Scientists believe they have discovered the first case in the world of a seal with bovine TB after being bitten by an animal infected with the killer disease.
The seal was washed up off the coast of Cornwall last year and taken to the Cornish Seal Sanctuary at Gweek.
Staff saw that the animal, which was around four months old, was suffering from several bite wounds.
When he failed to respond to treatment and deteriorated, staff at the sanctuary raised the alarm with vets.
Tests carried out by the Animal Health and Veterinary Laboratories Agency showed that the pup had bovine TB and in order to prevent any contamination the decision was taken to euthanase him.
The results of investigations suggested the strain originated from south west Wales and that the seal pup had probably been born there and swam to Cornwall.
The news has emerged at a time of massive controversy over a government cull of badgers as part of efforts to tackle the disease in cattle.
Some 5,000 badgers are set to be killed in two pilot culls in west Gloucestershire and west Somerset.
The pilots will assess whether sufficient badgers can be killed in an area to have an effect in reducing TB in cattle, following a long-term study which found that culling 70% of badgers in an area could reduce the disease in herds by 16%.
The Government said the cull is necessary as part of efforts to stop spiralling numbers of outbreaks of TB in dairy and beef herds, which saw 28,000 cattle slaughtered in England last year.
The seal was found injured on the beach in Porthleven on the south coast of Cornwall in January 2012.
But the discovery has only now emerged in the scientific journal, the Veterinary Record.
The journal's report states: "...this case is of significance as, to our knowledge, it is the first confirmed case of M bovis infection in pinnipeds".
Tamara Cooper, animal care supervisor at the Cornish Seal Sanctuary, said the seal was at first responding well to treatment and then suddenly went downhill.
"He developed a swelling on his lower abdomen. We had more blood samples taken by the vet and found that his white blood cell count was still high," she said.
"Over the next few weeks he started to deteriorate further and developed a cough.
"The vet took a swab and it came back as possibly bovine TB. With his prognosis being quite poor and the risk of infection, the vet took the decision to euthanase."
Ms Cooper said that report in the Veterinary Record was as a result of a post mortem examination carried out on the seal.
"As far as they know there is no other seal that has been found with bovine TB - it is the first of its kind," she said.
"They can only presume the seal caught it from another animal with TB.
"It looks like he caught it from a bite because they think it came through the skin and probably from the wounds he came in with.
"They just have no idea what it could have been from and no way of finding out. We had all the other animals here and the team tested and everything was negative.
"Hopefully a one off case that we are never going to see again."