Fear and mystery of cross-species killer
LONDON, England (CNN) -- It was a routine visit by a country vet to a dairy farm in southern England that set off the chain of events that was to become Britain's most notorious food scare.
Still mad cow disease, and its jump from cattle to humans, mystifies scientists and inspires public fear.
The illness was first identified in 1985 when the vet, puzzled by odd symptoms he had seen in cattle, consulted scientists at the Central Veterinary Laboratory in Weybridge, Surrey.
They found evidence of a new illness resembling the sheep disease scrapie. It was technically named Bovine Spongiform Encephalopathy (BSE) but it quickly acquired the mad cow tag because of the way infected cattle behaved.
A decade later in 1996 the British Government conceded people were falling victim to a degenerative new brain disease linked to BSE.
Despite previous denials that BSE could infect humans, ministers now accepted variant Creutzfeldt-Jakob disease (vCJD) was most likely caused by infected meat.
The devastating illness often starts with a bout of depression, progressively cripples the brain and always results in death.
But many questions still need answers -- how did BSE develop in cattle, how did the disease cross species to infect humans as vCJD and what factors affect whether a human will contract the disease.
The World Heath Organization says competing theories exist to explain how it develops.
The most accepted explanation is that the disease is caused by prions -- self-replicating proteins that contaminate neighbouring protein cells.
Other scientists claim vCJD is a virus-like illness able to carry genetic information that corrupts proteins.
The WHO says human sufferers of the disease usually experience psychiatric symptoms early in the illness, which most commonly take the form of depression and anxiety or less often, a schizophrenia-like psychosis.
Neurological problems like unsteadiness and involuntary movements usually follow and shortly before death, patients become completely immobile and mute.
Despite disagreement on exactly how it forms, scientists agree that the most likely link to humans came through people eating beef contaminated with BSE.
Experts believe BSE was created when cows were fed scrapie-infected feed manufactured from abattoir offcuts.
Cattle feed had been produced from animal remains since 1930, but in the 1970s and 1980s changes occurred in the way it was made.
Solvents thought to be a health risk to rendering workers were banned and lower temperatures were used in processing the feed.
Experts now believe those new manufacturing techniques allowed a resilient strain of scrapie to enter the feed and for it re-emerge in a new form in cattle disease -- BSE.
Cattle carcasses infected with BSE were then used in manufactured feed, which recycled the disease and rapidly worsened the epidemic.
As many as 500,000 contaminated beef carcasses are thought to have entered the human food chain. By the mid-1980s, large numbers of people in Britain were unwittingly eating beefburgers, cheap mince and pies infected with BSE.
The WHO says more than 168,000 cases of BSE have been reported in Britain and relatively small numbers of cases have also been reported in native cattle in France, the Republic of Ireland, the Netherlands, Portugal and Switzerland.
Small numbers of cases have also been reported in Canada, Denmark, the Falkland Islands, Germany, Italy and Oman, but solely in animals imported from the United Kingdom.
The BSE disease can be particularly concentrated in cow brains, spinal cords and certain organs such as the spleen.
Stringent measures introduced by the British Government in 1989 banned the use of this meat in stock feed.
Older cattle, which could have been exposed to feed containing the meat, were destroyed.
People with vCJD are thought to have been incubating the disease since before the new controls came into force.
And as it is not known how long the incubation period is, no one knows how many people are likely to die from the illness.
If the incubation period is very long -- some experts believe it could be as long as 30 or 40 years -- the epidemic may hardly have started yet.
In 1997 new animal studies suggested that vCJD could be transmitted through blood raising fears of a much wider problem.
White blood cells, which form part of the immune system and are found in the lymph glands, were isolated as one of the high-risk tissues for BSE infection.
As a result of this concern, the British Government required the removal of white blood cells from donated blood.
The fact that vCJD could enter the lymph system raised the further possibility that it might infect the tonsils or appendix, both of which contain large amounts of lymphoid tissue.
In August this year, new research indicated the existence of a hidden "subclinical" form of BSE, which produced no symptoms but could nonetheless be infectious.
This raised the frightening possibility that not only cattle but sheep, pigs and poultry exposed to BSE via animal feed may secretly harbour the disease.
Britain's Ministry of Agriculture said it believed current abattoir safeguards were adequate to deal with any potential threat from subclinical BSE.