The growing number of deformed frogs in recent years is caused at least partly by runoff from farming and ranching, new research indicates.

Nitrogen and phosphorous in the runoff fuel a cycle that results in a parasitic infection of tadpoles, resulting in loss of legs, extra legs or other deformities, according to researchers led by Pieter Johnson of the University of Colorado, Boulder.

Their findings are being published in this week's online edition of Proceedings of the National Academy of Sciences.

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The deformed frogs have been a puzzle for more than a decade, since a group of Minnesota schoolchildren discovered a pond where more than half of the leopard frogs had missing or extra limbs.

Suggested causes have ranged from pesticides and increased ultraviolet radiation to parasitic infection.

While parasite infection is now recognized as a major cause of such deformities, the environmental factors responsible for increases in parasite abundance had largely remained a mystery, Johnson said in a statement.

Here's how the cycle works:

The parasites, called trematodes, have a series of host species.

They grow in snails and become infectious when released by the snails into ponds, where they can infect frog tadpoles, forming cysts in the developing limbs.

Water birds eat the frogs and then excrete the parasites back into the ecosystem where they can infect the snails, he explained.

The increasing amount of runoff is fueling a boom in algae growth, the snails eat the algae and also undergo a population explosion, increasing the breeding places for the trematodes.

To test the idea, the researchers built 36 artificial ponds in central Wisconsin and introduced snails. Ponds with added runoff had a 50 percent increase in the snail population compared with those that did not have the extra nutrients.

The research was funded by the National Science Foundation.