Relationship between environmental, ecological and genetic drivers of emergence in amphibian chytridiomycosis.

在已经进行了全面评估的动物类别中，两栖动物被列为最受威胁的主要分类群，面临灭绝威胁的两栖动物物种几乎是鸟类物种的三倍。疾病已被确定为两栖动物衰退和灭绝的主要原因之一，其中一种病原体被单独列为迄今为止被确定为最危险的两栖动物疾病。树突巴氏杆菌是一种合胞真菌，与全球范围内的大规模死亡事件、种群下降和物种灭绝有关。世界自然保护联盟最近组织的两栖动物保护峰会认识到，目前对这种真菌的了解明显不足。针对这一研究差距，首脑会议发表了一项宣言，呼吁立即进行研究，以确定疾病的分布、如何实现这种分布以及病原体入侵对当地两栖动物群落的后果。在NERC的资助下，我们已经完成了对整个欧洲树突芽孢杆菌和合孢子菌病的三年监测，并首次表明感染是广泛存在的，存在于英国的野外，不幸的是存在于马略坎助产士蟾蜍身上，这是最濒危的两栖动物物种之一。我们现在需要增加我们目前对感染流行率的描述，并检验由这些模式产生的假设，以确定树枝杆菌导致种群崩溃的过程。我们将通过实地调查和实验室实验相结合的方式来实现这一点，以了解自然种群中感染的动态如何导致物种灭绝。首先，我们将对五个重点研究地点进行密集调查，这些地点存在疾病，但在两栖动物种群中造成不同的影响。我们将在三年内对这些群落进行采样，以跟踪感染如何在不同物种中传播，以及感染如何对两栖动物群落中的幼虫(蝌蚪)、变态和成虫产生不同的影响。使用我们最新开发的环境分子分析，我们现在能够测试环境中感染阶段的密度。这使我们能够直接测量两栖动物的暴露水平，并评估真菌是否能够在没有宿主的情况下在环境中生存。存在多个感染宿主的想法非常重要，因为当病原体从疾病宿主溢出时，易感物种更有可能灭绝。因此，我们将在实验室和中游系统中测试这些想法，在这些系统中，我们能够操纵树枝杆菌潜在储藏者的密度和类型。我们的第一笔NERC赠款的结果表明，有强有力的证据表明，两栖动物(特别是非洲爪哇和北美牛蛙)的国际贸易正在导致树蛙多次被引入英国和欧洲大陆。我们需要知道这些不同的病原体菌株在致病能力上是否存在差异，为了测试这一想法，我们将在我们的模式物种--常见的蟾蜍中华大蟾蜍--身上进行挑战实验。一旦完成了这些比较，我们将开发数学流行病学模型，明确目标是预测几个尺度上的疾病出现的动态。我们有强有力的证据表明，温度是壶菌引起的死亡率的关键决定因素，我们将开发统计模型来确定全球变暖对疾病在欧洲境内分布的潜在影响。我们的目标是，这些短期和长期的研究目标最终将使我们能够控制真菌传播或管理两栖动物种群，以防止与这种病原体的出现相关的种群下降。

Of the animal groups for which comprehensive assessments have been made, amphibians rank as the most threatened major taxon, with nearly three times as many amphibian species threatened with extinction as bird species. Disease has been identified as one of the major contributors to amphibian declines and extinctions, with one pathogen singled out as the most dangerous amphibian disease identified to date. Batrachochytrium dendrobatidis, a chytridiomycete fungus, has been implicated in mass mortality events, population declines and species extinctions around the globe. The Amphibian Conservation Summit recently organized by the IUCN recognized that the current state of knowledge regarding this fungus is manifestly inadequate. In response to this research gap, the Summit produced a Declaration calling for immediate research to determine the distribution of the disease, how this distribution was achieved and the consequences of invasion of the pathogen for local amphibian communities. Funded by NERC, we have completed three years of monitoring B. dendrobatidis and chytridiomycosis across Europe and have shown for the first time that infection is widespread, is present in the wild in the UK, and is unfortunately present in the Mallorcan midwife toad, one of the most critically endangered species of amphibian. We now need to add to our current descriptions of the prevalence of infection and test hypotheses generated by these patterns in order to ascertain the processes by which B. dendrobatidis causes populations to collapse. We will do this by using a combination of field surveys and laboratory experiments in order to understand how the dynamics of infection in natural populations lead to extinction. Firstly, we will intensively survey five focal study sites where the disease is present, but is causing different effects in the amphibian populations. We will sample these communities over three years in order to track how infection moves through the various species, and how infection differentially affects larval (tadpole), metamorph and adult stages within a community of amphibians. Using our newly developed environmental molecular assay, we are now able to test the density of infectious stages in the environment. This gives us the ability to directly measure the exposure-levels of amphibians and to assess whether the fungus can persist in the environment in the absence of its host. The idea that there are multiple reservoirs of infection is very important, as extinction is more likely in susceptible species when pathogen 'spillover' occurs from disease reservoirs. We will therefore test these ideas in laboratory and mesocosm systems where we are able to manipulate the density and type of potential reservoirs of B. dendrobatidis. Results from our first NERC grant have shown that there is strong evidence that the international trade in amphibians (specifically Xenopus and North American bullfrogs) is causing multiple introductions of B. dendrobatidis into the UK and mainland Europe. We need to know whether there is variation between these different strains of the pathogen in their ability to cause disease, and to test this idea we will perform challenge experiments in our model species, the common toad Bufo bufo. Once these comparisons have been completed, we will develop mathematical epidemiological models with the explicit goal of predicting the dynamics of disease emergence across several scales. We have strong evidence that temperature is a key determinant of chytrid-driven mortality, and we will develop statistical models to determine the potential effects of global-warming on the distribution of the disease within Europe. Our aim is that these short and long term research goals will eventually enable us to control fungal spread or manage amphibian populations in order to prevent the population declines that are associated with the emergence of this pathogen.