Did you think you left the tyranny of fractions behind in school? In truth, numerators and denominators still drive our view of the world, as well as the legislation and regulations that are changing our way of life today.
Nowhere are the two basic components of a fraction more important than in evaluating foodborne illness. To understand foodborne illness, we must evaluate the number of cases in relation to the exposed population over a period of time. The illnesses (number of people) are the numerator, and the exposure (number of meals consumed) becomes the denominator over the specified time period.
In March 2013, the Centers for Disease Control and Prevention (CDC) published a summary of foodborne disease outbreaks from 1998-2008. During this period, 13,352 foodborne disease outbreaks were reported in the U.S., causing 271,974 illnesses. Of these outbreaks, 37% (4,887) had an implicated food vehicle and a single causative agent.
In this paper, the CDC estimates the total burden of foodborne illness due to each commodity based on reported outbreaks and adjusted for factors such as under-reporting. The low end of the initial estimate range was established by outbreaks where the source could be attributed to a specific commodity. The high end consisted of complex outbreaks where multiple commodities could have been involved; therefore, the outbreak was attributed to each of the commodities.
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From 1998-2008, the CDC estimates there were 639,640 foodborne illnesses/year due to beef, including bacterial, chemical, and viral contamination. We’ll use this as our numerator. For a denominator, we’ll take the low value of reported U.S beef consumption from 2002 through 2008, which is 27 billion lbs. consumed. These data are available on the USDA Economic Research Service website.
Now we need to adjust for the amount of beef consumed per meal. It could be anything from 1-lb. steaks to the amounts contained in a beef hotdog or deli meat servings. If we pick .025 lb. as an average beef serving, that calculates to a total of 108 billion exposures/year. Our estimate of the number of beef-associated illnesses expressed as a percentage of beef “exposures” is the product of 639,640 divided by 108 billion, which is 0.0000059. This can also be expressed as 0.0006 %.
Since a percent is one in 100, this equates to a foodborne illness estimate for beef of 1 foodborne illness for every 166,667 beef meals. To look at it another way, during an average 80-year life expectancy, averaging 1 beef meal/day, that is 29,200 beef meals in a lifetime. Thus, our final estimate becomes one foodborne illness in six lifetimes.
Back-of-the-napkin approach
This is a back-of-the-napkin approach to calculations, and everyone reading this can substitute their own estimates. It’s interesting to note that 51% of the CDC-estimated outbreaks are related to plant commodities (4,924,877/year); in contrast, the beef-related cases were 6.6%.
Let me be clear: our goal is zero foodborne illness. My intent isn’t to trivialize human disease; I fully recognize the gravity of the hospitalizations and death resulting from foodborne illnesses. However, to discuss such cases without reference to consumption is irresponsible.
Of course, any effort by interests to drum up funding support is more effective if they cite 639,640 cases (the numerator) of foodborne illness, without considering the denominator and referencing that only 0.0006% of beef meals resulted in a foodborne disease. It’s also important to note that not all these cases are due to pathogens that came into the final food product source on the meat.
To read more health posts by Mike Apley, DVM, click here
Driving as much news exposure as possible on a single outbreak is the same strategy as publicizing lottery winners to give the impression that winning is common. In both cases, the goal is to give the impression that the event is quite common, when in fact it is not.
That’s why groups with goals of damaging animal agriculture only use the top half of a fraction or focus so much on single outbreaks. We must help our consumers see the whole picture.
Mike Apley, DVM, PhD, is a professor in clinical sciences at Kansas State University in Manhattan.
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