Increased Mortality in Groups of Cattle Administered the

The United States Food and Drug Administration (FDA) approved two b-adrenergic agonists (bAA) for in-feed administration to cattle fed in confinement for human consumption. Anecdotal reports have generated concern that administration of bAA might be associated with an increased incidence of cattle deaths. Our objectives, therefore, were to a) quantify the association between bAA administration and mortality in feedlot cattle, and b) explore those variables that may confound or modify this association. Three datasets were acquired for analysis: one included information from randomized and controlled clinical trials of the bAA ractopamine hydrochloride, while the other two were observational data on zilpaterol hydrochloride administration to large numbers of cattle housed, fed, and cared for using routine commercial production practices in the U.S. Various population and time at-risk models were developed to explore potential bAA relationships with mortality, as well as the extent of confounding and effect modification. Measures of effect were relatively consistent across datasets and models in that the cumulative risk and incidence rate of death was 75 to 90 % greater in animals administered the bAA compared to contemporaneous controls. During the exposure period, 40 to 50 % of deaths among groups administered the bAA were attributed to administration of the drug. None of the available covariates meaningfully confounded the relationship between bAA and increased mortality. Only month of slaughter, presumably a proxy for climate, consistently modified the effect in that the biological association was generally greatest during th