Recently, due to the increase in cattle diseases like Mad Cow and others, the feeding of cattle byproducts to other cattle was prohibited. However, farmers still can feed cattle porcine byproducts. It is therefore possible that the kosher beef I eat comes from a cow that was fed swine byproducts. Do you see this as a problem?
Fish and chicken eat worms and insects. There is no halachic problem in kosher animals that were fed non-kosher food.