I wouldn't worry about contracting mad cow disease from the gelatin used in the production of dietary supplements. The United States has stringent regulations in place designed to minimize any threat to humans from cattle with Bovine Spongiform Encephalopathy (BSE), the disease that affects the central nervous system of adult cattle. The U.S. Food and Drug Administration (FDA) prohibits the use of cattle materials with the highest risk of harboring BSE in human food, including dietary supplements. This rule applies to imported supplements as well as those produced in the United States.
According to the FDA, most ingredients used in the production of dietary supplements (and other food ingredients) come from cattle that are slaughtered when they are less than 30 months old, an age when they are considered at very low risk of harboring BSE. U.S. Department of Agriculture (USDA) rules governing cattle and cattle tissues used in human food also provide a safeguard against potentially infected tissue ending up in dietary supplements.
So far, only two cows in the United States have been diagnosed with BSE. The first was a dairy cow on a farm in the state of Washington. The cow was from a Canadian farm, but was found to be infected after it was slaughtered and after its meat already had entered the food supply. The meat was quickly traced and removed from the market. All of the cow's organs - in which the infectious material is found - were removed at slaughter and didn't enter the food supply. More recently, another cow, in Texas, was diagnosed with BSE but never entered the food chain. None of the other cows from the same ranch that were tested were infected.
Furthermore, the causative agent of BSE exists in nerve tissue of infected animals. Gelatin is purified collagen, a component of connective tissue that is not a problem. Still, if you would prefer to avoid gelatin from cattle, you can get dietary supplements that come in capsules made from plant materials called vegicaps. They contain no animal byproducts, preservatives, starch, wheat, or dairy products and are therefore an appropriate alternative to gelatin capsules for vegetarians and those with dietary restrictions.
Andrew Weil, M.D.