February 11, 2011
American Egg Board-Funded Review Scrambles the Science
The available science clearly shows cage housing systems are more likely to harbor Salmonella. A newly published review funded by the egg was only able to arrive at their “inconclusive” finding by ignoring nearly 90% of the data published over the last five years (at least 5198 of the 5907 flocks studied). The fatal flaw of the American Egg Board-funded review is that it fails to include the single most comprehensive study on the subject: the European Food Safety Authority analysis of more than 5,000 farms.”
This year, all 27 countries of the European Union are set to eliminate barren cage confinement in 100% of egg production. To understand the public health implications of this shift, an ambitious study was launched to compare the Salmonella prevalence in cage versus cage-free, large-scale, commercial egg production. Samples were taken from 5,310 operations across 25 countries. This massive study represents by far the most comprehensive investigation into the impact of different housing systems on the overriding issue of egg safety, yet the full results are conspicuously absent from the industry-funded review.
The “funding effect” is a well-known phenomenon in the public health field. It describes the uncanny correlation between the conclusion desired by a funding source, and the conclusion reached by the researchers being funded. To resist regulation, industries fund scientific reviews to downplay the risks of their products. Tobacco is the classic case, but producers have funded studies downplaying the risks of asbestos, beryllium, chromium, lead and a host of other hazardous products and processes.
The stated mission of the American Egg Board is "to increase demand for egg and egg products on behalf of U.S. egg producers." So when it funds a review of the food safety implications of the prevailing method of egg production—caged confinement of laying hens—you can bet it intends to get its money’s worth.
What did the European Food Safety Authority find? After analyzing more than 30,000 samples, EFSA concluded that the odds of finding Salmonella in a cage-free (barn-raised) flock were significantly lower than those of a caged flock—43% lower to be exact. Free-range flocks did even better, with just 2% the odds of Salmonella contamination compared to caged flocks, a highly significant finding.
That was for Salmonella Enteriditis, the type of Salmonella that triggered the half-billion egg recall this year. Rates of Salmonella Typhimurium were also measured, a less common, but deadlier strain. Compared to operations caging hens, there were 77% lower odds of finding Salmonella in cage-free systems and 93% lower odds in free-range systems. And for all other tested Salmonella strains combined, there were 96% lower odds of finding any of them in cage-free flocks and 99% lower odds in free-ranging birds.
All of these findings were statistically significant and completely consistent with one another. No matter what Salmonella strain they looked at, they always found significantly lower risk in cage-free environments. The conclusion reached by the European Food Safety Authority, the official governmental scientific body, was, “Cage flock holdings are more likely to be contaminated with Salmonella.” No wonder it’s missing from the Egg Board-funded review.
Even more damning, the authors can’t plead ignorance. It would be bad enough if the authors were simply unaware of the best available science on the central topic of their review, but they actually mention the EFSA analysis in their review. They mention how good EFSA’s sampling methods are, but they fail to mention the results of the 30,000+ samples that were taken. So this would appear proof-positive that they deliberately omitted the most authoritative body of evidence on the topic.
They also cite EFSA’s conclusion on vaccination from the report. The vaccination data is in the exact same tables as the housing data. The conclusion on vaccination is on the exact same page as the conclusion on cages—in fact the very next bullet point down. That isn’t cherry picking; that’s ignoring an entire orchard. What conclusion can be reached other than this industry-funded review represents a calculated distortion of the scientific record?
Also notably missing from the American Egg Board-funded review is a recent study of Salmonella infection in hens from hundreds of caged and cage-free flocks across five countries. Even after controlling for a number of factors, including the age of the buildings, hens confined in cages were found to have 20 times the odds of actively shedding Salmonella compared to cage-free flocks. The researchers conclude that "The housing of laying hens in conventional battery cages turned out to be a significant risk factor for Salmonella Enteritidis and/or Typhimurium." Is that why this large and highly significant study was omitted from the review? It’s not as if it was published too late for inclusion—they cited work published more than a month later.* Again, this casts doubt on the objectivity of their review.
Likewise, why was there was no mention of the published data noting faster clearance of Salmonella infection in experimentally infected cage-free hens compared to conventionally caged birds? Or the published evidence showing that exposure to bedding (as in many cage-free settings) may result in the acquisition of natural gut flora that could help block Salmonella colonization of the gut? These studies bear directly on the question at hand, yet are also inexplicably missing from the review.
