Nature To Blame, It Now Seems,
For Frog Deaths Blamed On Man

WASHINGTON — Dadgummit! There goes another eco-scare, shot down by facts.

For 20 years, environmental activists have been sounding an alarm about deformities and population declines among some frogs. The party line was that pollution, the "ozone hole", "global warming" or some other dastardly human-related cause — real or imagined — was to blame.

Now, researchers have tied the abnormality to a parasitic worm as small as the period at the end of this sentence.

Two studies published last week in the journal Science report that laboratory research shows a trematode — a simple parasitic flatworm — is infecting tadpoles that grow into frogs with deformed, missing or multiple hind legs.

The worm infection, says Stanley K. Sessions of Hartwick College in Oneonta, N.Y., causes the tadpoles to develop cysts where their hind legs grow, resulting in severe malformation that dooms the animal after it becomes a frog.

``Every single frog I have looked at with extra legs, and I have looked at hundreds, all have these cysts around the deformity,'' said Sessions, who co-authored one of the Science studies.

Pieter T.J. Johnson, a recent graduate of Stanford University now doing research at Claremont McKenna College, carried Sessions' early work one step further by collecting trematodes from snails in four ponds where deformed frogs were found. Laboratory tadpoles were then infected with the worms.

Johnson found that the more trematode infections the tadpoles acquired, the more the legs of the adult frog were deformed, multiplied or missing. And he also showed that the deformities developed in the laboratory experiments were the same as those seen in the four ponds in northern California.

Andrew Blaustein, an Oregon State University ecologist who conducted experiments to determine if ultraviolet radiation was causing the widespread problem, called Johnson's work ``the best experimental evidence showing a cause for the limb deformation in amphibians.''

Researchers worldwide have noted for years that many species of frogs are in decline, particularly in the northern and western United States, Central America and Australia.

Scientists have suspected pesticides, chemical pollution and excess UV radiation caused by a thinning of the atmosphere's ozone layer. A great deal of research has been devoted to the issue because the sudden demise of frogs was considered a possible warning about an unknown environmental problem.

The studies by Johnson and Sessions show that at least some of the frog problems are caused by Mother Nature, but the scientific community is not entirely willing to let go of the more sensational claims which have sustained lucrative research grants for two decades now.

Johnson conceded the increased deformities caused by the parasite could be part of a natural biological cycle. However, he said it is too early to hold humans blameless, speculating that fertilizer runoff may have caused an increase in water snails that are key hosts of the parasite.

The trematode has a life cycle that includes snails, tadpoles and frogs, and birds. A just-hatched form of the parasite is consumed or absorbed by a snail. The worm develops into a larvae that is deposited in a pond. The worm swims until it hooks onto a tadpole and then forms cysts in the leg buds of the developing amphibian. When the frog matures, its hind legs are either missing, multiplied or deformed. This makes the frog easy prey for birds, which become the next host of the parasite.

Digestive juices of the bird release trematodes from their cysts in the frog and the worms reproduce. Trematode eggs in the bird feces are then deposited in a pond and the cycle starts anew.

Johnson said even though trematodes are found throughout North America, the parasite probably is not the single cause for a worldwide decline in frogs and other amphibians first noticed two decades ago.

``It is far too early to say that this is the final answer for the amphibian decline,'' he said. ``Something different may also be going on.''

Researchers in Arizona have found one of those "something differents," and it turns out to be natural as well.

Since 1992, there have been at least eight known dieoffs of lowland leopard frog populations and two for Chiricahua leopard frogs caused by a fungus, said Mike Sredl, an Arizona Game and Fish Department biologist in Phoenix who studies frog populations.

The fungus has been involved in lowland leopard frog dieoffs ranging from 134 in 1992 at Big Spring near Safford to 27 sick or dead lowland leopard frogs in January in Rock Creek, northeast of Phoenix, and 110 the same month in Montrose Canyon north of Tucson, along with two canyon tree frogs, one of which died, Sredl said.

Ranid frogs, common throughout the country, have declined dramatically since the 1970s, Sredl said.

While habitat destruction, introduced species and pollution are commonly blamed for decline of frog populations, ``one of the factors that people hadn't looked at until recently was disease,'' Sredl said.

A team of University of Arizona scientists led by Phil Rosen first identified the chytrid (pronounced KIT-rid) fungus in 1997 as causing leopard frog deaths at a site in Cienega Creek, a nature preserve southeast of Tucson.

Sredl, who has studied frog dieoffs since 1992, said a reanalysis of the Big Spring deaths, first attributed to a possible bacteria-caused inflammation called red leg disease, showed signs of chytrid.

Chytrid fungus was officially identified as a disease-causing agent for frogs last summer in a National Academy of Sciences paper, Sredl said.

The fungus may allow an invasion by bacteria or even viruses, he said. It reportedly infects a variety of plants, but the frog is the first vertebrate for which it is known to be a pathogen, he said.

It forms balls of spores just below the frog's skin that eventually form ``like little volcanoes all over the skin,'' Sredl said. It's unknown whether the fungus causes a mechanical injury or secretes a toxin, or whether it has any natural enemies, he said.

The problem is potentially quite big, Sredl said. ``This past winter alone, there were five separate dieoffs that occurred in different parts of Arizona, and every one of those dieoffs have implicated chytrid fungi.''

Of Arizona's five known species of native leopard frogs, nearly every one is in decline across its entire or significant parts of its range, Sredl said.

The fungus also has been blamed for frog deaths in Central America and Australia.