Date of Award

12-2011

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Aram J.K. Calhoun

Second Committee Member

Joyce E. Longcore

Third Committee Member

Seth Tyler

Abstract

Chytridiomycosis, an emerging infectious disease caused by the chytrid fungus Batrachochytrium dendrobatidis, threatens anuran populations worldwide. This fungus colonizes mouthparts of amphibian larvae and superficial epidermis of post-metamorphic amphibians. Effects of chytridiomycosis on frog species are variable. Some species typically develop minor infections and may function as carriers; others typically develop lethal infections that can lead to population declines. Mild infections in the bullfrog (Lithobates catesbeianus) are well-documented. In contrast, recently metamorphosed wood frogs (L. sylvaticus) can die from chytridiomycosis. Here, the bullfrog and the wood frog serve as model species in which to investigate causes and consequences of the chytridiomycosis susceptibility spectrum.

Growth of B. dendrobatidis within host cells has been documented by light and transmission electron microscopy; however, entry of the fungus into host cells has not. To document how B. dendrobatidis enters and affects host cells in resistant and susceptible hosts, we inoculated bullfrogs and wood frogs, documented infection with transmission electron microscopy, and compared structure of epidermis between species with light microscopy. In bullfrogs and wood frogs, zoospores encysted on the skin surface and produced morphologically similar germination tubes that penetrated host cell membranes and enabled transfer of zoospore contents into host cells. Number of epidermal cell layers and thickness of epidermis were greater in bullfrogs than in wood frogs. We encourage further research on skin structure and function (e.g., supply of epidermal layers, pace of epidermal cell turnover) as potential determinants of susceptibility to chytridiomycosis.

The hypothesis that bullfrogs serve as vectors, transmitting B. dendrobatidis to susceptible species, has not been verified. We conducted an ex-situ experiment to determine whether B. dendrobatidis-infected bullfrogs could transmit the fungus to wood frog tadpoles when the two species shared a body of water. We tested for B. dendrobatidis infections with quantitative polymerase chain reaction (PCR) in a subsample of the wood frog tadpoles and in all metamorphosed wood frogs and compared risk of death of froglets exposed and unexposed to infected bullfrogs. B. dendrobatidis was sporadically detected in sub-sampled treatment tadpoles and frequently detected in treatment froglets. Pooled risk of froglet death was higher in treatment enclosures than in control enclosures. This underscores the importance of quantifying lethal and sub-lethal effects of bullfrog vectors on B. dendrobatidis susceptible species. Our results also indicate that, at the low infection loads bullfrogs tend to carry, swabbing for PCR analyses may underestimate prevalence of B. dendrobatidis in this species. We highlight bullfrog disease screening as a management challenge, especially in light of exotic bullfrog colonies on multiple continents and large scale global trade in this species.

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