The aquatic fungus, which has led to the extinction of several species of amphibians that spend all or part of their life cycle in water, also threatens terrestrial amphibians. In Brazil, researchers supported by FAPESP have found unprecedented mortality among a genus of tiny frogs known as pumpkin toads, which inhabit the rainforests of the Atlantic Ocean away from any aquatic environment. The animals were heavily infected with chytrid (Batrachochytrium dendrobatidis), which causes chytridiomycosis.
Research published in the journal Biological preservation shows that the fungus also poses a threat to land-breeding amphibians with important ecological functions, including combating disease-transmitting insects such as dengue, yellow fever and zika.
“The fungus affects the skin of an amphibian, where it exchanges gas with the environment. The infection causes a physiological imbalance and the animal eventually dies of a heart attack, ”said Diego Moura-Campos, first author of the article. The research was carried out during his master’s work at the Institute of Biology of the University of Campinas (IB-UNICAMP) in the state of São Paulo with a fellowship from the Brazilian Ministry of Education for the Coordination of Professional Development in Higher Education (CAPES).
The study was carried out under the auspices of the project “Chitrid mushroom in Brazil: origins and effects” associated with the FAPESP research program on the characterization, conservation, restoration and sustainable use of biodiversity (BIOTA-FAPESP) and coordinated by Luis Felipe Toledo, a professor at IB-UNICAMP and co-author of the article …
“We have studied the fungus from different angles, but rarely had the unpleasant opportunity to see animals die of fungal infections in the wild. This is the first study to demonstrate this phenomenon in Brazil. If an amphibian dies and becomes infected, this does not mean that the fungus caused its death. It can coexist with the pathogen without developing disease. In this case, we were confident that this was the cause of death, because the animals had the right symptoms, such as weight loss, severe peeling. skin and a very high infectious load, ”said Toledo, who is also the principal investigator for another project to understand how the fungus spreads in nature.
Researchers believe that direct-developing species (which breed on land and do not have tadpoles, and fully formed miniature adults hatch from ground eggs) are even less adapted to the fungus. Aquatic species have been in contact with the pathogen longer and may have developed some resistance to infection.
Moura Campos observed morbidity and mortality of infected frogs during a field survey conducted at the Serra do Japi Biological Reserve in Jundiaí, São Paulo, between May 2018 and May 2019. Curiously, dead and dying individuals of the species Brachycephalus rotenbergae were discovered after an atypical period of drought.
“These animals are very small and difficult to find. They decompose quickly after death. Finding nine of them dead or seriously ill in a short period of time, like us, suggests that others are likely dead, too, ”said Guilherme Becker, professor at the University of Alabama in the US and the last author of the article.
According to Becker, who is also a visiting professor at UNICAMP on his master’s program in ecology, the study shows that the acceleration of global climate change in the coming decades will increase the frequency of this type of disease, with pathogens that could become more dangerous as hybrids emerge, as already was shown in an earlier study by the group.
“The lack of moisture in the soil in the forest where they live may have caused these animals to seek hydration in streams and become more contaminated than usual with fungus,” he said.
Another hypothesis put forward by the researchers is that periods of drought could compromise the frogs’ immune systems and make them more vulnerable to the fungus.
Cosmopolitan pathogen
The fungus originated in Asia and probably spread throughout the world as a result of the frog meat trade. Species consumed by humans for this purpose, such as the American bullfrog (Rana catesbeiana), are resistant to the fungus and can be carriers without being infected.
According to an article published in the journal for 2018. The science with co-author Toledo, the mushroom originated on the Korean Peninsula and spread to other parts of the world in the early twentieth century.
Another study, which Toledo participated in, also found that the fungus caused a decline in populations of at least 501 amphibian species worldwide. In Brazil alone, at least 50 species or populations were affected, 12 became extinct and 38 declined.
“Amphibians are very important for the functioning of many ecosystems. Their biomass in forests is enormous. They serve as food for many other animals, feed on arthropods in the wild and control invertebrate communities, ”Becker said. “As for aquatic species, most of them are herbivores at the tadpole stage and consume phytoplankton, which could have destroyed the aquatic environment if not for the tadpoles. These animals traverse aquatic and terrestrial ecosystems, so the impact is significant during outbreaks of chytridiomycosis. “
As an example, Becker recalled a recent study in which scientists associated with institutions in the United States and Panama showed that declining amphibian populations due to B. dendrobatidis infection were associated with an increase in malaria outbreaks in the 1990s and 2000s in Panama. and Costa Rica. …
More observations over several years are required to obtain a more accurate estimate of the global impact of chytridiomycosis on amphibian populations, Becker, Toledo et al.
More information:
Diego Moura-Campos et al., Low Precipitation Cluster of Fungal Diseases in Tropical Land Frogs, Biological conservation (2021). DOI: 10.1016 / j.biocon.2021.109246
Simon J. O’Hanlon et al., Recent Asian Origins of Chytrid Fungi Causing Global Amphibian Decrease, The science (2018). DOI: 10.1126 / science.aar1965