Runny Egg Logic

Remember when Salmonella was a tomato thing? A Jalapeño problem? A peanut panic? (A Peter Pandemic?) Well, this month it's eggs under the infectious-disease spotlight, and you'd almost think the Humane Society of the United States has completely forgotten about all the vegetarian foods that can carry the bug.

By now we’re all aware of the ongoing recall of about 330 million eggs following a spike in cases of salmonella poisoning. And since every bit of news about animal protein has a corresponding HSUS press release, the animal rights group has already issued a statement from its favorite “Dr. Doom” spokesman.

Michael Greger, HSUS’s vegan public health expert, is calling for egg farmers to go “cage-free” in order to reduce the salmonella risk. Here’s the central claim in HSUS’s press release (emphasis added):

Every scientific study published in recent years shows that confining hens in cages results in significantly increased Salmonella risk, including a 2010 study that found 7 times greater odds of Salmonella Enteriditis contamination in operations caging hens.

“Every scientific study” is a pretty definitive statement. Not “most”studies, or “many,” but “every” study.

Is it true? Heck no. HSUS is ignoring lots of science. Maybe on purpose.

For starters, the 2010 study from the Veterinary Record journal—the only study that HSUS actually cites—doesn’t agree with the group’s unqualified conclusion. The study’s authors write that while there might be a (truly) tiny bit less bacteria present in “cage free” flocks, that's mainly because the flocks are smaller. And there’s actually no way to tell whether that translates into any health risk to people.

More on that study in a moment. But first, let’s look at some other recent research that blows a hole in that "every scientific study" nonsense, and argues against blaming chicken cages for making people sick.

  1. A 2009 Swedish study found that bacteria levels among hens kept in "floor housing systems" (i.e., "cage-free") were more than three times greater than those found where hens were kept in cages. This is because "bacteria concentration increase[s] with increasing dust concentration," and cage-free hens are exposed to a lot more dust.
  2. A 2008 study published in Japan’s Journal of Poultry Science compared traditional caged hens with those in larger “enriched” cages and cage-free “aviary” systems. It concluded: “No significant differences could be found in prevalence of Salmonella between laying hens reared in conventional and enriched cages and aviary.”
  3. A 2008 report published in World’s Poultry Science Journal declared: “Contamination of eggshells with aerobic bacteria is generally higher for nest eggs from non-cage systems compared to nest eggs from furnished cages or eggs from conventional cages.”
  4. A 2005 study published in the U.S. journal Poultry Science compared salmonella levels in “aviary” (cage-free) and “battery cage” housing systems with various methods of handling manure. It found: “The system with the lowest chance of infection was the cage system with wet manure.”
  5. A 2004 study conducted by the British government found “no statistically significant difference … between the prevalence of Salmonella contamination in samples from different egg production types.” This study sampled nearly 5,000 dozen eggs from England, Scotland, Wales, and Northern Ireland.

And there are more where these came from. But let’s return to this year’s Veterinary Record study, the one HSUS cites in its press release.

The authors use something called Population Attributable Fractions (PAFs) to measure how much human disease we can trace back to a specific risk factor on the farm. (Risk factors might include how many hens are in a flock, whether or not the animals’ feed is grown on the same farm, and how many field mice are milling about.) Here’s what they say about those PAFs (emphasis added):

PAFs assume a causal link between risk factors and disease. While causality is assumed between these factors and Salmonella in order to interpret the PAFs, it is impossible to verify a causal role through cross-sectional studies such as the present study, and caution is needed when interpreting the PAFs.

In other words, there’s no direct connection between hen cages and sick people.

Here’s how the authors conclude the study. Note how cages aren’t mentioned at all (emphasis added):

Combinations such as improved cleaning and disinfection, vaccination, the use of foot dips and brushes, and improved rodent control could have significant effects on reducing Salmonella levels on farms in Great Britain, and the findings of the present study support previous work emphasising the importance of maintaining good farm biosecurity, hygiene practices and pest control in reducing levels of Salmonella on layer farms.

Earlier in this same study, the authors calculate that if farmers simply conducted better rodent control, vaccinated their flocks, and harvested a whole flock before growing a new one, we might see “a 97 percent reduction” of salmonella in egg-laying hens.

None of this seems to matter to HSUS. The group recently published a “white paper” with its typical, apocalyptic claims about caged chickens. The group even has a scary infographic. And the visuals are based on a single study from Europe, conducted between 2004 and 2005.

And guess what? Yet again, the study itself doesn’t support the spin that HSUS is giving it.

Here’s what the European Food Safety Authority writes (emphasis added):

Cage production was found to be associated with a higher risk of positivity than for the other investigated laying hens production types. However, compared to the other production types, cage production was characterised by larger flock sizes. Organic flocks were on average of the smallest size, whereas the barn and the free-range standard flocks were of low to medium size. Consequently cage production as well as a larger flock size were associated with a higher risk of positivity. But it was not possible to determine which of these two factors was a true risk factor for positivity.

Translation: The more chickens you have, no matter where you keep them, the more likely it is that they’ll pass bacteria around. (Heck, the same is true for pre-school children.)

Why couldn’t HSUS and Michael Greger just have written that? At least it would have the virtue of being true. And while they're at it, some perspective is in order—or at least two common-sense reminders. (You may remember these from our discussion of the U.S. Constitution's "Commerce Clause" as it applies to California's "Proposition 2" egg law.)

  1. USDA scientists have found that only about 1 in every 20,000 eggs might be contaminated with Salmonella. At current consumption rates, the average American might encounter one of those eggs every 84 years.
  2. Even if an egg is contaminated with Salmonella, the only way it can translate into a human illness is if the egg isn’t refrigerated or cooked properly.

Feel scrambled yet? Us too.