WASHINGTON – 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. 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. 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. 160Want local news?Sign up for the Localist and stay informed Something went wrong. Please try again.subscribeCongratulations! You’re all set!