Although nothing comes close to the scientific rigor of the 5,000+ farm EFSA analysis, the large majority of other studies comparing Salmonella rates in cage versus cage-free systems similarly found significantly more risk in cage operations. In fact, not a single one of the 16 published studies published in the last 5 years comparing Salmonella rates in conventional cage to cage-free systems found higher rates in cage-free environments.,,,,,,,,,,, , , ,,
Even an industry trade publication concedes that "the majority of the studies clearly indicate that a cage housing system has an increased risk of being Salmonella-positive in comparison to non-cage housing systems" (from "Salmonella Thrives in Cage Housing," published in World Poultry).
The Egg Board-funded review cites four older sources purported to show greater risk in cage-free flocks. The first is a 15-year-old investigation into a single farm that actually found a lower infection rate in egg samples from cage-free (barn-raised) hens compared to eggs from caged hens (0.24% positive for Salmonella vs. 0.49%. respectively). How can they cite this to support the caging of hens? Higher risk was found for the free-range hens on the farm, but that was determined by the principle investigator to be due to exceptional circumstances, in that a creek "entirely composed of sewage effluent" bordered the property.
The second source they cite is a 16-year-old survey of just 17 flocks of cage-free hens in Germany whose findings, through bacterial culture and serology, didn’t even reach statistical significance (P = 0.73 and 0.49, respectively).** The third source they cite doesn’t address cage-free production at all, and the fourth source actually shows the opposite of what they assert when the model is applied to typical, U.S. cage systems. Using standard flock sizes,, caged flocks would have 1.9 to 6.7 times the chance of being Salmonella-positive compared to cage-free flocks. In the end, not a single one of the studies they cite actually shows significantly less risk in conventional cage systems compared to cage-free.
So having arrived at a combination of ignoring the best available science and citing a few small, old, ambiguous, and/or irrelevant studies, the reviewers are then able to set up the bogus comparison necessary to reach their funder-friendly conclusion that it is “unclear” whether different production systems impact Salmonella rates. Like climate change skeptics who concentrate on a few outliers and ignore the overwhelming body of evidence, only by pretending the EFSA data doesn’t exist can a controversy over the safety of cage confinement be fabricated.
Furthermore, their primary critique of the small fraction of the evidence showing greater risk with caging that they do manage to cite is fundamentally flawed. The reviewers concede that larger flock size increases Salmonella risk but mischaracterize it as a confounding factor, when it is in fact part and parcel of commercial cage production. It would be like arguing the tar in tobacco smoke confounds the link between smoking and cancer, contending that smoking is not to blame for increasing cancer risk, but rather it’s the inhalation of tar into the lungs. Just as tar is an intrinsic part of cigarette smoke, large flock sizes go hand-in-hand with cage confinement. Vertically stacking tiers of cages that allot only 67 square inches of cage space per hen—less space than a sheet of paper—on which to live her entire life enables industrial producers to cram literally hundreds of thousands of hens into a single shed. That is in fact one of the explicit reasons given by the industry for using cages: “more birds can be placed in a given house.” A large cage-free hen house may have as many as 28,500 birds; a large cage operation may confine more than 400,000 hens under a single roof. Flock size is a causal factor, not a confounding factor.
The reviewers also dismiss other aspects inherent to cage production under the banner of "confounding factors." Confining 10 or 20 times more birds inevitably means 10 or 20 times more feces, a known risk factor for Salmonella contamination. Cage operations are considered more attractive to Salmonella-carrying rodents because they can roam freely without interference from the birds who, by definition, are confined in cages. Rows of cages and associated complicated structures (feeder, drinker, and egg belt systems) may be stacked 12 tiers high, making cage operations "intrinsically difficult to clean and disinfect to a good standard." There are all factors inherent to commercial cage operations. They help to explain, not confound, the findings of greater Salmonella contamination in cage systems.
Salmonella can be an issue of life or death. In general, Salmonella has been found to be the leading cause of food-poisoning related fatalities in the United States. Before the half-billion egg recall, the Food and Drug Administration estimated that 142,000 Americans are sickened by Salmonella-tainted eggs every year. Even under the new egg safety rules, the FDA estimates tens of thousands of Americans will continue to be stricken by egg-borne Salmonella. Given that Salmonella can survive common cooking methods such as scrambling, over-easy, and sunny-side-up, it is the industry’s responsibility to decrease the risk at the source. The weight of evidence clearly shows cage systems have higher rates of Salmonella contamination. Instead of funding misleading reviews to confuse the issue, the U.S. egg industry should take food safety and animal welfare seriously by phasing out cage confinement.
* Miscited, actually. The Snow et al. study they cite is from Vet. Rec. 166(2010):579-586, not 163(2009):649-654.
** Fisher’s exact test
- Our letter published the the March 2011 issue of Poultry Science
- Unabridged letter submitted to the Editor-in-Chief of Poultry Science criticizing the fatally-flawed review [pdf]
- An HSUS Report: Food Safety and Cage Egg Production [pdf]