Brazil has two populations of American bullfrogs, an exotic species that transmits a deadly fungus

Genetic analysis of Aquarana catesbeiana, a species that originally came from North America and is now found in nine Brazilian states, shows that the lineage introduced in 1935 prevails in both captive and feral bullfrogs. Law enforcement to maintain sanitary standards is difficult. The invaders prey on native amphibians and transmit diseases to them.

By André Julião  |  Agência FAPESP – Researchers at the University of São Paulo (USP) and the State University of Campinas (UNICAMP) have conducted the most comprehensive genetic analysis ever of the American bullfrog (Aquarana catesbeiana) in Brazil, concluding that there are two populations of the species here, living on frog farms or invading local ecosystems. Bullfrogs are considered the world’s main invasive amphibian. 

An article on the study, which was supported by FAPESP, is published in Scientific Reports.

“We confirmed the existence of at least two different populations of bullfrogs. One probably descends from the first bullfrogs introduced into Brazil. This population is present in practically all the South and Southeast. The other is basically confined to the state of Minas Gerais, but occurs in small numbers in other states,” said Gabriel Jorgewich-Cohen, first author of the article. The study was part of his master’s research at the Institute of Biosciences (IB-USP) with a scholarship from Brazil’s National Council for Scientific and Technological Development (CNPq).

A. catesbeiana is native to North America and was brought to Rio de Janeiro in 1935 to produce meat. It is now farmed throughout the South and Southeast and has also spread in the wild, with adverse impacts on local ecosystems, such as diseases against which Brazil’s native species have no defenses.

“Our results show that captive and invasive bullfrogs are genetically indistinguishable, reinforcing the importance of preventing escape from frog farms,” said Taran Grant, a professor at IB-USP supported by FAPESP and principal investigator for the study.

If the populations were genetically different, it would be possible to know the provenance of each animal. In theory, analysis of a captured bullfrog could point to a region or frog farm from which it or a close relative had escaped, so that monitoring and law enforcement would be feasible. Compared with introduced populations of the species studied in other countries, however, the Brazilian groups have the least diversity. 

State policy 

The researchers analyzed specific genes in 324 tissue samples. Specimens came from 38 sites in seven of the nine Brazilian states where captive and feral bullfrogs are found. They concluded that the vast majority belong to the same population, which descends from the animals first brought from North America to Rio de Janeiro in 1935, after which the bullfrogs spread out across the country in response to incentives offered as a matter of state policy.

The other population descends from a batch of animals brought in the 1970s to Minas Gerais under a public policy implemented later in the state. These included breeding pairs probably imported from the United States. The species is native to the eastern US, as well as northern Mexico and southern Canada.

“The results of the genetic analyses match these two more well-documented introductions, although there’s anecdotal evidence of others in the 1980s and 2000s, and isolated initiatives by some producers. If there were other introductions, the animals concerned could have had the same origin or may have interbred and merged with the existing population. Alternatively, we simply didn’t collect samples from these individuals,” said Jorgewich-Cohen, currently a PhD candidate at the University of Zurich in Switzerland.

Bullfrog farming peaked in the 1980s in Brazil. Some 2,000 farms were producing at that time. The activity declined in the ensuing decades owing to a number of factors, such as lack of private investment and public incentives. Many farms were abandoned, and animals escaped into the wild.

“The species reproduces easily, laying many eggs and growing quickly until individuals reach 15 cm. In addition, it’s highly resistant to disease and can coexist with the fungi and viruses that have led to a global decline in populations of other amphibians, without necessarily seeing its development impaired,” said Luís Felipe Toledo, the other co-author of the article. Toledo is a professor at the State University of Campinas’s Biology Institute (IB-UNICAMP) and is supported by FAPESP.

American invasion

These characteristics are desirable in any breeder species but become a major environmental problem when the animals in question invade wilderness areas. In the case of A. catesbeiana, the adverse effects include competition with native species for food and other resources. The North American species is also a voracious predator, feeding on other frogs as well as snakes, birds, and even mammals, and its loud croaking interferes with the reproduction of native amphibians. “These alterations can have a significant impact on reproduction since most anurans [frogs and toads] depend on acoustic communication to locate, appraise and choose mates,” Grant said.

The most serious environmental problem, or at least the most well-documented to date, is the transmission of diseases. “Having spread through the Atlantic Rainforest biome from Rio de Janeiro to Rio Grande do Sul [Brazil’s southernmost state], bullfrogs affect native wildlife in various ways. The main problem is that they’re carriers of the amphibian chytrid fungus [Batrachochytrium dendrobatidis] and ranavirus. Native amphibians lack their resistance to these two pathogens, which have even led to species extinction,” Toledo said.

Chytrid fungus causes chytridiomycosis, an infectious disease that penetrates the skin of adult amphibians, which become unable to breathe and die from cardiac arrest. It has decimated the populations of at least 501 species of amphibians worldwide (more at: agencia.fapesp.br/37145/ and agencia.fapesp.br/30127/). 

Ranavirus is also associated with the decline in populations of these animals and has been detected in the Atlantic Rainforest (more at: agencia.fapesp.br/30808/). Brazilian law requires anyone detecting chytrid fungus or ranavirus in farmed animals to notify the Ministry of Agriculture, Livestock and Food Supply (MAPA) and perform “sanitary slaughter” (or depopulation), destroying all animals and disinfecting the facility before starting a new breeding cycle, but this is not what happens.

“We detected chytrid fungus in almost all the frog farms we visited. There’s a great deal of movement of bullfrogs around the country. Producers swap animals based on the mistaken idea that this increases their genetic diversity,” said Toledo, who works with MAPA and state departments of agriculture to try to improve the legislation and control bullfrog breeding and marketing.

The study shows that frog farms have merely consolidated the same populations with low genetic diversity by exchanging animals. The practice does not necessarily have economic or financial drawbacks. Brazil currently produces 400 metric tons of frog meat per year, all of which is sold on the domestic market. 

“Interest in preventing the diseases caused by chytrid fungus and ranavirus is very incipient. Many producers can’t sell all the meat they produce. Oversight and inspection need to be greatly improved. An alternative strategy would be to develop the industry if the large meatpackers were interested in the product. In that case, high sanitary standards would have to be enforced by both producers and consumers,” Toledo said.

The article “Genetic structure of American bullfrog populations in Brazil” is at: www.nature.com/articles/s41598-022-13870-2.