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High chytrid prevalence and infection intensities in tadpoles of Mixophyes fleayi

Matthijs Hollanders , Laura Grogan, Hamish McCallum, David Newell

Abstract

Context: The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused catastrophic biodiversity loss globally, but species and life-stages within species respond differently to the pathogen. Although tadpoles are often reported to be less vulnerable to disease, they can constitute important infection reservoirs in ecosystems. Aims: We aimed to describe Bd infection patterns of a long-lived tadpole in a species where post-metamorphic animals appear to exhibit limited mortality due to chytridiomycosis. We further investigated how oral dekeratinisation can be used as an indicator for infection. Methods: We conducted surveys over two years for tadpoles of Mixophyes fleayi (Fleay’s barred frog) at two rainforest streams on the east coast of Australia to assess patterns in Bd infection prevalence and intensity. We developed an integrated hierarchical model propagating pathogen detection errors and incorporated how Bd infections affect oral dekeratinisation. Key results: We found that Bd infection prevalence was strongly associated with lower temperatures and larger body size, consistent with Bd optimal thermal range and a cumulative risk of exposure for tadpoles. The individual probability of a tadpole being infected with Bd was estimated to be 0.58 [95% HPDI: 0.432, 0.713], approximately 8 times greater than for adults at the same sites. Tadpoles infected with Bd were 113 [29, 293] times more likely have oral dekeratinisation than uninfected tadpoles, where uninfected individuals were estimated to have a 0.05 [95% HPDI: 0.011, 0.11] probability of having mouthpart loss. Conclusions: Our results show that M. fleayi tadpoles are more likely to be infected with Bd than adults, suggesting that tadpoles could contribute to Bd maintenance in streams. We further show that sites can be rapidly assessed for Bd by visually checking for oral dekeratinisation. Implications: Long-lived tadpoles in general may contribute to Bd maintenance in ecosystems. We suggest continued exploration of Bd immunocompetence across amphibian life stages to further understand the vastly different infection patterns.

WR23126  Accepted 17 August 2024

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