New frog-killing disease may not be so new

A skin disease that savaged frogs and toads on opposite sides of the globe in the 1990s now looks like the killer in mass die-offs 20 years earlier.

Chytrid organisms (arrows) show up as rounded pouches in these photos (above and below) of cross sections of amphibian skin. CDC Emerging Infectious Diseases

An ongoing effort to test old museum specimens for the newly identified chytrid pathogens has turned up signs of outbreaks in Colorado and California in the 1970s, scientists said last week at the annual meeting of the American Association for the Advancement of Science in Washington, D.C.

One of the researchers, Cynthia Carey of the University of Colorado in Boulder, also presented the first results from a cross-check between unusual weather and die-offs of amphibians. Climate changes don’t seem to have nailed amphibians directly, she reported: “They didn’t boil to death.” Carey did, however, propose ways that weather might have conspired with killer diseases.

The chytrid skin disease made headlines in 1998 as the culprit in spooky disappearances of amphibians in remote wildernesses in Panama, Costa Rica, the U.S. desert, and Australia (SN: 7/4/98, p. 7: The disease at first baffled scientists because they had never known chytrids to attack vertebrates. Common in soil and fresh water, the funguslike microbes typically break down debris or attack live plants and insects.

The amphibian outbreaks of chytrids reminded Carey of mass die-offs among boreal toads and leopard frogs that she had witnessed as a graduate student. She sent what old specimens she could find to pathologist D. Earl Green at the U.S. Geological Survey’s National Wildlife Health Center in Madison, Wis. He had found chytrid infections in boreal toads collected last year (SN: 10/2/99, p. 219).

Green and Carey haven’t published details yet, but she says 2 of 12 preserved leopard frogs yielded chytrids. The pathogens also showed up in specimens of the toad Bufo canorus collected in California’s Sierra Nevadas in the 1970s.

In Australia, probing old specimens has revealed a chytrid infection as early as 1978, pathologist Lee Berger of the Commonwealth Scientific Industrial Research Organization in Geelong told Science News. The find came from a preserved dainty tree frog in southern Queensland.

That species is still common, but Berger points out that consequences might have been more severe for others. Southern Queensland was the sole home of the southern gastric brooding frogs, which incubate their young in their stomachs and give birth by regurgitating. No one has found one of these marvels since 1979, and Berger muses that perhaps a chytrid snuffed out the species.

Berger guesses that the chytrid entered Australia near Brisbane in the 1970s. “The simplest way for it to be spread is by other amphibians,” she speculates. She notes that thousands of frogs hitchhike around Australia in fruit boxes, and pet-seekers often visit grocery stores to capture the little stowaways.

Carey looks elsewhere for the pathogen’s vectors in the western United States. In one scenario considered, the disease spreads via the muddy boots of an ecotourist or, heaven forbid, a herpetologist. However, the die-offs are widespread. “You’d have to have a really active per son who had nothing else to do,” she says. Instead, she suspects birds with muddy feet or wind-blown insects.

The temperature and rainfall data that she and her colleagues gleaned from NASA link die-offs in three locations with slightly abnormal weather. Australian and Central American die-offs occurred during warm, dry periods. High-altitude Colorado species suffered during weather slightly colder or wetter than normal.

Other times of unusual weather hadn’t troubled amphibians, but perhaps the killer hadn’t arrived or needed special circumstances to turn ugly, Carey speculates. She looks to DNA comparisons of chytrid strains to illuminate this issue.

One of the discoverers of the recent chytrid outbreaks, Karen R. Lips of Southern Illinois University in Carbondale, emphasized at the meeting that diseases aren’t the whole picture.

Looking at all the species in trouble, she says, “the major cause of amphibian declines is habitat loss.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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