Mad cow disease is the common name for a condition known technically as bovine spongiform encephalopathy, or BSE.

Here are some quick facts about BSE, and its human offshoot, variant Creutzfeldt-Jakob disease.

What is BSE?

BSE is one of a group of brain and nervous-system diseases affecting various animals, such as chronic wasting disease in elk and deer in North America and scrapie in sheep. It's caused not by bacteria or viruses, but by rogue proteins called prions.

The disease affects the brain and the spinal cord tissues. BSE gets its scientific name from the microscopic holes in the brains of affected animals, giving the tissue a sponge-like appearance. There is no vaccine or treatment for BSE, according to the World Organization for Animal Health.

How is BSE spread?

The only known source of mad cow disease is from animal-based feed contaminated with tissue from a diseased animal. The original source of BSE is believed to have been feed containing tainted meat from sheep with a related disease called scrapie.

The disease gets into the human food supply when an infected cow is slaughtered for meat. Milk from infected cows doesn't spread the disease, according to the Canadian Food Inspection Agency.

Can it affect humans?

Yes. A prion disease called variant Creutzfeldt-Jakob disease (vCJD) was first diagnosed in humans in the United Kingdom in 1996 and has been linked to consumption of tissue from animals infected with BSE, according to the U.S. Centers for Disease Control and Prevention (CDC). The first person to develop symptoms of what turned out to be vCJD became ill in January 1994, according to the World Health Organization.

Scientific evidence indicates that vCJD is caused by the same agent that causes BSE in cattle, says the Canadian Food Inspection Agency. The CDC adds that research indicates that older adults are much less susceptible to vCJD than children and young adults.

Symptoms typically don't start showing up until several years after infection, sometimes taking nearly a decade to appear. Once someone is infected, there is no cure.

Early in the illness, patients usually experience psychiatric symptoms, which most commonly take the form of depression or a schizophrenia-like psychosis, according to WHO. Unusual sensory symptoms, such as "stickiness" of the skin, have been experienced by half of the cases early in the illness. Neurological signs, including unsteadiness, difficulty walking and involuntary movements, develop as the illness progresses and, by the time of death, patients become completely immobile and mute.

What are my chances of getting infected?

Work continues at a meatpacking plant in Toronto on May 22, 2003, despite several countries placing a temporary ban on Canadian beef after a case of mad cow disease was discovered on an Alberta farm. (Mike Cassese/Reuters)

In Canada, brain and spinal cord material are removed from the carcass of animals and do not end up in the food supply. No cases of vCJD have been linked to eating Canadian beef, according to the Canadian Food Inspection Agency, and it says "the risk of contracting vCJD in Canada is extremely small."

By October 2010, a total of 222 definite and probable variant CJD cases had been reported worldwide in residents of 12 countries, according to the Creutzfeldt-Jakob Disease Surveillance Network.

Canada's first case was identified in 2002 and a second case was identified in March 2011, according to the Public Health Agency of Canada. Both were thought to have been contracted abroad.

How do I avoid it?

Cooking doesn't break down prions. The best way to avoid BSE is to not eat products that contain animal brain or spinal cord tissue.

What's a prion?

It's a self-replicating infection agent made of protein that can replicate like a virus.

The word prion comes from "proteinaceous infectious particle," according to Scientific American, and it's an infectious agent that consists only of protein with no nucleic acid genome. (All previously known pathogens, such as bacteria and viruses, contain nucleic acids that enable them to reproduce.)

Mad cow disease starts when prions mutate and trigger a chain reaction, which eventually kills the original cell and then moves on to other cells. Then the process begins all over again. Holes form in the infected brain as cells die, crippling the affected animal and finally killing it.

Mutated prions are very hard to destroy because they don't break down under regular cooking temperatures as bacteria and viruses do.

What are the symptoms of BSE?

The problem for veterinarians is that animals with BSE can take four or five years to show signs of the disease. The symptoms also cover a wide range, including nervous or aggressive behaviour, depression, twitching, touch, hypersensitivity to sound and touch, strange postures, loss of co-ordination, difficulty standing up and weight loss, according to the World Organization for Animal Health.

How is it detected?

Tests for BSE are essentially a hunt for the presence of infectious prions. These tests are done on brain and spinal tissue of dead animals where prions are most highly concentrated. Clinical symptoms may raise suspicion that an animal has BSE, but a diagnosis can only be confirmed by microscopic examination of brain tissues taken on the dead animal, says the World Organization for Animal Health.

Has BSE been found in Canada?

Yes. In the most recent case, the Canadian Food Inspection Agency (CFIA) confirmed on Feb. 18, 2011, that it had detected a BSE case in a dairy cow born in Alberta in 2004. It reported that no part of the affected animal carcass entered the human food or animal feed systems.

The first case of BSE in a Canadian-born beef cow was in May 2003.

BSE is thought to have found its way into Canadian cattle in the 1980s or early 1990s through meat and bone meal that was ground up into a protein supplement for animal feed, according to the Canadian Food Inspection Agency.

Canada's first known case of BSE was on a farm near Red Deer, Alta., in 1993 in a cow imported from Britain. Two other cows imported at the same time died and were processed into feed and food. Since 1997, it has been illegal in Canada to feed this type of cattle-based meal back to cattle. The CFIA introduced enhancements to the feed ban in July 2007, and tissue considered risky is now banned from all animal feeds, pet foods and fertilizers.

How prevalent is BSE today?

According to the U.S. Centers for Disease Control, BSE surveillance since 2003 has identified three BSE cases in the U.S. and 19 in Canada. Since March 2006, each of the 15 cattle reported with BSE in North America were born in Canada and identified through the Canadian BSE surveillance system. There was one reported case a year in Canada in 2009, 2010 and 2011.

In Britain, the epicentre of the outbreak, there were 184,500 confirmed cases of BSE between 1993 and 2010. The numbers have been dropping, from 1,443 in 2000, to 225 in 2005, and 11 cases in 2010.

When was BSE first identified?

Mad cow disease began in England in 1984 when a cow developed strange symptoms and soon died. More unexplained deaths followed, and scientists began chasing down clues. It took two years, but in November 1986 the British made their first diagnosis of bovine spongiform encephalopathy.

Almost five million cattle were slaughtered to stop the spread of BSE, but not before almost one million had made it into the food supply.