The UK's only epidemic of BSE was officially blamed on the feeding to cows of an improperly treated offal meal that had become contaminated by the scrapie agent - found in sick sheep brains. Yet this same UK ingredient was exported in millions of tonnes to cattle in BSE-free countries all over the world. It was also manufactured in a similar way in the US without any BSE erupting there. Furthermore, 24,000-plus UK cattle born after the offal ban in 1988 have still gone down with the disease.

But there was something unique to UK dairy farming in the Eighties that may give the real reason for the BSE epidemic: the Government required dairy farmers to treat their cattle for warble fly with an excessive dosage of a systemic pesticide. It contains organo-phosporate - a chemical also found in military nerve gas used in Iraq. This mother of all chemicals also contained phthalimid - the basic unit of the infamous thalidomide.

This chemical had to be poured along the backs of cattle, whether infected with warble fly or not, and was designed to penetrate the skin, turning the whole internal environment of the cow into a poisonous medium. The whole purpose was to kill off the warble grub that could be found even inside the central nervous system itself.

Being an organic farmer, I won a High Court action in 1984 to debar the Ministry of Agriculture from treating my cows with this pesticide. I warned officials that this chemical could permanently damage proteins inside the central nervous system of both the treated cows and the farmers carrying out the treatment. It is worth remembering that there has been no case of BSE in home-reared cattle on organic farms.

I endorse US scientific opinion that BSE and similar diseases are caused by the presence of a misfolded version of a specific protein called prion protein, found inside the brain of all mammals. The protein can become corrupted due to a rare inherited genetic fault; this accounts for low background incidence rates of all these diseases (CJD, scrapie, BSE) that have run for light years. (Maff's chief vet in 1912 reports "Scrapie in Oxon!")

But a specific environmental trigger may mean that the folding process of the protein becomes artificially corrupted in large numbers of beings, so that the abnormal protein starts accumulating like a cluster bomb in the brain. An epidemic can then erupt.

This line of thinking should make us look again at conventional explanations for BSE. We can't simply blame it on feed .

Infectious though this prion agent is, the feeding of massive doses of these prions to misfortunate laboratory animals rarely passes the disease on. This suggests that some defence mechanism operates in the gut of healthy beings. So while we must consider a risk, however minute, from the intake of prions through beef consumption, the greater risk must come from exposing ourselves in early life - perhaps in the womb - to significant doses of chemicals that can corrupt this prion protein.

I believe it was the exposure of our cattle population, farmers and animal workers to these chemicals in the 1980s that switched on the epidemic of this disease. This could be the true cause of the BSE debacle.

Mark Purdey

The author is an organic dairy farmer in Somerset and a BSE researcher.