Fertilizers from farms and lawns are responsible for frog deformities cropping up in ponds and lakes across North America, a new study shows.

The finding not only has implications for worldwide amphibian declines, but could shine light on such diseases as cholera, malaria, West Nile virus and diseases affecting coral reefs, said assistant professor Pieter Johnson of the University of Colorado's ecology and evolutionary biology department.

©Pieter Johnson / University Of Colorado

Andrew Blaustein, zoologist from Oregon State University, hailed the CU finding as one of the first to connect the "drastic" problem of fertilizers with the proliferation of parasites and several diseases that can deform amphibians and sicken humans.

In 1995, Minnesota schoolchildren noticed that more than half the frogs in a pond had missing limbs or too many limbs.

Scientists since then have figured out that a parasite plays a key role.

The more nitrogen, phosphorus or cattle droppings in a pond, the more algae forms, said Johnson.

The algae boosts the population of snails, which host a microscopic parasite known as a trematode.

The snails release the parasites into the ponds, which then get into tadpoles and form cysts in their developing limbs.

It doesn't take many parasites in a tadpole to produce missing limbs or extra limbs in the adult frog.

After the parasites have caused deformities in frogs, large wading birds eat the infected frogs and spread the parasite back into the ecosystem.

The new study, led by Johnson, demonstrates that the fertilizers accelerate the cycle by stimulating the population of the snails, and thus the proliferation of the parasites.

The study didn't look at precisely how much fertilizer is too much - that could be the subject of a follow-up study, Johnson said.

"The next step is to figure out how to optimize the trade-off," using some amount of nutrients to boost food production but "minimizing the increase in human and wildlife disease."

The study appears online this week in the Proceedings of the National Academy of Sciences. Co-authors came from the Southern California Coastal Water Research Project and the universities of Washington, Wisconsin and Alaska.

It has implications for several human diseases.

For example, in parts of Central and South America, when a lot of nutrients collect in water, it leads to a proliferation of mosquitoes good at transmitting malaria, Johnson said.

Extra nutrients also likely boost the populations of mosquitoes in Colorado that carry the West Nile virus, he said.

The National Science Foundation funded the study to look at how parasites would respond to changes in ecosystems and in land use.

"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 researchers installed 36 artificial ponds in central Wisconsin, loaded them with parasite- carrying snails and parasite eggs, and then put different amounts of fertilizers into the ponds.

"It was all well within the range of what you might find in an agricultural pond - a lot less in many cases," he said.

In ponds with added nutrients, the snail population grew by 50 percent, and those snails collectively had eight times as many parasite eggs as the ponds with no extra nutrients.

The infection rates in frogs was two to five times higher in those ponds as in the ponds with fewer nutrients.

"Generally, there's over-application of these fertilizers," and that applies not just to farmers and ranchers, but to suburban homeowners, Johnson said.

It takes just a little fertilizer to stimulate growth of the grass in the yard, he said. The excess washes into the gutter and eventually to a body of water where an algae-loving snail may be lurking to start the unhappy cycle again in an unsuspecting frog.