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 mouthparts, 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.Abstract
Counts of post-metamorphic Rana muscosa (live and dead) and tadpoles at the intensive survey site before (2002) and after (2003) the appearance of Batrachochytrium dendrobatidis (A), and jaw sheath pigmentation scores for tadpoles at the same site (B). Numbers next to dots in (B) indicate the number of tadpoles represented by a single dot. The higher tadpole counts in 2003 than in 2002 are likely attributable to the fact that in 2002 all tadpoles were age-1, an age at which they are difficult to see due to their small size. In 2003, all tadpoles were age-2, were considerably larger, and as a result were much more visible. This R. muscosa population was extinct by 2004
Estimated relationship between the log odds of Batrachochytrium dendrobatidis presence and the degree of upper jaw sheath pigmentation (including approximate 95% confidence interval), as determined from the generalized additive model.Log odds on the y-axis were standardized to have an average value of zero. Hatch marks above the x-axis indicate observed values, with some marks slightly offset to allow display of identical values