Fertilisers blamed as researchers solve mystery of deformed frogs

American researchers claim to have answered the riddle of the deformed frogs that have been appearing in increasing numbers around the world.

Run-off from farmland drenched in fertilisers is behind the explosion in amphibians missing legs, or having extra legs and other deformities, according to the scientists.

Nitrogen and phosphorous from fertilisers are leaching into rivers, causing significant changes to the aquatic ecosystem. This prompts algae growth and increases numbers in the snail population, animals which play host to parasitic flatworms called trematodes. These parasites infect birds, snails and amphibian larvae, causing severe limb deformities and an increase in mortality.

"This is the first study to show that nutrient enrichment drives the abundance of these parasites, increasing levels of amphibian infections and subsequent malformations," said Pieter Johnson of Colorado University, who led the study.

These malformations include the growth of extra limbs, partly formed or missing limbs, skin webbing and bone defects. When examined, amphibians with these defects were often found also to suffer from life-threatening eye abnormalities and tumours.

Reports of these abnormal amphibians have risen sharply since the mid-1990s, when some Minnesota schoolchildren found a pond where more than half of the frogs had missing or extra legs. This has generated increasing concern from scientists and ecologists, who have since established that parasite infection is a major cause of these deformities.

Trematode parasites that affect amphibians have a series of host species. They can grow in snails and then become infectious when released. The parasites then infect tadpoles, forming cysts on new limbs. When the frogs are eaten by predators, they excrete the parasites back into the ecosystem.

With many of the causes of amphibian decline still poorly understood, this research, which appears in the latest edition of the Proceedings of the National Academy of Sciences of the USA, may be a major breakthrough. "Our results have broad ecological significance," said Mr Johnson.

Rapidly declining amphibian populations have been noted globally since the 1980s, and are now regarded as one of the most critical threats to global biodiversity.