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Bull­frogs may help spread deadly amphib­ian fun­gus, but also die from it

pub­lished 18 June 2013 | mod­i­fied 30 May 2014

BullfrogAmphib­ian pop­u­la­tions are declin­ing world­wide and a major cause is a deadly fun­gus thought to be spread by bull­frogs (Rana cates­beiana), but a two-​year study shows they can also die from this pathogen, con­trary to sug­ges­tions that bull­frogs are a tol­er­ant car­rier host that just spreads the disease.

When researchers raised the frogs from eggs in con­trolled exper­i­men­tal con­di­tions, they found at least one strain of this pathogen, Batra­chochytrium den­dro­ba­tidis, also called Bd or a chytrid fun­gus, can be fatal to year-​old juve­niles. How­ever, bull­frogs were resis­tant to one other strain that was tested.

The find­ings, made by researchers at Ore­gon State Uni­ver­sity (OSU) and the Uni­ver­sity of Pitts­burgh, have been pub­lished in the March issue of the jour­nal Eco­Health. They show that bull­frogs are not the sole cul­prit in the spread of this deadly fun­gus, and add fur­ther com­plex­ity to the ques­tion of why amphib­ians are in such seri­ous jeopardy.

About 40 per­cent of all amphib­ian species are declin­ing or are already extinct, researchers say. Var­i­ous causes are sus­pected, includ­ing this fun­gus, habi­tat destruc­tion, cli­mate change, pol­lu­tion, inva­sive species, increased UV-​B light expo­sure, and other forces.

At least so far as the chytrid fun­gus is involved, bull­frogs may not be the vil­lains they are cur­rently made out to be
Stephanie Ger­vasi, zool­ogy researcher, OSU Col­lege of Science »

“The con­ven­tional wis­dom is that bull­frogs, as a tol­er­ant host, are what helped spread this fun­gus all over the world. But we’ve now shown they can die from it just like other amphibians.”

The research sug­gests that bull­frogs actu­ally are not a very good host for the fun­gus, which first was iden­ti­fied as a novel dis­ease of amphib­ians in 1998. So why the fun­gus has spread so fast, so far, and is caus­ing such mor­tal­ity rates is still not clear.

“One pos­si­bil­ity for the fun­gal increase is cli­mate change, which can also com­pro­mise the immune sys­tems of amphib­ians,” said Andrew Blaustein, a dis­tin­guished pro­fes­sor of zool­ogy at OSU and inter­na­tional leader in the study of amphib­ian declines. “There are a lot of pos­si­ble ways the fun­gus can spread. Peo­ple can even carry it on their shoes.”

The aver­age infec­tion load of the chytrid fun­gus in bull­frogs, regard­less of the strain, is con­sid­er­ably lower than that of many other amphib­ian species, researchers have found. Some bull­frogs can reduce and even get rid of infec­tion in their skin over time.

While adult bull­frogs may be car­ri­ers of some strains of Bd in some areas, the researchers con­cluded, dif­fer­ent hosts may be as or more impor­tant in other loca­tions. Inter­na­tional trade of both amphib­ian and non-​amphibian ani­mal species may also drive global pathogen dis­tri­b­u­tion, they said.

(Source: OSU news release, 17.06.2013)

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