Spontaneous oral chytridiomycosis in wild bullfrog tadpoles in Japan

Batrachochytrium dendrobatidis (Bd) infects Anuran larvae (tadpole) mouthparts and causes oral chytridiomycosis, which can be diagnosed in tadpoles by detecting mouthparts deformities. However, oral chytridiomycosis may or may not be observable, depending on species, tadpole stage and season, and has never been reported in Japan. We aimed to observe oral chytridiomycosis characteristics in bullfrog (Lithobates catesbeiana) tadpoles, determine associated pathologic features and investigate the usability of bullfrog tadpoles in Japanese Bd field surveys. Wild-captured bullfrog tadpole mouthparts were examined macroscopically, histopathologically and by molecular biological examination. Macroscopic lesions were observed in 21 of 59 tadpole mouthparts. Lesions were most frequently located in the lower jaw sheaths and were mainly recognized by partial depigmentation (11 tadpoles; some were completely depigmented) and thinning of the pigmented layer (10 tadpoles). Partial defects of the tips and blunt cutting edges of the jaw sheaths were observed with severe jaw sheath depigmentation. Whitened tooth rows were observed in 7 tadpoles. Histologically, the stratified epithelium (pigmented epithelium) showed partial or diffuse hypopigmentation or pigment loss. Irregular stratified epithelium thickening with hyperkeratosis or parakeratosis was observed in the jaw sheaths. Bd infection was confirmed in 20 of 21 tadpoles presenting jaw sheath deformities, by histopathological examination and/or nested polymerase chain reaction. Depigmentation and thinning of the pigmented layers of jaw sheaths were associated with Bd infection. Thus, diagnosis of Bd infection by macroscopic observation of bullfrog tadpole mouthparts is feasible. This is the first report of oral chytridiomycosis in wild bullfrog tadpoles in Japan.