Amphibian-killing fungus is widespread throughout Atlantic Rainforest

Fungus that infects skin and is one of the main causes of global amphibian decline is already present in Brazil's Amazon and Cerrado biomes.

By Elton Alisson

Agência FAPESP – A lethal infectious disease has been identified as one of the main causes of global amphibian population decline and species extinction. Amphibians are the most endangered group of animals on earth.

The disease is chytridiomycosis, which infects keratin cells of adult amphibians, causing imbalances in the exchange of gas, water and electrolytes through the skin and leading to death from cardiac arrest. In tadpoles, the fungus degrades denticle keratin, impairing food intake and stunting growth.

“Because they remain in the tadpole stage for longer and their size at metamorphosis is smaller, these animals are more likely to be preyed upon in the wild, and this may be a cause of population decline,” said Luís Felipe Toledo, a professor at the University of Campinas’s Biology Institute (IB-UNICAMP) in São Paulo State, Brazil.

A study coordinated by Toledo and conducted by a consortium of researchers from universities and research institutions in Brazil and the United States as part of the project “Into the heart of an epidemic: a US-Brazil collaboration for integrative studies of the amphibian-killing fungus in Brazil”, supported by FAPESP, showed that Batrachochytrium dendrobatidis (Bd), the fungus that causes chytridiomycosis, is widespread throughout the Atlantic Rainforest biome and is already present in other Brazilian biomes such as the Amazon and Cerrado.

The Cerrado is Brazil’s savannah-like grassland biome. The Atlantic Rainforest is the Brazilian biome with the greatest diversity of amphibians and is where these animals are most threatened with extinction by factors such as habitat destruction and the introduction of invasive species.

The same study also showed that in addition to the native strain of the fungus, Brazil has a hybrid strain that is most likely more virulent than the global pandemic strain.

“We found that Bd is no longer confined to the Atlantic Rainforest and that at least two strains of the fungus are exclusive to Brazil,” Toledo told Agência FAPESP.

According to Toledo, Bd was first discovered in the Atlantic Rainforest in the early 2000s. The project supported by FAPESP, which set out to increase knowledge of the ecology and evolution of chytridiomycosis in the Americas, has now discovered that the fungus is present in a much wider variety of habitats and Atlantic Rainforest areas than previously thought. Outbreaks of chytridiomycosis are believed to have been more devastating in the Americas than in most other parts of the world.

Until the start of the project, the fungus had been detected in approximately 110 species of anurans (frogs) living in various habitats, especially in the southern and southeastern portions of the Atlantic Rainforest, and in two species of anurans in the Cerrado. These two biomes contain the largest number of species of endangered amphibians worldwide.

In an article accepted for publication in the journal Diseases of Aquatic Organisms, the researchers report that they identified species of Bd-infected anurans captured in the wild in the northern portion of the Atlantic Rainforest in the states of Bahia, Pernambuco and Minas Gerais.

In another paper accepted for publication in the North-Western Journal of Zoology, they also report the identification of endangered frog species from the Atlantic Rainforest infected by the fungus. This raises the number of Bd-infected anuran species in the Atlantic Rainforest to 128.

“The results of our study show that Bd is present throughout the Atlantic Rainforest and acts as a generalist pathogen in the biome, infecting families of anurans with a greater diversity of species,” Toledo said.

In the article, the researchers also report the first identification of a Bd-infected anuran species in the Amazon biome. In this case, the animal was captured in the wild in Pará State.

Hitherto, the only record of infected amphibians in the Amazon referred to tadpoles of the American bullfrog (Lithobates catesbeianus) obtained from a commercial frog farm, according to Toledo.

“In our assessment, the presence of Bd in the Amazon is recent and less abundant than in the Atlantic Rainforest,” he said. “In the Atlantic Rainforest, some 40% of amphibians have the fungus, whereas its prevalence appears to be lower in the Amazon.”

Brazilian strain

The researchers also found that in addition to the pandemic strain of the fungus, BdGPL, the Atlantic Rainforest also has a “genuinely Brazilian” strain, called BdBrazil, and yet another lineage called BdHybrid because it is a blend of BdGPL and BdBrazil.

The discovery of the hybrid strain, described in a paper published in the journal Molecular Ecology and further detailed in an article published later in Proceedings of the National Academy of Sciences of the United States of America (PNAS), surprised the researchers because it had always been assumed that Bd reproduced asexually (without recombining genetic material).

“The existence of this hybrid strain shows that both BdBrazil and BdGPL can reproduce sexually,” Toledo said.

