Tadpole mouthpart depigmentation as an accurate indicator of chytridiomycosis, an emerging disease of amphibians

Chytridiomycosis is an emerging infectious disease of amphibians caused by the fungal pathogen Batrachochytrium dendrobatidis, and its role in causing population declines and species extinctions worldwide has created an urgent need for methods to detect it. Several reports indicate that in anurans chytridiomycosis can cause the depigmentation of tadpole tnouthparts, but the accuracy of using depigmentation to determine disease status remains uncertain. Our objective was to determine for the Mountain Yellow-legged Frog (Rana muscosa) whether visual inspections of the extent of tadpole mouthpart depigmentation could be used to accurately categorize individual tadpoles or R. muscosa populations as B. dendrobatidis-positive or negative. This was accomplished by assessing the degree of mouthpart depigmentation in tadpoles of known disease status (based on PCR assays). The depigmentation of R. muscosa tadpole mouthparts was associated with the presence of B. dendrobatidis, and this association was particularly strong for upper jaw sheaths. Using a rule that classifies tadpoles with upper jaw sheaths that are 100% pigmented as uninfected and those with jaw sheaths that are <100% pigmented as infected resulted in the infection status of 86% of the tadpoles being correctly classified. By applying this rule to jaw sheath pigmentation scores averaged across all tadpoles inspected per site, we were able to correctly categorize the infection status of 92% of the study populations. Similar research on additional anurans is critically needed to determine how broadly applicable our results for R. muscosa are to other species.