202908 Animal feeding operations and infant mortality: No evidence of causality

Tuesday, November 10, 2009

Jennifer L. Greiner, DVM , National Pork Producers Council, Washington, DC
Kerry R. Leedom Larson, DVM, MPH , National Pork Board, Des Moines, IA
Louis A. Cox Jr., PhD , Cox Associates, Inc., Denver, CO
Sneeringer (1) raises the important public health issue of whether air pollution emissions from livestock operations increase infant mortality rate (IMR). She compares changes in IMR against changes in aggregate “animal units” (a weighted sum of cattle, pig, and poultry numbers) over time, for counties throughout the United States, while attempting to control for other relevant variables in a reduced-form, fixed-effect regression model. She concludes that “a doubling of production leads to a 7.4% increase in infant mortality.” We note that causal interpretation of reduced-form model coefficients is not justified. Reexamining the livestock production-IMR association using specific components of “animal units” (cattle, pigs, and broilers) at the state level, we find no statistically significant positive relations between livestock variables and IMR variables, after controlling for poverty and percent Black (not included in Sneeringer's models) and other variables. To the contrary, we observe statistically significant negative associations between changes in livestock production (especially, cattle production) and changes in IMR. We conclude that statistical associations between livestock variables and IMR variables are very sensitive to modeling choices (e.g., selection of explanatory variables, and use of specific animal types vs. aggregate “animal units) and that such associations do not warrant causal interpretation. We find no evidence of a valid causal relation between livestock production and any increase in infant mortality.

(1) Sneeringer S. Does animal feeding operation pollution hurt public health? A national longitudinal study of health externalities identified by geographic shifts in livestock production. Amer. J. Agr. Econ. 91(1) (February 2009): 124–137

Learning Objectives:
Identify known risk factors for infant mortality rate (IMR), such as race, poverty, and total mortality rate in the population. Examine whether changes in livestock production levels have any relation to IMR after controlling for demographic risk factors for IMR. Distinguish between statistical associations and evidence of causation, and understand how statistical modeling choices (such as which explanatory variables to include in a model) can change apparent statistical associations.

Presenting author's disclosure statement:

Qualified on the content I am responsible for because: I am trained as a veterinarian, and in my role as Science and Technology Director for the National Pork Producers Council I am very familiar with issues regarding livestock and public health.
Any relevant financial relationships? No

I agree to comply with the American Public Health Association Conflict of Interest and Commercial Support Guidelines, and to disclose to the participants any off-label or experimental uses of a commercial product or service discussed in my presentation.