Technically known as bovine spongiform encephalopathy, or BSE, mad cow disease is a chronic degenerative disease affecting the central nervous system of cattle. The concern for humans is the risk of eating meat or meat products that contain the BSE pathogen. Whether bovine-derived ingredients used in cosmetics can harbor the disease and cause health risks is unknown, but theoretically there is a remote possible risk. Some researchers believe that there is no evidence BSE can be contracted through the skin (Source: Cosmetic Dermatology
, December 2001, pages 43–47); however, neither cooking nor preserving nor any of the other processing that most cosmetics go through can eliminate BSE pathogens. That means that if animal by-products are used in cosmetics (in particular bovine placenta and spleen extracts), they can pose a risk, albeit remote, to the user. The British BSE Committee (www.bse.org.uk/), in various reports, has mentioned a concern that people could become infected if the creams are used on broken skin.
It is important to realize that very few products contain those kinds of ingredients. If you are thinking of buying cosmetics that contain animal organ extracts of any kind, you may want to reconsider, or discard them if you have already made a purchase.