US Long-Term Ecological Research Network

From Lakeland Times

By John Bates

Pieter Johnson, a doctoral candidate at the Center for Limnology at the University of Wisconsin-Madison, and already a recognized world expert on amphibian deformities, pulls on his rubber boots and leads me into an open field dotted with fifty 300-gallon tanks of water. The tanks contain varying amounts of nutrients, snails, and a parasitic flatworm, or fluke, called Ribeiroia ondatrae, all waiting for the mid-July introduction of the key component, frog eggs. Here is where Johnson hopes to demonstrate the link between the extraordinary increase in reports of frog deformities and the numbers of a particular snail, the Rams Horn snail or Planorbella tenuis. "The snails are the key to everything. They are the only species that serve as hosts for the parasites." Johnson says. "The more snails that are infected with parasites, the more deformities in frogs."

Nearly a decade has passed since eight middle school children in Minnesota discovered a pond full of severely deformed leopard frogs, triggering a media firestorm. Grotesque pictures of abnormal frogs with missing limbs, extra limbs, limbs jutting in odd directions, or webbing between their limbs briefly dominated the news, but the story quickly dropped out of the public eye.

While media attention disappeared, the issue piqued dozens of scientific studies, all demonstrating that the number of deformities continues to grow. Dozens of researchers have found high frequencies of amphibian deformities in at least 60 different species in 46 US states and parts of Canada, Japan, Australia, and several European nations. Johnson's 2002 broad-scale survey of more than 12,000 amphibians, representing 11 species from 101 sites in five western US states, recorded severe malformations at up to 90 percent rates.

The issue isn't just one of non-Midwestern species in far away places. It hits close to home - deformities are found throughout Wisconsin. The most adversely affected areas include the Midwest, along with the western US and southeastern Canada.

The first questions that arose was whether these abnormalities were normal. Johnson and others exhaustively studied historic records and found that while observations of amphibian deformities were documented for over two centuries, the proportion of abnormalities typically was under 5 percent in any population, and most often involved only a missing digit or part of a limb, not the bizarre spectrum of abnormalities reported in the last decade. Scientists normally expect that a few individuals in any healthy population of amphibians will exhibit aberrations, but nothing of the order and magnitude that researchers were finding around the world.

Dozens of studies since 1995 have narrowed the causes of the deformities down to three major sources:
1. Increased exposure to ultraviolet radiation
2. Pesticide and herbicide runoff
3. The parasitic fluke Ribeiroia ondatrae

All three have been shown to act in concert in a dynamic and complex process that contributes to amphibian deformities. But in laboratory and field experiments, ultraviolet radiation and chemical contamination proved to not be the smoking guns that many thought they might were. Still, the data clearly implicates them as part of a cocktail of stressors that enable infection by Ribeiroia flukes to occur more easily.

Instead, the research consistently has pointed strongly to the flukes as the major causative factor, and to over-fertilized waters as the hotspot sources for the flukes. In his 2002 study, Johnson reported that several geographical regions in which high numbers of deformed amphibians and the flukes were associated, were in habitats with a great excess of nutrients, such as farm ponds situated near large quantities of cattle manure and fields with heavy fertilizer use. The excessive nutrients produced masses of algae, which in turn produced higher growth rates and numbers of the snails needed by the flukes.

Early on, researchers realized they needed to know how the flukes spread into other waters, and they found the flukes have a highly complex life cycle requiring multiple hosts to support them from the egg stage to larvae to adults. The Planorbid snails function as the first intermediate hosts of the flukes. Here the flukes reproduce asexually within the snail, cloning themselves in high enough numbers that 40-1000 larvae are released every night per snail. The larvae swim out and look immediately for their next host, the tadpole of a frog, toad, or salamander.

The flukes target the tadpoles' limbs, burrowing into their skin and often forming hundreds of cysts. The cysts appear to obstruct the growth of the tadpole's limbs, causing in particular their hind legs to stop growing altogether, to split into two, or to jut off at an abnormal angle.
The tadpoles, if they survive the infection - and many don't - eventually morph into adult frogs. But the adult frogs are dramatically malformed, and thus far more susceptible to predators because they can no longer swim or hop at normal speeds.

A slow frog makes an easy meal for wading birds, the third intermediate host. "If the frogs were to die of old age, the flukes would die with them," says Johnson. "The only way for the flukes to finish their life cycle is to be eaten by a bird, primarily by herons, though nearly 50 bird species are known to eat frogs."

The flukes lodge in the bird's esophagus, mature, and then reproduce sexually, producing eggs that flow out in the bird's feces. If the feces land on the ground, the flukes die. If the feces land on water, however, the eggs can hatch and re-infect the snails by burrowing under their shells and through their skin.

Nobody knows how long the flukes live in the esophagus of birds, so there appears to be a constant source of new eggs coming into water systems. Johnson notes that, "Birds are vectors as they fly between lakes and rivers. No one knows if the birds are harmed by the flukes. We're currently doing research in Madison to understand more about the role birds play in this cycle."

Frogs, toads, and salamanders are considered "bioindicaters" of environmental health, the proverbial canaries in the coalmine. Since amphibians have permeable skin and shell-less eggs, they are highly sensitive to changes in their environment. Amphibians provide the function of biological monitors of local conditions because they don't wander far from where they were hatched. Deformities in frogs, and their continually decreasing numbers, serve as clear warnings of serious environmental degradation.

Johnson's current study asks the critical question of why the frequency and geographical range of malformed amphibians has apparently increased. He's hoping to discover the secondary factors that are responsible for the increased number of flukes, or the increased susceptibility of the snails. Johnson hypothesizes that habitat alteration, specifically an increase in highly eutrophic waters, lies at the heart of the matter. Higher plant growth increases the snail's growth rates, its survivorship, and its number of generation per season. Is it a clear equation of more algae blooms equals more parasitized snails which equals more deformed amphibians?

Johnson strongly believes we need to know the impact on amphibians as a species conservation issue. "What will be the impacts if amphibian populations continue to decline?" he asks.

He also notes that amphibians play a key role in how ecosystems function. "The adults help regulate insects. The tadpoles help regulate algal growth. And they all provide food for birds and other predators."

He wonders too if understanding what causes this parasitic infection "might shed light on the mechanisms of other emerging diseases. If we can account for the environmental conditions that drive frog deformities, maybe we can shed some light on the pathways of other new infectious diseases."

Johnson also knows that kids are connected to frogs - " They are our eyes and ears out there," he says. "Kids love frogs. They're the ones who usually find the deformed populations. Kids growing up with out frogs would be a very sad loss."

For more information on Pieter Johnson's work, go to his website at the University of Colorado at Boulder where he now is an Assistant Professor in Ecology and Evolutionary Biology. Specifically, you can learn more and see pictures of deformities here.