Another recent discovery by the same research group is that the hybrid lineage may be more virulent than the Brazilian and pandemic strains of the fungus, which are causing species loss and extinction in Central American countries such as Panama as well as in Australia and the western United States.

One reason for this enhanced virulence is that the hybrid fungus is larger and has more DNA content than the pandemic and Brazilian strains so that it can produce more proteins and enzymes to increase its efficiency in infecting amphibians.

The fungus propagates in water, producing motile aquatic zoospores (flagellated spores) that swim until they find an amphibian, on which they form cysts under the surface of the skin and become zoosporangia, enclosures in which more zoospores are formed, Toledo explained.

“There’s a link between the morphology of the fungus and its infectious potential. The pandemic strain, for example, can be larger, with zoospores and zoosporangia that have a larger diameter than those of the Brazilian lineage, making it more virulent,” Toledo said.

The existence of all three lineages in the Atlantic Rainforest, including the oldest, suggests that the pathogen may have emerged first in this biome or another part of South America, he went on.

However, this would be a premature conclusion from the research findings obtained so far. Indeed, at present it is impossible to specify the geographic origin of the fungus and how it spread around the world.

“We’re working with the hypothesis that the Brazilian strain originated and remained in Brazil, whereas the pandemic strain arrived here more recently and they hybridized,” Toledo said.

Possible origins

According to the researchers, since the discovery of Bd in 1998 and of its link with the decline in the world’s amphibian populations, several hypotheses have been raised to try to explain the emergence of chytridiomycosis and its transformation into a pandemic disease.

One of these hypotheses is that the global spread of the disease may have been due to international trading of American bullfrogs for human consumption. L. catesbeianus is considered highly resistant to chytridiomycosis and is frequently infected by the fungus.

However, the Brazilian researchers’ findings show that Bd was already present in Brazil long before the introduction of this species, which is native to North America and has become invasive in the western US, South America, Europe and Asia. L. catesbeianus was introduced into Brazil in the mid-1930s.

“Global trading of bullfrogs may have contributed to the global spread of chytridiomycosis, but it wasn’t the key factor. The problem now is that frog farms are exporting animals infected with the Brazilian strain of the fungus, which can reproduce sexually. This could lead to even greater difficulties in other parts of the world,” Toledo said.

During the study the researchers found farm-bred exemplars of American bullfrog traded in the US and infected with the Brazilian strain of Bd.

“The frogs may have been exported by Brazil or Uruguay. We don’t know, but there should be control and safety mechanisms in these countries to prevent the situation from getting worse as time goes on,” Toledo said.

Species harm

According to Toledo, there are no official estimates of the number of amphibian species that have declined or become extinct in Brazil and other countries owing to chytridiomycosis because the fungus was described only in 1998.

What we do know, Toledo said, is that Bd has infected more than 500 species of amphibians in a wide variety of aquatic and terrestrial habitats in the Americas and that the worst declines have been recorded in areas with the greatest species diversity.

“The species interact with the fungus and environmental conditions such as climate and topography, which influence the dynamics and propagation of the disease,” Toledo said.

In another study reported in Diseases of Aquatic Organisms with anurans in three Atlantic Rainforest areas with different elevations, the researchers found higher prevalence and intensity of infection by Bd at higher altitudes.

One explanation for this is that the conditions at higher altitudes, including cooler temperatures and rainfall, are more favorable to the growth and spread of the fungus.

Beside anurans, Bd also attacks caecilians and salamanders. However, Toledo noted, the fungus does not represent a hazard for humans.

“A better understanding of the ecology and physiology of the fungus, and of how the disease spreads, will enable us to identify areas for species conservation and monitoring, as well as other measures to contain the advance of chytridiomycosis,” he said.

The article “Chytrid fungus acts as a generalist pathogen infecting species-rich amphibian families in Brazilian rainforests” (doi: 10.3354/dao02845), by Valencia-Aguilar et al., is forthcoming in the journal Diseases of Aquatic Organisms.

The article “Complex history of the amphibian-killing chytrid fungus revealed with genome resequencing data” (doi: 10.1073/pnas.1300130110), by Rosenblum et al., can be read by subscribers to PNAS at www.pnas.org/content/110/23/9385/F2.expansion.html.

The article “Interaction between breeding habitat and elevation affects prevalence but not infection intensity of Batrachochytrium dendrobatidis in Brazilian anuran assemblages” (doi: 10.3354/dao02413), by Gründler et al., can be read in Diseases of Aquatic Organisms at www.int-res.com/abstracts/dao/v97/n3/p173-184.