The Facts
Bovine spongiform encephalopathy (BSE), commonly known as "mad cow disease," is a fatal disease that strikes the nervous system of cattle.
BSE is part of a group of diseases called prion diseases that occur in both animals and humans. The main animal forms are chronic wasting disease in deer, scrapie in sheep, and mad cow disease. In humans, there are three different prion diseases: Creutzfeldt-Jacob disease (CJD), also named sporadic CJD (sCJD), Gerstmann-Straussler-Scheinker disease, and fatal familial insomnia. In 1996, another form of CJD was identified; it has since been named "variant CJD" (vCJD). This variant form of CJD has been linked to the consumption of meat products infected with BSE.
Currently, no vaccine or treatment exists to treat BSE, and affected animals display a variety of neurological symptoms before they die (think of television reports showing cows having trouble standing up).
An animal with outward symptoms of BSE may survive for 2 weeks to 6 months, though it may have carried the disease for up to 8 years. BSE has an incubation period (the time between infection and development of symptoms) ranging from ranging from 4 to 6 years, which is a long time for a disease to remain undetected.
Causes
Scientists believe that BSE is most often spread through the practice of feeding cattle various meats (rendered material) from slaughtered animals such as sheep, goats, and other cattle.
During this process, an abnormal protein that is linked to BSE can spread from a slaughtered diseased animal to a healthy one. This abnormal protein, called a prion, can withstand high temperatures and does not get destroyed during the rendering procedure. Since the incubation period for BSE is so long, it is possible for an infected animal to enter the food chain before the symptoms appear.
Proteins are long molecules that are folded up into particular shapes. A prion is folded differently from a normal protein, and it can cause normal proteins to change and fold abnormally. When this happens, the proteins (normally found in liquid form in cells) begin to solidify.
The cells most often infected are the brain cells. The resulting solidification of the proteins causes the infected brain tissues to look like a sponge with several tiny holes, hence the name "spongiform encephalopathy."
Symptoms and Complications
Because BSE damages the brain tissue, it has a variety of symptoms ranging from behavioural changes to coordination problems. Cows with BSE may show nervousness or aggressive behaviour, difficulty with coordination, trouble standing up, decreased milk production, and weight loss. The disease is fatal, with death usually occurring 2 weeks to 6 months after symptoms start.
sCJD usually occurs in older people with an average age of 65, but it has been reported in teenagers and people in their 90s. More than half of people with CJD suffer from memory loss and confusion, which eventually affect all people with CJD. The disease is fatal after only a few months. vCJD usually occurs in younger people and they usually survive for about 18 months.
Making the Diagnosis
Live animals cannot be tested for the disease. The only way to confirm the presence of BSE is by checking the brain tissue of an animal after it dies. Upon examination, the brain is found to be full of small holes, like a sponge.
In humans, a genetic test exists to determine if a person might be susceptible to vCJD. The most useful test is magnetic resonance imagine, which can detect characteristic changes in the brain. Other tests include checking for abnormal electrical activity in the brain through electroencephalography (EEG) or looking for prions from a spinal fluid sample. A sample of brain tissue may be obtained through a biopsy or autopsy to confirm the diagnosis and type of disease.
Treatment and Prevention
There is no cure, treatment, or vaccine for BSE, CJD, or vCJD.
The best way to prevent the disease is to avoid feeding cattle rendered material from slaughtered animals, and to isolate and destroy all infected animals. Most countries have developed policies for monitoring BSE in their cattle herds and procedures for dealing promptly and thoroughly with BSE cases when they do arise.
Canada is continuing to work to prevent and control BSE:
- Health Canada works closely with the Canadian Food Inspection Agency (CFIA) as part of the agency's National Response Team.
- The 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.
- 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.
- In 1997, Canada banned the feeding of rendered protein products from ruminant animals (cattle, sheep, goats, bison, elk, or deer) to other ruminants.
- In 2007, Canada implemented an enhanced BSE-related feed ban. This regulation extends the original feeding ban to all animal feeds, pet food, and fertilizer products.
- In 2021, Canada was recognized as a “negligible” country for BSE by the World Organisation for Animal Health (OIE). Canada’s last case of BSE was found in 2015.
- A permit issued by the CFIA is now required for those needing to handle, transport, or dispose of cattle carcasses and certain cattle tissues.
- More detailed policies regarding the prevention and control of BSE are available on the Health Canada website.
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