Amphibian carriers of chytridiomycosis

North American Bullfrog (Lithobates catesbeianus)

The North American Bullfrog (Lithobates catesbeianus)

As has been mentioned in earlier posts, some species of amphibians are believed to be reservoirs, or carriers, of the dreaded amphibian chytrid fungus. One of the most well known of these is the North American Bullfrog (Lithobates catesbeianus), an invasive species in the Western United States, South America, Asia and Europe, among others. North American Bullfrogs are farmed for frog legs in some of these countries, and invasive wild populations may have started from escaped captive individuals. Poorly maintained bullfrog farms also provide ideal breeding grounds for amphibian disease, which is one of many reasons why some people do not support the frog leg trade (myself among them). An experiment was performed where several North American Bullfrogs and some Dyeing Poison Dart Frogs (Dendrobates tinctorius) were infected with Bd  while in the laboratory. The Bullfrogs were relatively unaffected by the chytrid, but the infected poison dart frogs succumbed before the experiment even ended. Why North American bullfrogs are so resistant is not yet known, but it is believed that the community of bacteria that live on their skin may play a role. (As a side note, the North American Bullfrog can be differentiated from the Northern Green Frog (Lithobates clamitans melanota) by the lack of dorsolateral folds of skin extending from the eye to above the groin that are present in the Green Frog. These species are sympatric in the Southeastern United States, including Virginia.)

A second North American species that has only recently been implicated as another carrier of chytrid is the Pacific Chorus Frog (Pseudacris regilla). Chorus frogs infected in the laboratory experienced greater resistance to chytridiomycosis, with only one individual dying at the conclusion of the test. It was also observed that the range of the Pacific Chorus Frog increased in the Sierra Nevadas of the Western United States, even as the Mountain Yellow-legged Frog (Rana muscosa/sierrae) populations were declining. Unlike with the North American Bullfrog, Pacific Chorus Frogs are not invasive in many areas of the world. If the factors that go into increased resistance of these species to the chytrid fungus can be identified, perhaps “cures” can be found for more susceptible species like poison dart frogs (Dendrobatidae), and toads (Bufonidae).
Frog: 1, Fungus: 3 [I am counting this for the fungus, even though the carriers are technically well off]

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