Mark Purdey was a maverick figure in the study of one of the largest public-health scares of the last century. A Somerset dairy farmer, he became convinced that BSE had been caused not by feed containing scrapie-infected sheep offal, as was the received wisdom, but by organophosphorus (OP) compounds used widely to treat warble fly infection.

One of the implications of his theory was that fears of transmission were unfounded, and that thousands of cattle had been slaughtered unnecessarily. He spent many years, and many thousands of pounds, trying to convince the Government.

It was Stanley Prusiner, Professor of Neurology at the University of California, who first put forward the idea that transmissible spongiform encephalopathy (TSE) diseases of all kinds, including CJD, BSE and scrapie in sheep, were caused by proteins called prions that folded abnormally and corrupted the molecules around them. He also demonstrated that when TSE-diseased brain tissues were injected into laboratory animals, those animals also contracted the disease. Fifteen years later, in 1997, he was awarded the Nobel Prize for his discoveries.

Purdey highlighted what he saw as flaws in the application of the theory by Ministry of Agriculture scientists to the BSE crisis. He claimed that 2,000 of the BSE-infected cattle never consumed the suspect feed; that there were numerous countries to which infected ruminant meat and bone meal (MBM) had been exported that had not been affected; that the US remained BSE-free despite also adopting the relaxed manufacturing techniques blamed for the scrapie agent surviving in UK feed; and that several trials in the US failed to induce BSE in cattle when it fed and injected them with massive doses of scrapie-contaminated brain tissue. In addition, in his view, more than 40,000 cows born after the ban in 1998 on MBM incorporation into ruminant feed developed BSE nevertheless.

Purdey set about studying several countries affected by TSEs and those where it was completely absent. His results appeared to suggest that a combination of high manganese levels, low copper levels and a high level of environmental oxidising agents such as ultraviolet radiation could bring about TSEs in susceptible genotypes. Research by biochemists at Cambridge University afterwards supported this theory: they found that when copper was substituted by manganese in prion proteins, the prions would adopt the distinguishing features of the infective agent in BSE.

Purdey pointed out that the BSE epidemic in the UK began two years after OP compounds were first used as sheep-dip pesticides; and that it was most intense in areas that were designated warble fly eradication zones in the 1980s. He also pointed out that during this period cows were also exposed to high levels of manganese in the form of poultry manure — hens were fed manganese to boost egg production — which was put into their feed.

In 1992 Purdey treated one of his cows, which had developed BSE after spending a few months on a chemically managed farm, with a mixture of drugs used as an antidote to the organophosphorous nerve gas to which soldiers were exposed during the Gulf War. The cow had improved within 90 minutes, and continued to improve until further drugs were withheld at the request of outside experts.

Mark Purdey was born in Much Hadham, Hertfordshire, in 1953, into the famous Purdey shotgun-making family, and educated at Haileybury College. Expelled for breaking out of school to visit some girlfriends, he was offered a place at London University to study zoology, but dropped out to set up an organic farming community in Ireland. By his mid-twenties he was running his own organic dairy farm in Somerset.

In 1984 an official from the Ministry of Agriculture visited the farm to insist that he use the OP warble fly treatment on his cattle. He refused and the case went to the High Court.

When he won, Purdey was featured on the front page of The Times and besieged with letters from farmers who suspected that OPs were to blame for their failing health. He started to read up on the subject in earnest and in 1987 was asked to write a paper on it for the House of Commons Agriculture Select Committee.

Support from the public and media was growing, and when in 1988 Purdey made a television documentary on OPs and human health detailing his finds, Ted Hughes wrote to congratulate him: “So simple. One bull’s-eye after another,” he wrote. “You’ve planted a big bomb. They can’t hide from the camera, can they? They’re as scared as we are.”

Purdey began research into the environmental prerequisites of transmissible spongiform encephalopathies after BSE erupted in four cows he had purchased. None of his farm-bred cows ever contracted BSE. When in 1992 he wrote to the Ministry of Agriculture, Fisheries and Food (MAFF) explaining his theory he was told that there was no evidence to support his claims. In 1994 he was invited to a five-hour meeting with MAFF scientists, and in 1995 the Medical Research Council toxicology unit carried out experiments to test the theory. But they were inconclusive, as was a meeting in 1996 with the EU Farm Commissioner Franz Fischler.

In 1997 Purdey, helped by donations, commissioned trials at the Department of Neuroscience at the Institute of Psychiatry. The results, which showed that the OP pesticide Phosmet increased susceptibility to BSE, were later presented to the Spongiform Encephalopathy Advisory Committee (SEAC). The following year it was announced that the Government would provide funding for research into his BSE theory.

The BSE Inquiry, 1998- 2000, by Lord Phillips of Worth Matravers, conceded that organophosphorus compounds might make cattle susceptible to BSE. Purdey was praised for his many years of work in the public interest, but his theories on manganese, copper and radiation levels in the environment as a factor in BSE, were found unconvincing.

Purdey continued his work, however, and was still giving lectures in February this year. A brain tumour had been diagnosed last December.

Purdey’s first marriage, in 1974, to Carol MacDonald, was dissolved. He is survived by his second wife, Margaret. There were eight children from his two marriages.

Mark Purdey, farmer, was born on December 25, 1953. He died on November 12, 2006, aged 52.