Oral Chytridiomycosis in the Mountain Yellow-Legged Frog (Rana muscosa)
The chytrid fungus Batrachochytrium dendrobatidis was originally reported in wild frog populations in Panama and Australia, and from captive frogs in the U.S. National Zoological Park (Washington, DC). This recently described fungus affects the keratinized epidermis of amphibians and has been implicated as a causative factor in the declines of frog populations. We report here the presence of B. dendrobatidis in larval and recently metamorphosed mountain yellow-legged frogs (Rana muscosa) in or near the Sierra Nevada Mountains of California, an area where declines have been documented in all five species of native anurans. Forty-one percent (158 of 387) of larval R. muscosa examined in the field with a hand lens and 18% (14 of 79) of preserved larvae had abnormalities of the oral disc. Twenty-eight larvae were collected from 10 sites where tadpoles had been observed with missing or abnormally keratinized mouthparts, and 24 of these were examined for infection. Sixty-seven percent (16 of 24) of these tadpoles were infected with B. dendrobatidis. Batrachochytrium dendrobatidis was cultured from both tadpoles and recent metamorphs from one of these sites. Tadpoles with mouthpart abnormalities or confirmed chytrid fungus infections were collected at 23 sites spanning a distance of > 440 km and an elevational range from 1658–3550 m. Life-history traits of R. muscosa may make this species particularly susceptible to infection by Batrachochytrium. We recommend that biologists examine tadpoles for oral disc abnormalities as a preliminary indication of chytridiomycosis. Further, we believe that biologists should take precautions to prevent spreading this and other amphibian diseases from one site to another.Abstract
Oral discs of a Rana muscosa tadpoles collected 25 1999 May at Y-764, Dry Creek, Mono County, California. (A) Note that there would normally be three toothrows on top and four on the bottom. In this individual, there are only short, broken, asymmetrical toothrows. The beak still retains most of the black pigment. The oral papillae do not show well, but they are neither swollen nor noticeably red. (B) All toothrows are missing in this individual; the beak is pale rather than jet black, and the oral papillae are swollen, and some of the papillae are noticeably red. (C) Normal oral disc in sagital section. Both jaw sheaths and two anterior and three posterior toothrows are present. Note the presence of black pigment at the tips of the jaw sheaths and toothrows and that the jaws and toothrows terminate in a point to various degrees. H&E stain. Accession number 98RaMu011. (D) Buccal cavity and oral disc in sagital section with oral chytridiomycosis. Note absence of black pigment at tips of jaw sheaths and toothrows, rounding of the tips of the jaw sheaths and toothrows, and increased thickness and cellularity in the epithelium of the same structures. H&E stain. Accession number 98RaMu141. (E) Posterior (lower) jaw sheath (same tadpole as panel C) showing marked oral chytridiomycosis. Note the complete absence of black pigment, the loss of a cutting edge, fraying of the surface epithelium, and the presence of numerous small clear spaces (chytrid zoosporangia) in the surface cells (see panel F). H&E stain. (F) Epithelium of posterior (lower) jaw sheath. Note the numerous round and ovoid clear chytrid zoosporangia in the pale thickened surface epithelium, absence of black pigment, and absence of an inflammatory cell reaction in the underlying dermis. H&E stain. Accession number 98RaMu143