Briggs Cheryl J, Knapp Roland A, Vredenburg Vance T
Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA 93106, USA.
Proc Natl Acad Sci U S A. 2010 May 25;107(21):9695-700. doi: 10.1073/pnas.0912886107. Epub 2010 May 10.
Chytridiomycosis, the disease caused by the chytrid fungus, Batrachochytrium dendrobatidis (Bd), has contributed to amphibian population declines and extinctions worldwide. The impact of this pathogen, however, varies markedly among amphibian species and populations. Following invasion into some areas of California's Sierra Nevada, Bd leads to rapid declines and local extinctions of frog populations (Rana muscosa, R. sierrae). In other areas, infected populations of the same frog species have declined but persisted at low host densities for many years. We present results of a 5-year study showing that infected adult frogs in persistent populations have low fungal loads, are surviving between years, and frequently lose and regain the infection. Here we put forward the hypothesis that fungal load dynamics can explain the different population-level outcomes of Bd observed in different areas of the Sierra Nevada and possibly throughout the world. We develop a model that incorporates the biological details of the Bd-host interaction. Importantly, model results suggest that host persistence versus extinction does not require differences in host susceptibility, pathogen virulence, or environmental conditions, and may be just epidemic and endemic population dynamics of the same host-pathogen system. The different disease outcomes seen in natural populations may result solely from density-dependent host-pathogen dynamics. The model also shows that persistence of Bd is enhanced by the long-lived tadpole stage that characterize these two frog species, and by nonhost Bd reservoirs.
壶菌病是由壶菌真菌——蛙壶菌（Batrachochytrium dendrobatidis，简称Bd）引起的疾病，它导致了全球两栖动物数量的减少和灭绝。然而，这种病原体的影响在两栖动物物种和种群之间差异显著。入侵加利福尼亚内华达山脉的某些地区后，Bd导致蛙类种群（内华达山黄腿蛙、山黄腿蛙）数量迅速下降和局部灭绝。在其他地区，相同蛙类的受感染种群数量减少，但在低宿主密度下持续存在多年。我们展示了一项为期5年的研究结果，表明在持续存在的种群中，受感染的成年蛙真菌负荷较低，年复一年存活下来，并且经常感染和清除感染。在此，我们提出一个假设，即真菌负荷动态可以解释在内华达山脉不同地区以及可能在全世界观察到的Bd不同的种群水平结果。我们开发了一个纳入Bd与宿主相互作用生物学细节的模型。重要的是，模型结果表明，宿主的持续存在与灭绝并不需要宿主易感性、病原体毒力或环境条件存在差异，可能只是同一宿主 - 病原体系统的流行和地方病种群动态。在自然种群中看到的不同疾病结果可能仅由密度依赖的宿主 - 病原体动态导致。该模型还表明，这两种蛙类特有的长寿蝌蚪阶段以及非宿主Bd库增强了Bd的持久性。
