Four days earlier, Damson had betrayed all the symptoms of BSE - mad cow disease. Her ears were swivelling, she was staggering like a crab or was sprawled out. In Mr Purdey's words, she was 'like a frog on ice'.
On Thursday he injected her with oxime, a chemical carried by troops in the Gulf war as an antidote to nerve gas shells. Within 90 minutes Damson had shown remarkable signs of recovery.
Mr Purdey, an organic farmer, said at his 100-acre dairy farm near Elworthy, Somerset: 'The photographer from Farming News was there and said it was a miracle. Yes, I think I've discovered a cure for BSE because I've rectified the biochemical balance of Damson's body in an astounding way; and that itself pinpoints the root of the problem - these organo-phosphate pesticides that farmers are told to use.'
Mr Purdey, 38, thinks Damson - 'a divine cow, a cow of cosmic radiance' - became infected when he placed his herd of 35 Jerseys on a neighbouring 'chemical farm' for three months, earlier this year. Another of his cows, Mustard, was also infected but recovered after treatment with magnesium sulphate.
Damson's recovery revives, not for the first time in Mr Purdey's 19 years as a farmer, the debate over the use of pesticides. He believes his own life reflects some of the horrors of Natural Lies, the BBC's recent television thriller about big business and gangsters in the world of BSE. 'In 1985 I took the Government to the High Court - and won - because they wanted to treat my animals forcibly with organo-phosphate for warble fly.
'We'd moved farms to Devon and this strange person had been following us around, buying the farm next door. We came under this regime of absolute terror. The police came out many times. He was firing at us, not to hit us, basically just to undermine our validity as organic farmers. He went under several different names. I got the feeling he was from the criminal fraternity. That's the sort of vibes I got off him. One policeman told me 'you realise people are employed to behave in this way'. '
Mr Purdey said a house he had in Pembrokeshire was set on fire the night before he moved in. 'Then last Christmas a stone barn collapsed on to my caravan while we were away . . . We get odd people turning up here pretending to be chemical victims and trying to find out dates of meetings. And yet to me they seem to have memories like computers and if you're poisoned with pesticides you've no memory at all.'
Mr Purdey says he believes a government vet who visited him this week wanted to put Damson down. 'Then she saw the media here and drove off in a cloud of dust. Another vet arrived and took photographs of the media.'
On Friday, John Cohen, another government vet, turned up. He clapped his hands in front of Damson, a test that would have made a BSE bovine jump. She merely moved closer to investigate. Did Mr Cohen still believe she was suffering from BSE and should be put down? 'That's a leading question,' he said.
Mr Purdey said: 'The natural instinctive methods of farming that people are born with have all been kind of lobotomised out of their brains at agricultural college . . . You realise someone is making a lot of money manufacturing chemicals that are having serious side-effects and then they make more money by manufacturing pharmaceuticals to rectify that.'
A Ministry of Agriculture spokesman said: 'BSE can only be diagnosed by post-mortem though one can never be sure whether a particular animal has got it. Meanwhile, Mr Purdey has a form A movement restriction order on the animal as a precaution.' Did the Government think Mr Purdey had found a cure for BSE? 'At this early stage we can say nothing of significance.'