Recent studies also have shown similar relationships between host diversity and the risk of disease in some plants, mammals, birds and coral. A decrease in vertebrate host species for ticks carrying Lyme disease, for example, can increase the risk of Lyme disease in humans, said Johnson.
"It could be that the most dangerous parasites occur in greater numbers in disturbed environments," said Hoverman, who recently accepted a position as assistant professor at Purdue University's forestry and natural resources department. "If we are trying to minimize disease risk in humans or in threatened groups of animals like amphibians, studies like this will be able to tell us which scenarios are most likely to occur."
The new study has implications for declining biodiversity being seen across the planet as a result of human activities, including amphibians, said Johnson. Roughly 40 percent of amphibian species around the world are in decline, and more than 200 have gone extinct since the 1970s, some as a result of the often-fatal chytrid fungus that infects amphibian skin. Some scientists argue that rapid global amphibian decline seen today is driving the next great mass extinction event, he said.
Trematodes have a complex life cycle that involve snails, amphibians and predators. Host snails release parasite larvae in the water, infecting amphibians and causing deformities that include extra or missing legs. Deformed frogs and toads rarely survive long because of their susceptibility to predators like wading birds, which ingest them and later release trematodes that infect other snails, completing the life cycle.
Deformed frogs first gained attention in the mid-1990s when a group of Minnesota schoolchildren discovered a pond where more than half of the leopard frogs had missing or extra limbs, said Johnson. Since then reports of deformed amphibians have been widespread in the United States, leading to specu
|Contact: Pieter Johnson|
University of Colorado at Boulder