Frog Decline Linked to Agri-Chemicals

The global decline in frog populations has been attributed to an increase in infectious diseases. However findings in Nature reveal a link between parasitic infection and local interaction between phosphate fertilisers and herbicides.

The study focused on leopard frogs from wetlands in Minnesota, USA, and these frogs were examined for trematode larvae - a type of parasitic flatworm. Trematodes are known to cause kidney damage and occasionally deformities in amphibians. The researchers also looked for indicative signs of water pollution, by measuring the level of melanomacrophage liver cells, which are involved in amphibian immune response.

Of the many possible factors contributing to these ailments in the frogs, the strongest predictor of larval infection was the herbicide atrazine combined with high phosphate levels. High levels of the atrazine herbicide were correlated with low levels of melanomacrophage cells, indicating the herbicide suppressed the frogs' immune response, increasing susceptibility to the trematodes.

A probable cause of the interaction was revealed, when the frogs were placed in tanks exposed to supposed environmental levels of these chemical in the U.S. Atrazine reduced phytoplankton growth, creating clearer water and higher levels of nutrients. This stimulated algal growth, which in turn encouraged gastropods. Gastropods play host to trematodes, acting as a vector to the amphibians. Since wetland birds are the primary host of trematode eggs, as the authors suggest, these must be present along with the elevated phosphate levels and herbicide for the interactive effect to take place.

The findings highlight problems in the European and American systems for registering chemicals. Although the chemicals have no direct effect on mortality independently, the combined effect results in this pollution-disease pathway and would not be identified under existing pollutant control tests.

Many European countries banned its use even before EU legislation withdrew support for its use in 2004, because of concerns over concentrations in groundwater. Atrazine is used widely across the world for corn and sorghum production. There is still some limited support for its use in European countries, although the UK, Ireland, Spain and Portugal no longer have any support for it.

Amphibians are becoming increasingly threatened across the globe, with many species expected to become extinct by 2050. Amphibian fungal infections, particularly Chytridiomycosis are on the rise across the globe, possibly exacerbated by climate change. Chytridiomycosis is thought to have originated in South Africa, although nobody is entirely certain where it began.

Adapted from the Science Environment Policy bulletin, Source: Rohr, J.R., Schotthoefer, A.M., Raffel, T.R., et al. (2008). Agrochemicals increase trematode infections in a declining amphibian species. Nature. 455: 1235-1240.