A highly lethal pathogen is responsible for the mass death of tadpoles and the rapid decline of amphibian populations in the Atlantic Forest. A group of researchers supported by FAPESP demonstrated for the first time in South America that recurring ranavirus outbreaks caused an 83% drop in monkey frog ( Phyllomedusa distincta ) reproduction in a pond in the Brazilian state of Santa Catarina. The disease is caused by a virus that affects reptiles and fish as well.
The study, published in the journal Biodiversity and Conservation , provides the first robust evidence that ranavirus can cause the collapse of wild South American populations – a documented risk of extinction in Europe. The data also reveal that this threat worsens during dry periods because low air humidity favors disease outbreaks.
Ranaviruses commonly infect bullfrogs ( Aquarana catesbeiana ) on commercial farms and in the wild without causing disease. However, in recent decades, there has been an increase in reports of suspected mass die-offs – possibly triggered by this pathogen – among native species in North and South America. Researchers and environmentalists fear that other cases of mass mortality caused by the virus may be occurring without anyone’s knowledge ( read more at agencia.fapesp.br/30808 ).
In tadpoles, the infection progresses rapidly and devastatingly, causing tissue destruction and necrosis in vital organs, such as the liver. Visible clinical signs include abdominal swelling (edema), extreme redness, ulcers, and multiple hemorrhages, which can occur inside the animals’ eyes.
“Despite several suspected deaths documented in the Americas, this is the first time that ranavirus has been proven to be the cause of death in a native species through [ histological ] analysis of tissue samples and viruses isolated from the animals’ livers,” says Aline Fonseca, who conducted the study as part of her doctoral research at the Federal University of Paraná (UFPR).
At least three outbreaks
The first mass mortality event in the pond occurred in January 2024, followed by two others, between November 2024 and January 2025 and between November 2025 and March 2026. In total, 324 dead tadpoles were found, including 315 monkey frogs and nine Bischoff’s tree frogs ( Boana bischoffi ).
Of the monkey frogs, 279 (88.6%) tested positive for ranavirus, and 8 (88.9%) of the Bischoff’s tree frogs were infected. The viral load in the monkey frogs was considered high, at nearly 10 billion copies of the virus per microliter of sample. In the Bischoff’s tree frogs, this figure was about 2 billion. However, the number of dead Bischoff’s tree frogs was too small to constitute a mass die-off.
To confirm the clinical signs of the disease, the livers of thirteen tadpoles were analyzed histologically, a technique that examines tissue and cellular damage using a microscope. The lesions found confirmed the severity of the infection.
The deaths occurred at the Rã-Bugio Sanctuary Private Natural Heritage Reserve (RPPN) in the municipality of Guaramirim, Santa Catarina.
The study is part of the “From the Natural History to the Conservation of Brazilian Amphibians” project, which is supported by FAPESP and coordinated by Luís Felipe Toledo , a professor at the Institute of Biology at the State University of Campinas (IB-UNICAMP). Funding for the study was also provided by Brazil’s National Council for Scientific and Technological Development (CNPq) and Spain’s Ministry of Science, Innovation, and Universities.
Population decline
In addition to confirming ranavirus as the cause of death, the researchers compiled climate data from two weeks before and during the mass mortality events to identify possible associations.
“We performed statistical analyses to identify environmental and climatic changes that could have contributed to the deaths, but no factor had as significant an impact on the population decline as ranavirus,” explains Karla Campião, a professor at UFPR who advised Fonseca’s doctoral studies and obtained CNPq funding for the project.
The population decline was identified thanks to continuous monitoring of the lagoon conducted since 1999 by Germano Woehl Jr. and Elza Nishimura Woehl of the Rã-Bugio Institute for Biodiversity Conservation. They are also authors of the study.
Every day during the breeding season, from June through February, researchers count the number of egg masses around the pond. The egg masses are deposited on leaves, which the female folds around the eggs to protect them from drying out. When the eggs hatch, the tadpoles fall directly into the water ( read more at agencia.fapesp.br/37215 ).
From August 2025 to February 2026, shortly after the mass die-offs, the number of egg masses was substantially lower than in the same months between 1999 and 2004. Based on the data, the population decline was estimated at 83%.
Virus present
Although no single environmental factor can be significantly linked to the deaths over the years, the researchers point to a sharp increase in the risk of an outbreak when humidity drops from 90% to 65% two weeks before the die-off.
“The virus is likely present in the environment at all times without causing disease, but low humidity may cause it to increase its own replication and trigger an outbreak. It’s also known that amphibians’ immune systems can be compromised at low humidity levels, which facilitates infection,” Fonseca suggests.
Additionally, less water in the pond causes the tadpoles to become concentrated in a smaller area, facilitating transmission of the virus from one individual to another.
Monitoring of the pond at the Rã-Bugio Sanctuary continues. In the future, the researchers at UFPR plan to collect samples in nearby areas to assess the prevalence of ranavirus.
“This virus could also pose a significant threat to amphibian conservation. For more than a decade, we’ve been studying the chytrid fungus [ Batrachochytrium dendrobatidis ] as a cause of amphibian mortality and extinction. But this virus, which is already widespread in the Atlantic Forest, may be doing the same. So we need to deepen our studies to understand the real danger posed by the presence of ranavirus,” says Toledo of UNICAMP.
In addition to searching for the virus in wild animals, Toledo emphasizes the importance of examining amphibians housed in zoological museums to determine the impact of ranaviruses in the past.
In a recent study, his group found that the chytrid fungus was already infecting Brazilian amphibians as early as 1916, even before the bullfrog was introduced to the country. Until then, the invasive species had been considered the most likely source of the fungus’s introduction to Brazil ( read more at agencia.fapesp.br/56935 ).
“Perhaps the ranavirus has also been here for a long time. It remains to be seen what changed for it to become a threat,” Toledo concludes.
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at www.fapesp.br/en and visit FAPESP news agency at www.agencia.fapesp.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.fapesp.br/subscribe.
Biodiversity and Conservation
17-Jul-2026