SCIENCE -- October 25, 2011 at 5:16 PM ET
Researchers Battle Parasite-Driven Frog Deformities
At a University of Colorado lab, a leopard frog named Flipper flaps a half-formed seventh leg as he tries to navigate his aquarium. Another frog kicks himself onto his back and, burdened with two useless extra legs, can't manage to turn himself over again.
These frogs in Pieter Johnson's lab are just a handful of thousands of multi-limbed frogs in North America. Johnson, a professor of ecology and evolutionary biology, has been studying these malformations since they first gained national attention in the mid-1990s, when a group of school children in Minnesota found that a local pond had an abnormally high number of amphibians with deformities. The deformed frogs began popping up all over the place - California, Oregon, and Quebec, to name a few hot spots.
The main culprit is a parasite, or flatworm, called Ribeiroia ondatrae, according to Johnson.
"This parasite, this tiny little invertebrate, is manipulating frogs and changes body plans to alter development and increase its own spread," he said.
Johnson and his team are now trying to understand factors that drive the various infections and malformations in these frogs, while searching for ways to reduce the number of deformities.
"We are running experiments looking at shifts in temperature, runoff of nutrients and pesticides, and biodiversity loss to evaluate their potential influence," he said.
This is just one of many challenges endangering frogs worldwide. They are faced with habitat loss and threatened by toxins from pesticides and UV radiation. Other diseases like chytridiomycosis, a deadly skin disease caused by a fungus, have caused amphibian declines on five continents. All of this adds up to make amphibians the world's most endangered class of vertebrates.
Ribeiroia starts its life cycle in freshwater snails, stripping the mollusks of their reproductive organs and turning them into parasite-breeding machines. The parasite gets released into the water, where it seeks out tadpoles. And it's from there that things get really "macabre," says Johnson.
"It's almost like they're tasting the skin until they find the right spot and burrow in," he said. "What happens next is like you jammed a golf ball into the space where the limb grows."
No two deformities are alike, said Brandon Ballengée, a biologist and artist who photographs the frogs. And tadpoles undergoing metamorphosis are incredibly susceptible to infections and predators.
"[The tadpoles] are like Play-Doh," he said. "All these cells are telling them to grow very quickly ... but when you have something going in and mechanically altering cells and moving them around, you're going to get an enormous diversity of deformities. The tadpole body turns around and tries to restructure itself to survive."
With too many or too few legs, the infected frogs, toads, and salamanders are easy prey for birds. And when a bird or other host consumes an infected frog, the parasite can live, sexually reproduce and hatch eggs inside of that bird. The parasite eggs are then deposited via the bird's droppings into another water system, and the cycle starts anew.
The parasite has so far only been confirmed in North America, Ballengée said, and research shows that its distribution is changing. Some populations of Pacific tree frogs have infection rates as high as 95 percent, while places that had high rates in 1999 have seen their infection rate drop.
Andrew Blaustein, an ecologist at Oregon State University, said environmental factors are driving the spread of parasite infections. Nitrate fertilizers used in agriculture drain into nearby wetlands. Nitrogen from that runoff feeds algae, causing it to bloom, providing a food source for the snails that host the parasite.
Nitrate runoff is linked to a multitude of other problems, including oxygen depletion and nitrate poisoning in well water, which can be harmful to humans as well. Frogs with Ribeiroia, Blaustein said, are just one sign that things aren't right.
Amphibians are under a lot of stress from diseases to habitat loss, said Kerry Kriger, an ecologist and founder of Save the Frogs, a non-profit dedicated to protecting frogs and amphibians, and their survival is crucial to the vitality of whole ecosystems. He added that frogs are a good indicator of the health of an ecosystem because their life cycles are dependent upon a balanced wetland system.
"Everything has evolved to have frogs in the environment," Kriger said. "Right now, we're driving them rapidly to extinction. If frogs are removed from the ecosystem, then everything is slightly out of whack."
Scientists and researchers are looking for ways to stop the parasite spread and restore the habitat, but so far there isn't a simple solution. Ballengée said that eliminating Ribeiroia is more like triage than surgery, and each wetland needs to be assessed and treated individually.
"We can come up with ideas and hopefully ideas work for the larger scale issue," he said. "Inevitably, [Ribeiroia] is a natural creature ... this demonstrates our human manipulation of wetlands gives it an upper hand."
The parasite doesn't infect humans or livestock, Johnson said, but studying its transmission and life cycle may unlock the key to preventing the spread of similar parasites, like the trematode that causes schistosomasis in Africa. They are also studying species that act as 'parasite sponges' to determine whether they could help to reduce parasite-driven deformities.
"The real challenge is trying to understand patterns of transmission," he said. "It's not like a nice, clean, normal distribution. How do you find the super-spreader hosts or habitats -- those small minorities that are causing most of the transmission. If you can manage those, you can control things in a much better way."
Photos by Rebecca Jacobson.