Cane toads help spread parasites to frogs

By Natalie Muller 27 May 2011
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Two native parasites spread by cane toads may be responsible for the population decline of 10 frog species.

RESEARCHERS HAVE IDENTIFIED TWO two new parasites spreading disease in critically endangered Australian frogs.

According to researchers from the University of Sydney, two myxosporean parasites (Myxidium spp.) have infected at least 10 native frog species.

The findings, published in the science journal PLOS ONE, also suggest these parasites might be native, and, contrary to popular belief, were not brought over by South American cane toads in 1935.

The team found the pathogens in the threatened green and golden bell frog (Litoria aurea), the southern bell frog (Litoria raniformis) and even the yellow spotted bell frog (Litoria castanea), a species that was, until recently, presumed extinct.

Ashlie Hartigan, the Sydney University PhD student leading the research says these parasites, if left unchecked, could infect other frogs around Australia. The two parasites are similar in appearance and wreak havoc on the frog’s brain and liver function.

“Infected frogs lose weight, are lethargic, and some can’t move their back legs, making them more vulnerable to predators,” says Ashlie. “Infection could also be reducing the number of tadpoles that become adults, with affected tadpoles more likely to delay metamorphosis and die from liver disease.”  

Cane Toads loaded with parasites 

Ashlie and her colleagues identified the parasites in both native frogs and cane toads collected in New South Wales and Queensland, but they couldn’t find traces of the same myxosporea in cane toads from Brazil and Hawaii.

“We are 99 per cent sure the cane toad did not bring it in,” says Ashlie. “It’s possible these are an Australian parasite. So the cane toad might have been blamed when it shouldn’t have been.”

But the invasive toad is not completely blameless. The researchers believe the cane toad became infected by the parasites in Australia and was responsible for spreading them quickly across the east of the country.

Dr Michael Mahony, herpetologist from the University of Newcastle, says the research raises some interesting questions about the role of cane toads.

“The cane toad could have played a role not in competing or eating the native species, but by breeding the parasite,” he says. “The invasive species arrived here clean, now it’s loaded with myxosporea and affecting native frogs. The message is the amplification and that is worthy of further exploration.”

Professor Ross Alford, herpetologist from James Cook University, says that while the research is interesting, there should be more work done to determine whether the parasite might have been spread earlier by some of the other species it has been found in.

“I am not quite as convinced…that they have conclusively demonstrated it is an emerging parasite and must have been facilitated by the cane toad,” he says.

Golden Bell Frog conservation

Infectious diseases are affecting frog populations in Australia and around the world. The iconic green and golden bell frog, once commonly found in eastern Australia, is now one of the country’s most threatened frog groups. A 2005 recovery plan produced by the NSW Department of Environment found the species was missing from 90 per cent of the areas it previously inhabited.

Ashlie says it’s important to learn more about the life cycle of the parasites so they can be prevented from causing further disease in endangered frogs.

“We will be able to screen frogs for infection and control the spread to captive breeding populations and threatened populations in the wild,” she says. “If people aren’t looking, these parasites go undetected and can spread.”