Scientists have found that different bovine tissues contain different amounts of the BSE agent. It is generally believed that the highest amounts of infectivity are found in the brain and spinal cords from animals in the final stages of clinical disease.
Some tissues such as skeletal muscle and milk have never been shown to have any infectivity. However, the slaughtering and butchering methods used to obtain tissues and prepare materials can affect the amount of infectivity that may be present.
Also the production processes used to prepare bovine-derived materials (such as heat sterilization and chemical treatment) may reduce or remove infectivity.