Creutzfeldt-Jakob disease (CJD) is a very rare, fatal disease that attacks the nervous system. There are two types of CJD: classical CJD and variant CJD.

Unlike other diseases that are caused by bacteria or viruses, this disease is caused by abnormal pieces of protein called prions. These abnormal prions damage cells of the nervous system, forming holes in the tissue and eventually leading to severe brain damage and death.

Classical CJD is very rare, affecting about 1 person in 1 million each year. About 60 people are diagnosed each year in Canada, most of them are between the ages of 45 and 75 years. The three types of classical

CJD include:

• sporadic: this form of the condition occurs with no known cause and affects 85% to 90% of people with classical CJD
• familial: this form appears in families that seem to have a greater likelihood of developing this disease due to their genetics
• iatrogenic: affecting less than 1% of classical CJD cases, people with this form of the condition develop classical CJD from accidental contamination by the abnormal prions, often through contaminated medical equipment

Variant CJD (vCJD) typically affects younger people less than 30 years of age and is associated with eating meat from cows infected with bovine spongiform encephalopathy (BSE), popularly known as "mad cow disease."

The causes of variant and classical forms of CJD seem to be different. Variant CJD is associated with eating meat from cows infected with BSE. Classical CJD is an older form of CJD, which usually appears spontaneously without any apparent cause and has not been linked to eating contaminated meat. It may be associated with genetic factors, and in some cases it may be caused by contaminated medical equipment.

The link between vCJD and eating infected meat was discovered when 23 people in Britain developed CJD in the mid-1990s, about 10 years after Britain was struck by an epidemic of bovine spongiform encephalopathy in 1984 to 1986. The total rate of CJD was only about 15% higher than usual, but these victims stood out because they were young (the average age was 29) and had different symptoms than were thought to be typical of CJD. Because of these unusual features of this variant CJD, scientists started to investigate. It was already known that some neurodegenerative diseases (diseases that destroy the brain or nervous system) can be caught by eating animals that had the disease.

Scientists began to suggest that British cows had been infected with BSE because they had been fed the rendered remains of sheep. Sheep suffer from a neurological disease called scrapie, so-named because the affected sheep would try to scrape off their coat of wool by rubbing against fence posts and trees. It was then that scientists started to explore the theory that if cows could catch BSE by eating sheep who had scrapie, then maybe people could catch variant CJD by eating cows who had BSE. Evidence began to accumulate that confirmed the ability of the disease to cross the species barrier when cases of degenerative neurological disease started to emerge in zoo animals in Europe. These animals had also been fed remnants of sheep.

Previous tests of scrapie and CJD in mice and monkeys showed that it took about 10 years for symptoms to appear after eating nerve tissue from an infected animal. This matched the British timeline perfectly.

There was a problem in the theory, however. No one had found a bacterium or virus that caused any of these diseases. In fact, scrapie-infected tissue was still known to be infectious even after being exposed to radiation that would destroy DNA, and thus kill any bacterium or virus. Finally, it was discovered that a protein called a prion seemed to be causing the damage.

Proteins are long molecules made of smaller parts called amino acids that are folded up into particular shapes. A prion is folded differently from the normal protein. It also has the ability to make normal proteins that touch it fold the same abnormal way, leading to a domino effect, causing abnormal protein folding that spreads from one protein to the protein beside it. This is a completely new form of disease, caused by a protein that's not alive but is capable transforming the proteins around it.

It now seems that scrapie, BSE, and CJD (classical and variant) are all caused by prions transmitted from one affected brain to another brain.

At this time, it is not believed that prions are spread through casual contact, kissing, or sexual intercourse.

CJD is a very rare disease, and it is difficult to diagnose. Although some symptoms might look like they are due to CJD, they are more likely to be caused by another condition. Only close observation over time might give the doctor clues that point to a CJD diagnosis. For instance, dementia appears much more quickly in CJD than in conditions like Alzheimer's.

A detailed medical history will be taken. Clues towards a diagnosis of CJD may be gained from tests that monitor brain wave activity (EEG), special genetic tests looking for susceptibility to vCJD, magnetic resonance imaging (MRI) to obtain images of the brain, and spinal fluid tests ("spinal taps") to test for a specific protein.

The only test to confirm a diagnosis of CJD is a brain biopsy, in which a sample of brain tissue is removed, or an autopsy, where the brain is examined after death.

There is no known cure or treatment for CJD.

Classical CJD cannot be completely prevented or avoided, but it occurs so rarely that the odds are greatly against being infected with it. Health care providers can minimize risk by handling tissues and bodily fluids with extreme caution, and using effective sterilization methods consistently to disinfect equipment. People with classical CJD can have genetic testing done to see if the disease runs in their family. Genetic counselling may be recommended before this testing is done.

The risk of variant CJD can be decreased by not eating the brains and nervous system tissue of prion-infected animals. Obviously, you can't guarantee that a particular piece of meat is prion-free. However, when reports of infected cows occur, countries and officials are typically quick to isolate and trace the affected animals, making human infection very unlikely.

In North America, there is a prion disease affecting deer and elk, called chronic wasting disease. If you hunt, you might want to think twice before shooting or eating an elk or deer that appears confused, deranged, emaciated, or unconcerned about its own survival. By law, animals with signs of chronic wasting disease must be reported to provincial authorities.

Prions tend to accumulate in nerve cells, and cuts of meat that come from near the spinal cord and brain are the most dangerous. You are probably less likely to eat prions if you eat cuts of meat from other parts of the animal (e.g., steaks, roasts). Keep in mind that prions are not destroyed by cooking meat.

Canada is taking the following steps to prevent the spread of BSE in cattle, which will decrease the risk of variant CJD in humans:

• Health Canada is working closely with the Canadian Food Inspection Agency (CFIA) as part of the agency's National Response Team.
• Health Canada is assessing the implications of BSE and any emerging information from trace-back studies.
• The Canadian Food Inspection Agency (CFIA) has taken various precautions to prevent the introduction and spread of BSE, including creating a surveillance program in which the brains of cattle are tested for the disease.
• Since 1997, Canada has banned the feeding of specific tissues that are known to accumulate BSE-causing prions (specified risk material; SRM) from ruminant animals (cattle, sheep, goats, bison, elk, or deer) to other ruminants.
• In 2009, Canada banned SRM from all animal feeds, pet foods and fertilizers. By 2017, it is expected that BSE should be completely eliminated from Canadian cattle.
• The CFIA has mandated that all suspected BSE cases be reported to a federal veterinarian. The CFIA has also created a Canadian Cattle Identification Program for cattle and bison, making it possible to trace individual animal movements from the herd of origin to the time of slaughter.

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