Deadly to amphibians, a fungal strain emerged in Brazil and spread around the world

Batrachochytrium dendrobatidis is responsible for the decline of toad and frog populations across the globe. The origin of the fungal strain was the subject of a study led by researchers at the State University of Campinas.

By André Julião  |  Agência FAPESP – The chytrid fungus (Batrachochytrium dendrobatidis), or Bd for short, is considered one of the causes of the worldwide decline in amphibian populations. In recent years, researchers have identified different genetic strains of the fungus that causes chytridiomycosis in various parts of the world. This has already led to the decline of at least 500 species of anuran amphibians, including toads and frogs.

The global spread of the fungus is attributed to the trade of bullfrogs (Aquarana catesbeiana), a North American species farmed in several countries for its meat. The species was first introduced to Brazil in 1935, and a second population was introduced in the 1970s (read more at agencia.fapesp.br/30127 and agencia.fapesp.br/39532). 

One of the fungal strains, Bd-Brazil, was named after the country where it was originally found in 2012. However, its exact origin was later disputed. In 2018, an article published in the journal Science suggested that the strain originated on the Korean Peninsula. This led to the renaming of the genotype to Bd-Asia-2/Bd-Brazil.

A study now published in the journal Biological Conservation and supported by FAPESP confirms that the strain, which has also been detected in the United States, Japan, and the Korean Peninsula, originated in Brazil. The study was led by researchers from the State University of Campinas (UNICAMP) in the state of São Paulo, Brazil.

The Bd-Brazil strain was already present in Brazil by 1916, two decades before the first bullfrogs arrived, as a 2014 article in the journal Molecular Ecology by São Paulo researchers supported by FAPESP indicates. In that study, the authors identified strains of the fungus in frog samples deposited in museums since the 19th century through genotyping.

This recently published article cross-referenced data from scientific literature, such as the aforementioned article, with the detection of Bd in amphibians in museums around the world, genotyping of the fungus in Brazilian bullfrog farms, and genetic analyses of these animals sold in other countries. The researchers concluded that the fungus spread from Brazil to the rest of the world through the international trade of frog meat, in which Brazil is a major supplier.

“This genotype is highly prevalent in different native Brazilian species, with very old records. When we look elsewhere, the records are much more recent and occur only in bullfrogs and other exotic species. Here, however, the strain is present both in frog farms and in the wild, including some native species that do not develop the disease,” says Luisa P. Ribeiro, the first author of the study. She conducted the study as part of her doctoral research at the Institute of Biology (IB) at UNICAMP with a scholarship from FAPESP. 

The work is part of the project “From Natural History to the Conservation of Brazilian Amphibians”, which is supported by FAPESP and is coordinated by Luís Felipe Toledo, who is a professor at IB-UNICAMP and Ribeiro’s doctoral advisor.

“We were unable to identify the exact strain in a large sample of amphibians deposited in museums, since conservation isn’t always ideal for maintaining this information. Therefore, we only identified the presence or absence of the fungus in these individuals and sought other evidence that could indicate whether or not Bd-Brazil originated in Brazil,” says Toledo.

Microscopy of the chytrid fungus Batrachochytrium dendrobatidis (Bd), the causative agent of chytridiomycosis, shows zoosporangia and zoospores responsible for infection in amphibians (photo: Mariana Pontes)

History of a fungus

To assess its historical distribution, foreign collaborators examined 2,280 amphibian specimens collected between 1815 and 2014 and deposited in zoology museums around the world.

“Even without knowing the strain, we found records older than those previously reported in the literature and presented a review of historical records of Bd worldwide,” says Ribeiro, who is currently pursuing postdoctoral studies at the Federal University of São Paulo (UNIFESP) with a fellowship from FAPESP. 

Of the 2,280 museum specimens tested, 40 tested positive for Bd. The oldest record of infection by the fungus was found in five individuals of the species Alytes obstetricans. These frogs were collected in 1915 in the Pyrenees in France and are the oldest known in the country. The second oldest specimen was a frog of a species currently found in Rio de Janeiro, Megophrys goeldii, collected in 1964.

To reinforce the hypothesis of dissemination from Brazil to the rest of the world through the export of bullfrogs, the researchers resorted to historical records of trade in the species, genotyping of the fungus in frogs from Brazilian frog farms, and genetic analyses of bullfrogs from markets outside the country.

Bd-Brazil is prevalent in Brazil, with more than half of the occurrences found on frog farms. It is less virulent than the second most common strain, Bd-GPL, which probably originated in Asia.

The researchers analyzed 3,617 international frog meat trade routes involving 48 countries. Only 12 were identified as exporters, 21 as importers, and 15 as both. They integrated data from routes involving countries with confirmed Bd-Brazil occurrences, the earliest detection dates in each country, and genetic evidence from commercialized bullfrogs. Thus, the most plausible dispersion routes for the Brazilian strain could be established.

The hypothesis of spread from Brazil involved eight identified routes. Brazil exported directly to the United States between 1991 and 2009, and the United States exported to South Korea in 2004 and 2008. Since there are no documented exports from the Asian country where Bd-Brazil was confirmed to other affected countries, it is highly likely that Brazil is indeed the origin of the genotype.

The authors conclude that these results underscore the urgent need for preventive measures, such as import controls, pathogen screening, and quarantine protocols, to safeguard native species, with global regulation and monitoring.

: Brazilian frog farm dedicated to breeding bullfrogs (Aquarana catesbeiana). The use of untreated water from the surrounding area and high animal density may contribute to infection by environmental fungi (photo: Luisa Ribeiro)

The article “Origin and global spread of an endemic chytrid fungus lineage linked to the bullfrog trade” can be read at sciencedirect.com/science/article/pii/S0006320725005841.