Maintenance and transmission of the amphibian pathogen, Batrachochytrium dendrobatidis, during metamorphosis

Background/Question/Methods

Transmission is a key component of infectious disease dynamics.  When a pathogen can infect multiple life stages of a host, transmission among these stages may greatly influence the effects of the pathogen on populations and communities.  In the emerging infectious pathogen of amphibians, Batrachochytrium dendrobatidis (Bd), infection at the larval and post-metamorphic stages is localized in the mouthparts and epidermis, respectively.  Even if an individual is infected at the larval stage, it must become re-infected during or after metamorphosis to maintain infection.  The chances of re-infection may depend on the species, its environment, and the presence of other hosts.  In experimental mesocosms, we manipulated the presence of Bd, larval density, and larval species richness in two species of amphibians (Rana cascadae and Pseudacris regilla).  We then monitored infection of both species through metamorphosis. 

Results/Conclusions

Both species demonstrated a reduction in infection prevalence during metamorphosis.  While a high percentage of our larvae were infected (up to 100%), we detected Bd on only 18.2% of R. cascadae after metamorphosis.  In contrast, 81.5% of P. regilla remained infected after metamorphosis.  R. cascadae had consistently lower infection levels at both the larval and post-metamorphic stages compared to P. regilla.  There was a trend for the presence of larval P. regilla to increase infection prevalence in post-metamorphic R. cascadae when the two species were together, indicating pathogen spillover from P. regilla.  We also found that infection in post-metamorphic P. regilla was influenced by the density of conspecifics.  When P. regilla larvae were reared at low densities, they took longer to complete metamorphosis leading to higher post-metamorphic infection prevalence, presumably due to increased time in contact with infectious water.  These results indicate that amphibian larvae can recover from Bd infection as they undergo metamorphosis, but the chances of maintaining infection may be affected by larval infection severity, density of infected hosts, and the presence of other host species.