More frogs are being found with deformities in North America because farm water runoff contains excess nutrients—nitrogen and phosphorus—which makes more algae grow and increases snail populations that host microscopic parasites called trematodes.

"This is the first study to show that nutrient enrichment drives the abundance of these parasites, increasing levels of amphibian infection and subsequent malformations," said Pieter Johnson, a water scientist at the University of Colorado in Boulder.

Johnson said that he and his colleagues' work, which is detailed in the Sept. 24 issue of the Proceedings of the National Academy of Sciences, could also explain "a wide array of diseases potentially linked to nutrient pollution."

Deformed frogs first gained international attention in the mid-1990s, after schoolchildren found a pond where more than half of the leopard frogs had missing or extra limbs, Johnson explained. Parasite infection is now recognized as a major cause of such deformities, but the environmental factors responsible for increases in parasite abundance have largely remained a mystery.

"What we found is that nitrogen and phosphorus pollution from agriculture, cattle grazing and domestic runoff have the potential to significantly promote parasitic infection and deformities in frogs," Johnson said.

The trematode life cycle involves three host species. The tiny parasites form cysts in the developing limbs of tadpoles, causing missing limbs, extra limbs and other malformations, Johnson said. Aside from this stage and an infectious one in snails and the cyst stage in frogs, predators complete the trematode life cycle by eating infected frogs and disseminating the parasite back into the ecosystem.

To discover the link between farms and the trematode infections, Johnson and his team built 36 artificial ponds similar to farm stock tanks, where frogs and salamanders often breed and deposit their eggs.

The researchers then stocked each tank with snails and green frog tadpoles and, in addition to adding nutrients, they dropped in parasite eggs. In ponds with added nutrients, Johnson said, the total mass of snails was 50 percent greater and parasite egg production was eight times as great.

The infection rate in frogs rose between two to five times in those tanks, he added.

(Agencies via Xinhua News Agency September 26, 2007)