The origins of the BSE epidemic that devastated the UK beef industry could be revealed in a new series of tests that raise the prospect of the complete eradication of mad cow disease from the British Isles.
Government scientists are preparing to analyse material taken from animals that developed the disease despite being born after the 1996 ban on meat-and-bone meal that was supposed to have stopped the epidemic.
The genetic sequencing will either confirm or rule out the possibility that a small population of mutant cows predestined to develop the disease remain in the UK herd.
According to a hypothesis championed by Malcolm Ferguson-Smith, the scientist who jointly presided over the BSE inquiry and head of the veterinary cytogenetics research group at Cambridge University, these animals could be direct descendants of the first cow to contract the disease following a germline mutation of the gene that creates the prion protein.
Such a discovery could explain the recent rise in the number of BSE cases in cattle born after the feed ban and provide an explanation for isolated cases in Japan and Canada in the past two years. It could provide the basis for a strategy to mop up the last infected animals in the UK.
Professor Ferguson-Smith recommended the test in a letter to the government's Spongiform Encephalopathy Advisory Committee (SEAC) in February 2002. Although the SEAC did not discuss the matter when it met last week, the Department for Environment, Food and Rural Affairs confirmed on Wednesday that a sufficient number of samples had now been collected "to examine these cases as a series using molecular genetic sequencing".
The overall number of BSE-infected cows has fallen year on year since its peak at 36,700 cases in 1992. There have been 508 confirmed cases so far in 2003, mostly in animals born while the epidemic was still raging.
But 70 infected cows born after the ban on meat-and-bone meal have been identified.
This has troubled some scientists as it implies an undetected way of transmission.
A Defra spokesman said most of these cattle had been born shortly after the ban was introduced and could have contracted the disease after exposure to imported feed. A European Union ban on meat-and-bone meal was enforced only two years ago. He said the number of these cases was not expected to fall until 2006.
The spokesman supported the conclusion reached in Gabriel Horn's review on the possible origins of BSE that a genetic mutation was no more a likely cause than other suggestions such as the emergence of a new version of the sheep disease scrapie.
Professor Ferguson-Smith said: "We need to at least exclude this possibility."
He said that if the sequencing confirmed the hypothesis, it might present the government with an opportunity to ensure "this particular mutation was wiped out".