Protozoan hunted in widespread death of frogs Maryland pathologist finds vital clue in global mystery

September 18, 1997|By NEW YORK TIMES NEWS SERVICE

When Dr. Karen Lips trekked deep into the Panamanian rain forest last Christmas to continue her field study of tree frogs, she found a horrifying scene: dead frogs were everywhere.

"I'd go in the morning and see them sitting on the ground along the stream," said Lips, assistant professor of biology at St. Lawrence University in Canton, N.Y. "They looked perfectly alive, as if they were asleep." But their color was faded, and when Dr. Lips picked up the dead animals, they felt hard and leathery, as if they had been turned into stone by a wicked witch.

Lips picked up 50 dead frogs representing several species, froze 30 of them and put the others in formalin, a preservative, and sent them to Dr. Earl Green, a veterinary pathologist in Maryland, for examination. What he found may offer a vital clue to a global mystery.

Frogs and toads have been disappearing worldwide for the last 15 years, most often from habitat destruction or from agricultural pollutants. But why they have been dying in near pristine environments in Central America and other protected highlands had remained unknown. The Panamanian frogs, it turns out, had been attacked by a protozoan that may also be killing frogs elsewhere.

Finding the evidence

Despite widespread frog deaths, this is the first time that a field biologist has come across a large number of frogs in the process of dying, said Dr. David Wake, an evolutionary biologist at the University of California in Berkeley and authority on amphibian extinctions. In the tropics, dead animals are immediately consumed by ants, bees and other creatures, and leave no trace on that spot, he said.

But now the detectives had finally found a corpse. Green, the pathologist who examined the bodies, said in a telephone interview: "They appeared normal internally and externally. I could find no evidence of widespread viral, bacterial or fungal infections. But it looks like a protozoan of some sort had infiltrated their skin."

Green, who recently left the Maryland Department of Agriculture, said the identity of the protozoan remains unknown, although it resembles a pathogen that kills oysters in Chesapeake Bay. Because amphibians breathe and drink water through their skin, the skin damage caused the frogs to suffocate and dehydrate, he said. How long the process takes is not known, but since entire populations can disappear in months it is not a lingering disease.

Lips and others believe that the lethal protozoan is sweeping across Central America in a "death wave" moving through the mountains, from one range to the next. Biologists conjecture that the disease first broke out in the Monteverde Cloud Forest Reserve in northwestern Costa Rica in the late 1980s. It has since moved south and east into Panama and may have traveled north into Nicaragua.

Australian connection

In July, a tourist found dead and dying frogs on an island in the middle of Lake Nicaragua, but none of the animals were preserved for autopsy. In a finding that only serves to deepen the mystery, Australian biologists last year reported that they believe a similar "death wave" killed amphibians in the lush mountain forests of Queensland in the 1980s and 1990s. They suspected a virus. But when Green recently shared his slides of Panamanian frog skins with an Australian pathologist, she sent him e-mail saying what he had found was exactly what she was seeing in dying Australian frogs.

Could the same disease be killing frogs and toads in Central America and Australia? If so, where did it come from?

With financing from the Brookfield Zoo in Chicago, Green and his counterparts from several countries plan to meet at Loyola University in Chicago next month to examine slides of frog tissue under a powerful electron microscope. They will look at the most recent specimens as well as older tissues preserved in museum or private collections with the hope of finding common parasites.

Field biologists on all continents first started noticing amphibian declines about 15 years ago, Wake said. They would go back to their sites and find that once abundant animals were gone or greatly diminished in number. The animals evidently died in their burrows or were carried off by carrion eaters.

But the amphibian decline that most galvanized international attention occurred in Costa Rica's Monteverde Reserve, the home of the spectacular golden toad. In 1987, the last year the population was at a normal level, biologists saw hundreds of thousands of animals gleaming like jewels in the dark green forest, Wake said.

Two years later, they only found five animals. Since then, not one has been seen. In a paper to be published in December in the journal Conservation Biology, scientists report that the golden toad is almost certainly extinct; its disappearance is not due to some natural fluctuation of amphibian life cycles. Twenty other species of frogs and toads are also missing from the region.

Seeking explanations

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