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Deadly virus detected in wild frog populations in Brazil

  • Researchers have detected the first case of ranavirus infection in both native frog species as well as the invasive American bullfrog in the wild in Brazil.
  • While the study cannot attribute ranavirus as the cause of death for the observed American bullfrog tadpoles, the findings suggest that ranavirus is spread in the wild, the researchers say.
  • Ranavirus infections could be far more widespread in Brazil, and may have simply gone unnoticed until now, the researchers add.

In November 2017, Joice Ruggeri and her colleagues came upon a pond with several dead tadpoles and a few dead fish in the state of Rio Grande do Sul, in southern Brazil. All the dead and dying animals had skin ulcers, with signs of hemorrhaging and edemas.

When the researchers analyzed some of the dead tadpoles, they found that the animals were infected with ranavirus, a pathogen known to have contributed to mass die-offs of amphibians, fish and reptiles across the world. In Brazil, though, ranavirus infections have been linked to mass die-offs of only farmed tadpoles of the North American bullfrog (Lithobates catesbeiana).

A new study has now recorded the first case of the deadly virus in wild frog populations in the country, researchers report in the Journal of Wildlife Diseases.

“There are not many reports on dead frogs in the wild, at least in Brazil, and I personally don’t recall of any report on a mass mortality event,” Ruggeri, a postdoctoral researcher at the University of Campinas and lead author of the study, told Mongabay. “However, as ranavirus infection usually leads to a quick death, I wonder how many events like this one we have been missing.”

Two killer diseases have been wiping out amphibians across the world. The chytrid fungus, Batrachochytrium dendrobatidis (Bd), has contributed to the decline of more than 500 amphibian species. And now, ranavirus is emerging as yet another deadly threat to wildlife; the virus is known to infect at least 175 species of amphibians, reptiles and fish.

The American bullfrog, a species native to North America and introduced to more than 40 countries, has been implicated in the spread of both chytrid and ranavirus around the world.

Brazil is a major producer of the bullfrog, with most farms located around the Atlantic Forest between Rio de Janeiro and Rio Grande do Sul. Several farms were abandoned in the early 1990s, however, and numerous captive bullfrogs are said to have escaped into the environment. Ruggeri and her colleagues suspect that feral bullfrog populations could be spreading ranavirus to frogs in the wild.

After they chanced upon the pond with dead tadpoles, the researchers collected 18 seemingly healthy tadpoles of native species and four American bullfrog tadpoles — two dead and two lethargic individuals — from two separate ponds, and found that both the native and bullfrog tadpoles tested positive for ranavirus DNA. But whether the virus is definitively harming the animals is hard to say.

“Our findings indicates that ranavirus is spread in the wild,” Ruggeri said. “But, no, we cannot attribute ranavirus as the cause of death [in case of the bullfrogs] without histopathologic examination.”

Moreover, while the researchers detected the presence of ranavirus in tadpoles of native species, nothing is known about how susceptible they are to ranavirus infection. “We still have a lot to investigate in order to understand the real threat of ranavirus in Brazil,” Ruggeri said.

Amanda Duffus, an associate professor and ranavirus expert at Gordon State College, who was not involved in the study, said the sample size of tadpoles analyzed was small but “diverse in terms of what families of amphibians were found to contain ranavirus DNA.”

“It is not uncommon to find multiple species infected with ranavirus in an amphibian community and I am not surprised by this finding,” Duffus told Mongabay. “It is likely that ranavirus infections in Brazil are far more widespread and are potentially causing mortality events that are going unnoticed. Ranaviruses are globally distributed infections and with the global trade in amphibians, fish, and reptiles, no area is likely to be truly safe from this group of pathogens.”

Ruggeri agreed that ranavirus infections may have gone undetected until recently because no one was looking for them. “Especially as chytridiomycosis has been the major concern to conservationists worldwide,” she added.

Ruggeri and her colleagues are examining their samples in greater detail and hope to have some answers soon. “This ranavirus lineage could potentially be native to Brazil, which might explain why we detected low viral copies on native specimens,” she said. “We are working on genotyping some samples to see its phylogenetic position in the group.”

Despite the small sample sizes in the study, the results are useful, Duffus said.

“A high prevalence of Ranavirus in invasive populations of bullfrogs is a problem,” she said. “Ranavirus infection can lead to severe disease. It has been likened to ebola for ectotherms. In amphibians, the emergence of this infection can lead to population declines and even has the potential to lead to extinctions. This is something that needs to be taken quite seriously, as we are only beginning to understand the full extent of the effects of ranavirus infections and disease, and their potential interactions with other disease causing agents.”

Researchers say that American bullfrogs that escaped from captivity in Brazil could have something to do with the spread of ranavirus in wild populations of frogs. Image by Carl D. Howe via Wikimedia Commons (CC BY-SA 2.5).

Citation:

Ruggeri, J., Ribeiro, L. P., Pontes, M. R., Toffolo, C., Candido, M., Carriero, M. M., … Toledo, L. F. (2019). First case of wild amphibians infected with Ranavirus in Brazil. Journal of Wildlife Diseases. doi:10.7589/2018-09-224