The impact of this chemical is boosted in the wild by phosphate fertilisers, the investigators believe.

Runoff from fertilisers into ponds encourages the proliferation of snails that are a natural host to the flatworm parasites, they say.

The flatworms, called trematodes, are notorious for causing limb malformations, kidney damage and sometimes death in several species of frog.

The new study points the finger at atrazine, an active ingredient in several herbicide products manufactured by a Swiss-based company, Syngenta.

Atrazine was banned in the European Union in 2004 after the chemical showed up in drinking water, but has over the last 15 years become a leading farm chemical in the United States, especially in corn-growing regions.

In a field survey led by Jason Rohr of the University of South Florida, scientists measured more than 240 variables in 18 Minnesota wetlands that could account for the rate at which frogs are infected by trematodes.

The strongest link by far was with atrazine concentrations, which accounted for more than 50 percent of the likelihood that the amphibians would become diseased.

When the presence of atrazine was combined with traces of phosphate fertilizer -- runoff from nearby agricultural plots -- the rate of diseased frogs went up to 75 percent.

Seeking to find out more, the researchers raised tadpoles for four weeks in several 290-gallon (1,100-litre) tanks containing snails, leaves and insect larvae, to approximate a natural environment.

In tanks where atrazine was added in concentrations found in wetlands, four times as many snails grew compared with the population that was in water free of the herbicide. The population of the parasitic flatworms exploded too.

Green frogs used in the experiment showed significantly higher levels of trematode infection, while pickerel frogs experienced high rates of mortality.

"The wetlands survey was highly suggestive that atrazine was causing the increase in larval trematode load," Rohr told AFP by phone.

"The follow-up experiment really demonstrated that it was indeed a causal link."

He cautioned, though, that these findings did not by themselves explain a massive slump in American frog populations, a fall that began in the mid-1990s and is mirrored by shrinking populations of amphibians elsewhere in the world.

Global warming, inflicting a loss of wetland habitat, has been blamed as one of the causes.

Syngenta, asked to reply by AFP, said in a statement: "50 years of use and a vast amount of research has shown that (atrazine) can be used safely with no long-term detriment to ecosystems."

The concentrations of the chemical in wetlands reported in the Rohr study were well below the "level of concern" thresholds established by the US Environmental Protection Agency (EPA), it noted.

Rohr added that there could be other chemicals in addition to atrazine and fertilizers that affected disease risk.

"Many chemicals can be immuno-suppressive, and standard toxicity tests used to register chemicals in the United States and Europe are conducted on isolated individuals, ignoring interactions with other species, such as their parasites.

"Thus, our findings are likely the tip of the iceberg for pollution-induced disease emergence in both humans and wildlife."

The study was published on Thursday in the London-based journal Nature.

Rohr said that a senior biologist from the EPA, Thomas Steeger, had requested a copy of the study.

In its latest evaluation of atrazine, the EPA concluded in 2006 that the product posed no threat to human health.

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