INDEPTH: MAD COW|
Science and symptoms
CBC News Online | Updated December 29, 2003
BSE, bovine spongiform encephalopathy, also known as mad cow disease, affects the central nervous system of its victims. It may be present in an animal for four to seven years before symptoms appear. During the final stages, infected animals become aggressive, lack co-ordination, and are unsteady on their hooves – hence the nickname for the condition – mad cow.
Victims of the disease usually die within a year after the first symptoms appear. Veterinarians can only tell for certain if an animal died of BSE by examining its brain after death. Brains of "mad cows" are spongy, as holes have developed in the nerve cells. A deposit of fibrous protein is also apparent in the brain's tissues.
Scientists originally thought BSE made its way into cattle through feed made from sheep offal (guts, basically). The sheep were infected with a similar condition called scrapie, and researchers thought that by eating feed made from sheep that had scrapie, the cows developed BSE.
But in October 2000, at the British BSE Inquiry, it was concluded that though the conditions are similar, BSE likely didn't originate when cattle dined on sheep with scrapie.
Instead, it was concluded that BSE is a new disease, possibly arising from a mutation in one cow's genes.
The assumption is that the remains of this one cow was used in cattle feed, thus starting the infection of Britain's herds.
Creutzfeldt-Jakob disease (CJD)
Creutzfeldt-Jakob disease, CJD (pronounced "KROYTS-felt YAK-ob"), is often referred to as the "human form of BSE." It has a long incubation period, up to 30 years, and CJD also degenerates vital parts of the brain. Symptoms include dementia, weakened muscles and loss of balance. Autopsies of human brains show the same spongy appearance as cow and sheep brains affected by BSE and scrapie. CJD is always fatal.
The disease has been present in human populations for many years. An average of one person in a million dies of CJD in a year. Most of them are elderly people. CJD has been shown to be transmitted through contact with infected people's brains, via insufficiently sterilized surgical instruments and electrodes applied directly into the brain, and through growth hormone created from infected corpses. But since the BSE scare began, there has been fear that humans could catch CJD by eating infected beef.
For the first 10 years of the epidemic in cattle, both the British government and many scientists believed there was no chance for people to contract CJD through food.
Then, in 1995, the first case of a teenager infected with CJD occurred in Britain. Other unusual cases had cropped up by this point; four slaughterhouse workers had died in the past three years from CJD, a dairy farmer whose herd had BSE died of CJD.
A panel of independent scientists was formed to examine 10 such new cases of CJD. In 1986, they came to a startling conclusion – a link between CJD and BSE appeared to be established. Calling the new strain "new variant CJD," or vCJD, they made their results public.
There is still no concrete proof of how exactly BSE, scrapie, CJD and vCJD are transmitted. The current theory is that a mysterious agent called a prion may be the culprit.
The prion connection
While studying scrapie in 1982, Dr. Stanley Prusiner, a neurologist at the University of California at San Francisco, said he had found a mysterious substance containing protein, but no genes. He called the substance a "prion" (pronounced pree'-on) or infectious protein.
Prusiner won the 1997 Nobel Prize in Physiology or Medicine for his discovery.
Prion specialists think there are two kinds of these proteins. Normal prions are found in all brain cells. Abnormal prions have a different shape, and are the suspected agents of diseases like BSE. When they make physical contact with the normal prions, they can flip them over into the abnormal shape, changing them into infectious agents.
The catch is that proteins need genes in order to make new proteins. So, if prions are just proteins, with no power to create anything new, scientists want to know how these abnormal prions can flip normal prions, and how they cause them to start causing disease.
Researchers think the abnormal prions are likely a product of a mutant gene, but they still don't know exactly how they cause disease.
Sheep prions closely resemble cow prions, lending credence to the original theory that BSE arose from cattle eating meal contaminated with scrapie. But human prions are significantly different from both sheep and cow prions, leading many researchers to believe humans don't contract CJD, or even vCJD, from cows - at least maybe not through prions.