British scientists are beginning to seriously consider a new theory about the cause of Mad Cow Disease, proposed by organic farmer Mark Purdey, who feels that the conventional explanation-that BSE is caused by animals beingfed infected meat and bone meal-is wrong.
Purdey believes that Mad Cow Disease is caused by cattle being exposed to high levels of the metal manganese, as well as a common insecticide, phosmet. He also believes that people who are exposed to these substances are susceptible to CJD, the human form of BSE.
David Brown, of Cambridge University, thinks the theory could be valid. "I think the general view of what caused BSE is wrong," he says. When the disease was first identified in 1986, he noticed that it corresponded with areas where phosmet had been used against the warble fly.
In experiments, Mr. Purdey and Dr. Stephen Whatley found that phosmet increased the number of prions produced in human and animal brains. Usually these prions exist for only a few hours, but in BSE, they become almost indestructible, building up in the brain cells and killing them.
Purdey has studied areas with high levels of BSE and in each one he found high levels of manganese and low levels of copper. Dr. Brown found that prions starved of copper and dosed with manganese change their shape into the dangerous form of the prion.
Dr. Steven Collins, who studies CJD in Australia, feels this explanation is inadequate because it does not explain why zoo animals that were fed bone meal developed BSE, even though they were not exposed to manganese or phosmet. "He has to have a whole explanation," says Collins. "It has tocover all dimensions of the epidemic. Why has the epidemic diminished? Has there been a change in pesticide use or manganese levels?"
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