Leap-frogging over science

Deconstructing the transatlantic concern over deformed frogs' legs.

Bill Durodié

Topics Politics

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Is humanity threatened by the existence of five-legged frogs? Do pesticides cause deformities that could spell danger for human health? Some environmental activists think so: and now the scientific establishments of America and Europe have responded to their concerns.

Two reports published in 1999, by the US National Research Council (NRC) and the European Commission’s Scientific Committee on Toxicity, Ecotoxicity and the Environment (CSTEE), looked into the reasons behind a supposed rise in limb abnormalities found in various frog species – including supernumerary (extra), deformed and missing legs. Such widespread deformities as these have been reported in the scientific literature for years. But recent speculation has raised the possibility of a correlation with ozone depletion, and/or increased use of chemicals by industry and agriculture.

One view has centred on the belief that some pesticides can mimic the effect of retinoids, which occur naturally in frogs and other vertebrates, and act as essential signalling molecules for limb development. Fears have been raised that whatever had been harming the frogs could potentially harm other wildlife and humans too, as retinoids interact with steroid hormone-like receptors and include some of the most powerful known human teratogens (substances which can cause birth defects). If proven, this idea would enhance influential theories surrounding so-called ‘endocrine disrupting chemicals’ (EDCs) – known in the popular media as ‘gender-benders’ and the primary focus of the two reports. These theories focus on the danger that chemicals could interfere with the ‘endocrine system’ – a complex of processes in which fundamental bodily functions are kept in check through the action of an appropriate balance of hormones – and thus disrupt the natural hormones responsible for homeostasis, reproduction, development and/or behaviour.

Yet the official reports reveal little evidence of any risk to human health coming from endocrine disrupters caused by pesticides. They recognise that the major human intake of endocrine disrupters actually comes from naturally occurring oestrogens found in foods, like peas, beans and celery. This is several orders of magnitude higher than the exposure to EDCs due to pesticides: a point which appears to be studiously ignored by the environmental campaigners. Oestrogen flavonoids in food are found to represent 102 micrograms per day, while daily ingestion of environmental organochlorine oestrogens released by human activity account for a mere 0.0000025 micrograms.

The original intention of CSTEE’s work was ‘to finally produce a report that covers human health and environmental effects of EDCs’. But the final product placed a far greater emphasis upon wildlife, ‘due to the fact that it is where the greatest impact is felt’. In other words, unable to come up with sufficient evidence for effects upon humans, the committee decided to play this down, rather than highlight the absence of risk. At the official level, the goalposts seem to have shifted. Meanwhile, the NRC report has been criticised for producing a ‘vacillating conclusion’, as its attempts to ‘achieve a consensus document’ from a ‘balance of views’ have simply produced a 400+ page fudge calling for even more research.

Among the many case studies examined, both documents repeated the concerns over the mystery of frogs’ leg deformities and their possible association with chemical contaminants. But between the release of the CSTEE and the NRC reports, two articles by American researchers published in Science magazine expressly refuted simplistic correlations between these widespread abnormalities and man-made environmental toxicants. In the first study, morphological (shape) analysis of some 400 frog specimens collected from 12 localities indicated no possible link to the expected action of retinoic acid, suggesting that the misshapen or extra limbs were more consistent with a physical, rather than chemical, effect. The second study showed conclusively that frogs grown from tadpoles kept under laboratory conditions with a microscopic parasitic flatworm developed abnormalities, while a control group did not. These trematode parasites, derived from a species of aquatic snail, would burrow into the tadpoles’ limb buds, physically rearranging them and causing abnormal limb development.

The idea that pesticides were behind an epidemic of deformity was now described by one of the scientists involved in the study as ‘dead in the water’. By autumn 1999 Gail Charnley, president of the prestigious US-based Society for Risk Analysis, had independently described the endocrine disruption hypothesis as ‘a theory based on anecdotes’, and compared it to ‘a conclusion looking for data’, before going on to criticise Congress for acting ahead of the scientific process by proposing a multimillion-dollar screening programme to investigate the supposed hormone-disrupting effects of up to 15,000 industrial chemicals.

Clearly simplistic associational evidence, rather grandly described by some as ‘ecoepidemiology’, is insufficient to rigorously explain biological and chemical processes best revealed through objective analysis of causal mechanisms and metabolic pathways. Yet rather than accepting the evidence, environmental campaigners – and even one of the scientists whose research refuted their claims – have shamelessly shifted the terms of the debate. They now argue that a species weakened by parasites may be more susceptible to the effects of climate change or the use of chemicals. They are also asking why there would appear to be an increase in the number of trematode-carrying water snails in the ponds and lakes investigated. Apparently they believe that this may be associated with an increase in the prevalence of organochlorine EDCs in the environment released due to human activity.

Despite the fact that none of their specific evidence has yet to stack up, by asserting a general need to act on a precautionary basis, such activists are running rings around a nervous establishment desperate to legitimise itself in the eyes of consumers. While frogs’ legs and snails may be off the menu for now, the wider endocrine disrupting chemicals saga looks set to run and run.

Bill Durodié is director of the International Centre for Security Analysis at King’s College London. He is the author of Poisonous Dummies: European Risk Regulation after BSE, European Science and Environment Forum, 1999 (download this book (.pdf 679 KB)). He is also a contributor to Science: Can You Trust the Experts?, Hodder Murray, 2002 (buy this book from Amazon (UK) or Amazon (USA)); and Rethinking Risk and the Precautionary Principle, Butterworth-Heinemann, 2000 (buy this book from Amazon (UK) or Amazon (USA)).

Reproduced from LM, issue 126, December 1999/January 2000

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Topics Politics


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