Two papers out this week on the population dynamics and epidemiology of the chytrid pathogen of amphibians, Batrachochytrium dendrobatidis (Bd). This is work from the Vredenburg and Briggs labs that includes several decade-long studies of frog declines and the prevalence of Bd.
In the Briggs et al paper, they describe a 5-year study on the fungal load in surviving populations of frogs in Sierra Nevada mountain lakes. They find that adult frogs that have low enough fungal load escape chytridiomycosis and can actually lose and regain infection. They propose that fungal load dynamics are the reason behind differential survival of various populations of mountain frogs. They conclude that:
“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.”
So they propose that differences in the populations that are coming down with the disease is due only to “density-dependent host–pathogen dynamics” not that some populations are resistant. They go on to provide a detailed model of persistence if the host and pathogen, chance of reinfection, and survival of the host which is derived from the long-term study data. There are many more interesting findings and models proposed in the paper. It also further reinforces (for me) the need to know more about the molecular basis of the host-pathogen interactions and more about how the fungus persists without a host, lifestyle of how it overwinters, and the details of the microbe-host interactions, and the infection dynamic when zoospores disperse from infected frogs.
The Vrendenburg et al paper adresses the dynamics of population decline in the mountain yellow-legged frogs over a periods of 1-5 and 9-13 year study in 3 different study sites at different sampling intervals. The authors were able to catalog the species decline and conduct skin swabbing to assess Bd prevalence. They found that the fungus spread quickly as it could detected in virtually all the lakes over the course of a year starting with a 2004 survey. The dramatic declines of frog populations in these lakes followed in the years subsequent to the initial detection. This sadly predicts that most if not all of the mountain lakes will go extinct for the frogs as the current tadpoles develop into frogs in the next 3 years and then fall victim to Bd. Based on their sampling work, the authors were also able to correlate what fungal burden predicted a subsequent decline – in populations where more the ~10,000 zoospores were detected in a swab from frog skin, then the frog population was about to experience a sharp decline. The take-home from this work is that finding ways to keep the intensity of fungal infections down could provide a meaningful intervention that could prolong the viability of the population.
Briggs, C., Knapp, R., & Vredenburg, V. (2010). Enzootic and epizootic dynamics of the chytrid fungal pathogen of amphibians Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0912886107
Vredenburg, V., Knapp, R., Tunstall, T., & Briggs, C. (2010). Dynamics of an emerging disease drive large-scale amphibian population extinctions Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0914111107