The fungus Batrachochytrium dendrobatidis infects toads and frogs and is largely responsible for the global decline in amphibians. One of the most common strains of the Bd fungus is Bd-Brazil. Biologists have now discovered where this genotype originally comes from and how it was able to spread worldwide. Accordingly, the pathogen was brought to the USA via the bullfrog trade from farms in Brazil – and from there to South Korea and many other countries. This case shows how unregulated animal trade can endanger native amphibian populations.
The chytrid fungus (Batrachochytrium dendrobatidis), or Bd for short, infects amphibians and causes the usually fatal disease chytridiomycosis. This pathogen has already decimated the population of 500 different amphibian species worldwide, primarily toad and frog populations. 90 species have already become extinct because of it. In recent years, researchers have identified various genetic strains of this fungus. One of the five now most widespread strains was first found in Brazil in 2012 and was therefore named Bd-Brazil. Biologists later found evidence that this strain has also been present on the Korean peninsula since 2014 at the latest and could have originated there. But this strain has also been found in frogs in the United States and Japan since 2009, later tests revealed. Since then, there has been scientific debate about where the fungus really came from and how it could lead to a global epidemic.
Preserved amphibians reveal historical distribution of the Bd fungus
Researchers led by Luisa Ribeiro from the State University of Campinas (UNICAMP) in the Brazilian state of São Paulo have now investigated these questions. To do this, they first re-evaluated the specialist literature on the topic. To verify the historical distribution of the Bd-Brazil strain, Ribeiro and her colleagues also conducted extensive analyzes of their own. They examined 2,280 preserved frog specimens collected from museums around the world between 1815 and 2014 for infection with the Bd fungus. They also used a PCR test to determine which genotype the fungus belonged to, provided the condition of the samples allowed this. “Preservation is not always ideal for obtaining this information,” explains senior author Luís Felipe Toledo of UNICAMP.
The result: Of the 2,280 newly tested museum specimens, 40 were positive for Bd. The five oldest of these infected frogs all belonged to the species Alytes obstetricans and were collected in the French Pyrenees in 1915. The second oldest infected frog belonged to the species Megophrys goeldii and was preserved in Brazil in 1964. The literature comparison also showed that the world’s oldest Bd cases occurred before 1900, all in America, including Brazil in 1894. However, in these infected amphibian specimens, it was not possible to determine exactly which fungal strain it was. However, other samples and studies showed that the Bd-Brazil strain is the most widespread in frogs in Brazil today – and probably was in the past. “This genotype is very common in various native Brazilian frog species, with very old evidence,” reports Ribeiro. Another research team found this strain in Brazilian museum samples from 1916, as the literature search revealed.
“In Brazil, the fungal strain is now present both in frog farms and in the wild, including some native species that do not develop the disease,” adds Ribeiro. Bd-resistant species include bullfrogs (Aquarana catesbeiana). In samples from other countries, however, the researchers were only able to detect the Bd-Brazil fungal strain in younger specimens and in fewer amphibian species overall. There the strain “only occurred in bullfrogs and other exotic frog species,” says Ribeiro. Taken together, the researchers conclude from these findings: The Bd fungus probably originally comes from America and the specific Bd-Brazil fungus strain actually comes from Brazil.
Bd-Brazil: About the frog trade in foreign countries
But how did this strain of fungus spread worldwide? Researchers have long suspected that the fungus reached foreign territories through the international trade in bullfrogs. This species of frog is native to North America, but is farmed for its meat in several countries and the legs are sold globally as food. The frog species was first introduced to Brazil in 1935. The Bd-Brazil fungal strain existed in Brazil around two decades earlier in other frog species, as the literature analysis showed. Ribeiro and her colleagues therefore suspected that the fungus jumped from wild frogs to farmed frogs and was brought from Brazil to the rest of the world through the international trade in frog meat.
To test their hypothesis, the biologists then also examined skin swabs from live bullfrogs from farms in Brazil and other countries for this fungus and the exact fungal strain. Additionally, they analyzed historical records of trade in this frog species. These papers documented 3,617 international frog meat trade routes involving 48 countries, of which only 27 exported frog meat. Ribeiro and her colleagues compared these data with findings of Bd-Brazil occurrences in bullfrogs and used them to reconstruct the most plausible distribution routes of this fungal strain. The result: Bd-Brazil could have spread from Brazil through the trade in bullfrogs via eight different routes. Among other things, the fungal strain probably arrived from Brazil to the United States between 1991 and 2009 and from there to South Korea between 2004 and 2008. In addition to the USA, other possible routes include Ecuador, Italy, Thailand and China as stopovers in international trade starting from Brazil.
“Although our study provides strong evidence for a Brazilian origin of Bd-Brazil, alternative scenarios cannot yet be ruled out,” emphasize the researchers. It would also be conceivable, although less likely given the current state of knowledge, that the fungal strain could spread internationally from Asia. The tribe could have traveled via the frog trade from Japan via three routes to the USA and further to Brazil. In order to reconstruct the exact historical events, further samples and measurement data are necessary. According to the biologists, their study already shows that the trade in live animals such as frogs needs to be more strictly regulated and controlled. They call for screenings for possible pathogens and quarantine protocols for food animals and pets to prevent the spread of pathogens such as the Bd fungus. Native amphibians worldwide would then benefit from this.
Source: Luisa Ribeiro (State University of Campinas, UNICAMP) et al.; biological conservation, doi: 10.1016/j.biocon.2025.111547