Wednesday, 12 May 2010

Endemic v Epizootic Bd in the US Sierra Nevadas

Two papers in PNAS document 13 years work on the emergence of Bd in the Sierra Nevadas by Vance Vredenberg and Cheryl Briggs. The papers contrast two montane scenarios, one where Bd is causing a year-on-year extirpation of mountain yellow-legged frogs Rana muscosa (see picture), and one where Bd has formed a stable persistent association with the frogs. Cheryl and Vance argue, and show using mathematical models, that these dynamics can be explained by density-dependent host-pathogen dynamics. So, if Bd hits a naive high-density population, then individuals are overwhelmed by high-intensity infections and the population is lost. Conversly, lower-density populations can tolerate lower Bd loads and persist. The conclusions from these studies parallel what has been observed in other regions, Australia (Taudactylus eungellensis) and Europe (Alytes obstetricans), where low-density populations persist past the epizootic 'hit'. An idea that is mooted from this work: that mitigation of infection may be possible by manipulating infection loads in the wild, perhaps by removing asymptomatic reservoirs (such as overwintering larvae). So, perhaps the best way to conserve a species is to decrease it to below a predicted critical density in advance of the wave of spread. Could this be used in the Pyrenees where infection is still very local? And, are such population manipulations ethically defensible? Many of these ideas will no doubt be discussed in the upcoming Bd mitigation workshop in Zurich this autumn.