Vredenburg Vance T, Knapp Roland A, Tunstall Tate S, Briggs Cheryl J
Department of Biology, San Francisco State University, San Francisco, CA 94132-1722, USA.
Proc Natl Acad Sci U S A. 2010 May 25;107(21):9689-94. doi: 10.1073/pnas.0914111107. Epub 2010 May 10.
Epidemiological theory generally suggests that pathogens will not cause host extinctions because the pathogen should fade out when the host population is driven below some threshold density. An emerging infectious disease, chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd) is directly linked to the recent extinction or serious decline of hundreds of amphibian species. Despite continued spread of this pathogen into uninfected areas, the dynamics of the host-pathogen interaction remain unknown. We use fine-scale spatiotemporal data to describe (i) the invasion and spread of Bd through three lake basins, each containing multiple populations of the mountain yellow-legged frog, and (ii) the accompanying host-pathogen dynamics. Despite intensive sampling, Bd was not detected on frogs in study basins until just before epidemics began. Following Bd arrival in a basin, the disease spread to neighboring populations at approximately 700 m/yr in a wave-like pattern until all populations were infected. Within a population, infection prevalence rapidly reached 100% and infection intensity on individual frogs increased in parallel. Frog mass mortality began only when infection intensity reached a critical threshold and repeatedly led to extinction of populations. Our results indicate that the high growth rate and virulence of Bd allow the near-simultaneous infection and buildup of high infection intensities in all host individuals; subsequent host population crashes therefore occur before Bd is limited by density-dependent factors. Preventing infection intensities in host populations from reaching this threshold could provide an effective strategy to avoid the extinction of susceptible amphibian species in the wild.
流行病学理论通常认为，病原体不会导致宿主灭绝，因为当宿主种群数量降至某个阈值密度以下时，病原体应该会逐渐消失。一种由真菌病原体蛙壶菌（Batrachochytrium dendrobatidis，简称Bd）引起的新发传染病——壶菌病，与数百种两栖动物物种近期的灭绝或严重衰退直接相关。尽管这种病原体仍在持续传播到未受感染的地区，但宿主与病原体相互作用的动态情况仍不为人所知。我们利用精细尺度的时空数据来描述：（i）Bd在三个湖盆中的入侵和传播情况，每个湖盆都有多个黄腿山蛙种群；（ii）随之而来的宿主 - 病原体动态变化。尽管进行了密集采样，但在研究的湖盆中，直到疫情开始前不久才在青蛙身上检测到Bd。Bd到达一个湖盆后，疾病以波浪状模式以每年约700米的速度传播到邻近种群，直到所有种群都被感染。在一个种群内部，感染率迅速达到100%，个体青蛙的感染强度也同步增加。青蛙的大规模死亡仅在感染强度达到临界阈值时才开始，并多次导致种群灭绝。我们的结果表明，Bd的高生长速率和毒力使得所有宿主个体几乎同时被感染并积累高感染强度；因此，随后宿主种群的崩溃发生在Bd受到密度依赖因素限制之前。防止宿主种群中的感染强度达到这个阈值，可能提供一种有效的策略，以避免野生易感两栖动物物种的灭绝。
