Thursday, February 8, 2007

Mad about BSE

Greetings, fellow nerds.

This will be just a quick post about the relative risks between the "mad" and the "cow" in mad cow disease. In truth, the danger "tainted beef" poses to us is so small that as a consumer, you should forget about mad cow disease entirely: it's a waste of neurons. I'd even feel guilty making you read this post if it weren't for the fact that it exposes the sensationalism behind mad cow disease reporting.

Assessing Mad Cow Lethality

Mad cow disease in cows is called bovine spongiform encephalopathy (BSE), and to date there have been at least 188515 reported cases of it worldwide. Common sense suggests that there are a whole lot of unreported cases too, considering the incentives farmers have to keep things quiet. The disease humans contract by eating infected cows is called variant Creutzfeldt-Jakob disease (vCJD), and there have been a total of 170 confirmed cases worldwide. This number is probably not exaggerated (in fact the false positive rate for vCJD might be rather high), so you might think of 170/188515 (or about 900 per million) as an upper bound on the chances of contracting vCJD from eating an entire mad cow. However, probably not all of the 188515 cows were consumed in their entirety, which means that 900 per million might be a good ballpark figure as to the actual transmission rate from eating a whole cow.

If there are roughly 250 lbs of meat per cow, and if a typical beef meal has a quarter pound of meat, the odds of catching vCJD from a single meal of infected beef are 900 in a billion. When you consider that in the time period the 188515 cows were detected, the UK (the country hardest-hit by BSE) produced about 100 million cows, the risk of getting vCJD from eating one randomly-chosen UK beef meal is about 2 in a billion.

No Consensus among Academics

Why aren't I just reporting what the medical journals say about BSE to vCJD transmission rates? Because these estimates are all over the place. This study in the journal "Risk Analysis" by Eric Grist shows that there's nothing like consensus regarding the risk of a human contracting vCJD from eating an infected meal: published transmission rates vary from 0.9 in a billion to 7 in a thousand (c.f. my estimate of 900 per billion based on case numbers). The "Risk Analysis" article continues on to point out that the 7 in a thousand measure is obviously inconsistent with observed vCJD infection rates. (Thank you, Dr. Grist!) Perhaps even researchers are prone to sensationalism, especially if the said sensationalism might result in extra funding.

New Risk Metrics

We need better units for analyzing risks. It's hard to grok these raw numbers without an everyday context, so I'm going to introduce two new metrics:
  1. Life Expectancy Decrease (LED)
  2. Equivalent Driving Distance (EDD)
The LED measures the expectation of the decrease in life expectancy based on taking the risk once, assuming an 80 year lifespan. So for example, the LED from eating a meal with UK beef at the height of the mad cow scare is (80 years * 2 per billion) = 5 seconds. Compare that to the LED associated with the extra calories in your fast food meal: (80 years * 94 per million) = 2.7 days. The extra risk of vCJD is insignificant next to the threat of the extra calories involved with taking a big bite of beef.

Driving cars is a risky activity that most of us have come to terms with. Driving therefore provides an ideal reality check that helps us put new risks in perspective. This DOT report (page 8 of the .pdf) shows that there are about 14.6 vehicular fatalities for every billion miles traveled. The EDD of a new risk is the length of the car trip with the equivalent level of risk. For example, the EDD of the untested UK beef meal is (2 per billion * 1 billion miles / 14.6) = 720 feet; less than one Manhattan city block. Again, if the beef was in a fast food meal, the EDD from the extra calories is over 6400 miles: greater than the radius of the Earth.

Conclusion: Calories Matter More than Prions

If you spent more than 5 seconds worrying about contracting vCJD from beef, even at the height of the mad cow scare in the UK, you've been had. Also, if you think a fast food meal saves time, you should consider the 2.7-day lifespan decrease it brings (unless you're underweight - see my blog entry). It's true that the risk was unknown at the time (so maybe caution was indicated), but I'm getting sick of people worrying about false alarms, especially when there are more pressing issues which aren't getting enough attention.

I'm also disillusioned with the media, although I can see their point. Nobody will buy a paper saying "Get Off Your Butt and Get Healthy," but "New Plague Risk Sweeps the Nation: Will YOU Die?" has more zing.

As new crises come up in the news, I'm going to try to keep up with the LED and EDD metrics for them. With any luck, they'll catch on as popular reality checks for how risky a new terror actually is.


noopur said...

excellent analysis, p! loved reading it.

Jason Michel Lefebvre said...

Wow. You are so right on. Ever since 9/11 it hit me how ridiculous our fear as a nation is. We watch the news telling us to fear Osama Bin-Ladin while we sit and eat McDonalds. I was the guy who posted on the Wikia site - "anonymous 8" I think. Unfortunately, I still don't have any cures to the absurdity, save for not buying into it myself.

Knaldskalle said...

Being European, I'm excluded from donating blood in the U.S. because I've lived in Europe for more than 12 years. Why? Fear of BSE transmission... What are the odds of me contracting vCJD and then passing it on to some innocent recipient of blood, when the odds of me contracting vCJD in the first place are less than 1 in a billion (especially being a non-Brit.)???

LeDopore said...

Hi Knaldskalle,

I know, it's pretty crazy. I wonder if eating lots of animal parts correlates with other lifestyle factors which make blood donation actually *more* risky. It wouldn't have to be a big effect to overshadow the BSE risk.