🐸 Why are we witnessing a global hecatomb of frogs and toads?

An alarming decline in frog and toad populations is being observed worldwide. For decades, scientists have suspected a pathogenic fungus, Batrachochytrium dendrobatidis (abbreviated Bd), of being responsible for this die-off, but the origin of this agent remains unclear. The origins of this devastating microorganism have long divided the scientific community.

A team from the State University of Campinas in Brazil conducted a thorough investigation to unravel this enigma. Their work, published in Biological Conservation, combines several methods to reconstruct the journey of a specific strain of the fungus. The results indicate a Brazilian origin, contradicting some previous hypotheses.


To go back in time, scientists examined more than 2,000 preserved amphibian specimens in museums around the world. These archives, some dating from the 19th century, allowed for the detection of the fungus in old samples. In Brazil, traces were identified as early as 1916, well before the introduction of bullfrogs, an exotic species.

The international trade of bullfrogs, native to North America but farmed in Brazil since the 1930s, appears as a major vector. This economic activity likely facilitated the spread of the Brazilian strain to other continents, via the export of frog meat, a market where Brazil plays an important role.

The researchers analyzed genetic data from Brazilian breeding farms and foreign markets. The Bd-Brazil strain is widespread locally, with a high prevalence in farms. Its detection in the United States, Japan, or South Korea coincides with documented trade routes, reinforcing the hypothesis of a spread from Brazil.

Unlike another more virulent strain of Asian origin, Bd-Brazil is less aggressive. It is present in both native and exotic species in Brazil, without always causing disease. This particularity could explain its discreet spread via trade, making its identification more difficult in new environments.

A silent killer

The fungus Batrachochytrium dendrobatidis, or Bd, belongs to a group of aquatic fungi called chytrids. It infects the skin of amphibians, disrupting their ability to regulate water and electrolytes. This disruption can lead to heart failure and death, often within a few weeks, without obvious external signs at the start of the infection.

Unlike many pathogens, Bd can survive in the environment without a host, thanks to resistant spores. This ability allows it to persist in bodies of water and spread easily between individuals. Infected amphibians can thus contaminate other animals through simple contact or via water, creating rapid infection foci.

Chytridiomycosis, the disease caused by Bd, has been identified as a major cause of the global decline of amphibians. Hundreds of species of frogs, toads, and salamanders have been affected, some on the brink of extinction. The speed of the spread has surprised scientists, leading to intensive research efforts to understand its biology.

The genetic diversity of the fungus, with strains like Bd-Brazil and Bd-GPL, influences its virulence and distribution. Understanding these differences helps develop conservation strategies, such as selecting resistant individuals or managing habitats, to mitigate the impacts on vulnerable populations.