by iii) promoting regrowth of vegetation and thus attracting herbivores, or iv) consuming all of the food and thus displacing them. We aim to determine which of these direct and indirect, positive/negative effects are most important, and their mechanistic underpinnings. We will do so by examining the impacts of highly mobile wildebeest on the parasite abundance in the environment and in four wildlife species: topi, hartebeest, African Cape buffalo, and Grant's gazelle. In wildlife species, we will examine these different mechanisms using a combination of tracking data (GPS collars, camera traps, spatial distribution models) and molecular analyses of parasite infection dynamics in animals. This work will be combined with experiments that manipulate two of the main vegetative mechanisms that alter parasite availability in the environment: density of feces and grazing intensity on grasses. We will use this information to develop a general model of infection dynamics in mobile and resident hosts that will allow insight into the conditions in which infections are most likely to increase or decrease. The same dynamics that occur in resident wildlife hosts are also likely to play out in resident livestock, however in many systems around the world there is a lack of understanding of whether wildlife and livestock even share the same parasites. Thus, a broader impact of our project is to examine the extent to which mobile wildlife change the infection intensity in resident livestock, whether the two groups share the same populations of parasites and whether mobile wildlife benefit livestock by acting as a refugia for parasites that are sensitive to anti-parasite drug treatment (i.e. a natural buffer to anthelmintic resistance). Given that gastrointestinal parasites have an environmental stage to their life cycle, ecology plays a central role in their infection and transmission dynamics, opening the door for a range of exciting future research questions in disease ecology, animal health and wildlife-livestock interactions, as well as training opportunities with local animal health students and authorities.