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SMALL doses of pesticide are enough to hit frogs where it hurts them most—in the immune system. This might explain the mysterious disappearance of amphibians all around the globe.

Researchers have been puzzling over this problem for years. The usual suspects are climate change, pollution or disease.

Pesticides are believed to break down under UV light to form poisons that trigger deformities in amphibians (¿ìè¶ÌÊÓƵ, 13 September 1997, p 18), and weedkillers can turn male frogs into females. But Brian Dixon of the University of Waterloo in Ontario, Canada, believes he has discovered the real reason why pesticides are bad news.

Dixon injected northern leopard frogs (Rana pipiens) with small amounts of the pesticides DDT, malathion and dieldrin. Then he measured the frogs’ immune response. He found that antibody production dropped to only a few per cent of normal after the pesticide injections, or even disappeared entirely. In fact these pesticides created almost as marked an effect as cyclophosphamide, a powerful drug designed to suppress the immune system, says Dixon.

When he studied wild frogs from polluted water in Ontario, Dixon found similar levels of pesticides in their bodies, and discovered that their immune systems were just as damaged as the lab frogs’. Although DDT and dieldrin have been illegal in Canada for years, they are slow to break down and malathion is still used for mosquito control.

However, the wild frogs’ immune systems were not entirely destroyed. For instance, they could still make antibodies to antigens they had encountered before they were exposed to pesticides. And he found that their immune function also improved after eight weeks in clean water.

Dixon believes the effect of pesticides on frogs’ immune systems could explain their disappearance worldwide, since it would leave them wide open to disease. It could also make them vulnerable to parasitic infection—which could lead to increasing numbers of frogs with missing or deformed limbs. His results will appear in Environmental Toxicology and Chemistry later this year.