University of Michigan
Department of Ecology and Evolutionary Biology

North American bullfrog. (stock image)

The global trade in bullfrogs, which are farmed as a food source in South America and elsewhere, is spreading a deadly fungus that is contributing to the decline of amphibians worldwide, according to Professor Timothy James.

Amphibian populations are declining worldwide at an alarming rate, and the spread of the deadly chytrid fungus is believed to be a contributing factor. The fungus infects the skin of frogs, toads and salamanders.

In a study to be published in an upcoming edition of the journal Molecular Ecology, University of Michigan evolutionary biologist James and his colleagues examine the role of bullfrog farming in spreading the chytrid fungus between the forests and frog farms of Brazil and then to the United States and Japan.

The researchers collected and analyzed bullfrogs sold at Asian food shops in seven U.S. cities and found that 41 percent of the frogs were infected with chytrid fungus, which is harmless to humans. Frogs in these shops are imported live primarily from farms in Taiwan, Brazil and Ecuador and sold as food for their legs.

James and his colleagues also analyzed bullfrogs from frog farms in Brazil and several native frog species from Brazil's Atlantic Forest, one of the most amphibian-rich regions in the world. Their DNA sequencing studies identified the various strains of the chytrid fungus, Batrachochytrium dendrobatidis or Bd for short, present in the frogs.

The studies revealed four previously unknown strains of chytrid, including one that was present on a live bullfrog sold for frog legs in an Asian market in southeastern Michigan. The team determined that the Michigan bullfrog probably came from a frog farm in Brazil's Atlantic Forest. Sampling of native frogs in that region revealed that the four closely related strains, known collectively by the researchers as the Brazilian strain or Bd-Brazil, are common in Brazil. By comparing their results to data in a previously published study, the researchers showed that the Brazilian strain is also present in bullfrogs in Japan.

The data suggest that the Bd-Brazil chytrid strain probably originated in Brazil among native frogs, rather than being introduced to the country by imported bullfrogs. The Bd-Brazil strain likely spread from infected native frogs to a Brazilian bullfrog farm and from there to other locations in bullfrogs shipped globally.

"Our data suggest that chytrid strains have been vectored across the globe by bullfrogs, which may have ultimately led to the disease being so widespread," said James, an assistant professor in the Department of Ecology and Evolutionary Biology.

"A lot of the movement of this fungus is related to the live food trade, which is something we should probably stop doing," James said. "We don't need to have millions of live frogs being shipped from foreign countries into the United States."

University of Michigan News Service press release