Chytridiomycosis. Bill Kenoi, a member of the Coqui Frog Working Group and administrative assistant to Big Island Mayor Harry Kim, stumbles over the word as he introduces Richard Speare, a professor from James Cook University in Queensland, Australia.
But when Speare begins his talk to the Hawai`i Island Coqui Frog Working Group, the word trips from his lips as if he invented the term – which, in fact, he and his colleagues did. The term derives from two Greek roots – chytrid, referring to a type of microscopic fungus, and mycosis, referring to parasitic fungal infection. Speare, who has doctoral degrees in medicine, veterinary science, and public health, is one of the world’s foremost experts in epidemics among amphibians. That expertise grew out of concerns over precipitous losses in Australia’s native amphibian populations resulting from introduced diseases.
In Australia, Speare’s work is directed at efforts to save native frogs from the fungus causing the infection (Batrachochytrium dendrobatidis). He was invited to Hawai`i, however, as part of an effort to develop means to control coqui. With chytridiomycosis having such lethal effect elsewhere, Department of Agriculture staff thought, the fungal infection might be just the silver bullet needed to knock down Hawai`i coqui populations.
Whether chytridiomycosis (pronounced kit-RID-ee-oh-my-KO-sus) can be used as a tool against the coqui is not certain at this point. In Australia, New Zealand, Central and South America, and the continental United States, the disease has contributed to mass die-offs of frogs and has even resulted in the extinctions of the last known populations of two Australian frogs.
Yet not all species of amphibians are vulnerable to infection by the fungus, Speare said. One of the most notorious of Australia’s amphibian invaders – the cane toad (introduced to Australia from Hawai`i) – has not been slowed at all by the disease in its march across the continent. Whether the coqui has similar resistance is something to be determined in lab tests.
And such tests might be a long time coming, according to Eloise Killgore, a plant pathologist with the Hawai`i Department of Agriculture. Killgore, who specializes in fungi, had identified the chytrid fungus as a possible biocontrol agent for coqui. She was instrumental in bringing Speare to Hawai`i to help Hawai`i researchers learn more about it.
Before the fungus gets the green light for use on the coqui, a number of hurdles have to be surmounted. First, the Institutional Animal Care and Use Committee has to approve of the tests. The IACUC, based at the University of Hawai`i, reviews all federally underwritten experiments that involve vertebrates.
The next hurdle is obtaining the fungus. “Labs where work is being done on it are reluctant to spread it around,” Killgore says. “Because it’s so devastating, they’re not freely giving away cultures. Most people working on the disease are trying to stop its spread. Many of them told me, ‘Don’t go there, Eliose, it’s a bad disease.’ Even Richard Speare has been reluctant to send me a culture. Here in Hawai`i, of course, we’re going after it from a different point of view.”
A third hurdle is the requirement that any potential biocontrol agent be tested on a variety of animals that it might infect, the so-called host-range test. “We haven’t decided what to include in the test,” Killgore says. Speare “eliminated a lot of animals as possible hosts – birds, because their body temperature is too warm, and fish, because they don’t have keratin or chitin.” Still, there remain lots of other animals that would have to be tested, including aquatic arthropods, “of which we have quite a few,” she notes.
If all goes without a hiccup, the soonest that the fungus might be released in Hawai`i would be on the time scale of a year or more, Killgore says.
It is far easier, of course, for organisms to be introduced accidentally. And in the case of the fungus that causes chytridiomycosis in frogs, Hawai`i may already have it. Speare noted that from 1996 to 1998, amphibians at the Honolulu Zoo were lost to the disease. Rebecca Choquette, an animal keeper at the zoo, says the outbreak “occurred just as the disease was being identified. It was hearbreaking. It burned through everything.” If chytridiomycosis were to be used as a biocontrol agent in Hawai`i, Choquette adds, she has concerns of the same thing happening again. “It spreads very easily,” she notes. “Some animals can be carriers without being affected,” including cane toads, which, she adds, have pretty much free run of the zoo grounds. Although frogs in the zoo collection are kept caged, screens on top of their enclosures to allow for ventilation would mean that they would once more be vulnerable to exposure. Use of chytridiomycosis as a biocontrol agent, she says with a sigh, “would certainly make my life more difficult.”
Killgore says that she has people across the islands who are on the lookout for chytridiomycosis, collecting tadpoles and frogs. “We do import amphibians in the pet trade,” she says. “Chytridiomycosis could be here and we wouldn’t know it.”
In the world of amphibious diseases, chytridiomycosis is a relative newcomer. The fungus that causes it was not identified until 1998. The disease itself has been determined to have emerged from Africa, with the earliest known case traced back to the 1930s.
The global reach of the disease was fostered through trade. According to Speare, the discovery in the 1950s that the South African clawed frog (Xenopus laevis) was suited for use in pregnancy tests (in the days before home test kits) led to a worldwide trade in that animal, which might have spread the range of the fungus. That frog can carry the fungus without apparent harm. More recently, the robust global pet trade and the popularity of water gardens (with resident frogs) probably continued its dispersal.
Climate change has been proposed as another factor in propagating the disease. Droughts and prolonged dry periods might cause amphibians to crowd around water sources in greater concentrations, making the disease spread all the more rapidly.
Chytrid organisms are common in soil, where they help in the breakdown of leaf litter, feathers and the like. The Batrachochytrium variety feeds on keratin, the hard protein found in hooves, toenails, claws, frog skin, and the like. Although some chytrids are parasites of plants and insects, the infection of frogs with B. dendrobatidis marks the first time any chytrid has been found on a vertebrate host.
— Patricia Tummons
Volume 13, Number 12 June 2003
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