Originally published in The Ottawa Citizen January 23, 2005
Mad cow disease (bovine spongiform encephalopathy, or BSE) remains in the news, with two more cases reported in cattle this month in Alberta.
What is the incidence of this disease in Canada? How does it spread and what is your risk of contracting the disease if you eat beef? What is its relationship to Creutzfeldt-Jakob disease (CJD)?
Should consumers stop eating beef?
CJD is a fatal infectious disease that literally creates holes or gaps in brain tissue. The causative agent is the prion (proteinaceous infectious agent). Prions are normally found in small quantities on the surface of many cells, including nerve cells. A distorted form of this protein is responsible for CJD.
There are two forms of CJD: classical and variant.
Classical CJD is not linked to eating beef. It is a human disease occurring in older individuals (mean age of 65 years) whose incidence is about one per million people per year. People die about five months after disease onset.
Variant CJD (vCJD) appears in younger individuals. Indeed, a study out of the United Kingdom reported a mean age of onset of 29 years. People succumb about 14 months after diagnosis. It frequently causes psychiatric disturbances, cognitive impairment, involuntary body movements, immobility, unresponsiveness and mutism, among other nervous system damage.
In the United Kingdom, several factors contributed to the development of vCJD in humans: The cattle that people ate were exposed to the infection from the offal or feed that contained bovine brain and nervous-system ruminants, and slaughter practices contaminated the meat with the BSE prion. Fifty thousand infected cattle entered the human food chain. Despite this number of cattle, human infection rates were low.
There have been 155 reported cases of vCJD worldwide as of January 2004, including 145 from the U.K., six from France, and one each from Ireland, Italy, Canada and the United States.
Why, despite the level of contamination in some countries, has the number of worldwide cases remained small? Some of the proposed possible explanations include:
- A low level of the prion in milk and meat.
- The prion has difficulty causing infection when ingested orally.
- The cattle prion is not efficiently designed to spread to other species; there is a species barrier.
- There is a low incidence of certain genetic factors in humans that can increase the susceptibility to vCJD.
The single case of vCJD in Canada was related to this person spending time living in the U.K. There has never been a documented case of vCJD from a Canadian source of beef.
The latest reports from Health Canada and the Canadian Food Inspection Agency (CFIA) is that there are three confirmed cases of BSE in cattle in Canada. So, how does this affect the chance of contracting vCJD?
Health risk assessment is a mix of fact and perception. Indeed, Dr. William Leiss, scientist at the University of Ottawa McLaughlin Centre for Population Health Risk Assessment, has produced a thoughtful essay on the assessment of BSE risk in Canada.
Dr. Leiss states that, to accurately calculate risk, one must include not only the probability of infection but the consequences of the infection to society.
In other words, the first report of one cow with BSE in Canada in 2003 was enough for the U.S. to close the border to our live cattle and increase the public’s concern about infection (even though the documented individual health risk is negligible).
As Dr. Leiss writes, “BSE risk, however, is a classic case of the type known as ‘low-probability, high-consequence’ risk — others of this type are, for example, nuclear power plant accidents, severe earthquakes in populated areas, and terrorism events. In all cases of this type, the final risk estimation is driven overwhelmingly by consequences rather than frequency or likelihood (probability).”
Indeed, there have been health problems due to the impact of BSE: Farmers and people whose livelihoods depend on the beef industry have suffered from depression and other stress-related disorders. This in turn can introduce family stresses, damage relationships, and worsen existing health disorders.
There have been various classifications of risk. Sir Kenneth Calman, the chief medical officer of health in Great Britain, classified events into these categories:
- Moderate: Less than one in 100 but greater than one in 1,000 (smoking 10 cigarettes a day, parachuting, for example).
- Low: Less than one in 1,000 but greater than one in 10,000 (influenza, highway accidents).
- Very Low: Less than one in 10,000 but greater than one in 100,000 (leukemia, playing soccer, accident at work, murder).
- Minimal: Less than one in 100,000 but greater than one in 1,000,000 (railway accidents, horseback riding, fishing).
- Negligible: Less than one in 1,000,000 (hit by lightning or radiation leak from nuclear power station).
From an individual BSE infection-rate perspective, our risk is negligible. But from a societal health and economic perspective, the consequences of the disease have been, in Dr. Leiss’ calculation, “catastrophic.”
In short, it’s all about how you look at it.
Dr. Leiss’s report – http://www.leiss.ca/bse/155
Health Canada – http://www.hc-sc.gc.ca/english/diseases/bse/
© Dr. Barry Dworkin 2005