Possible Causes of BSE
Science has not found a definitive cause for bovine spongiform encephalopathy (BSE, mad cow disease), nor is the means of transmission absolutely known.
The leading theory is that the infectious agent is not a bacteria, virus, or other microorganism. Rather, many scientists believe that proteins in the animal's brain called prions that somehow become abnormal. They are changed so that they resist protease, an enzyme that would normally break down proteins, and are camouflaged so that the animal's system does not recognize them as foreign or abnormal.
Some scientist believe that BSE is caused by a slow-acting virus with unusual characteristics, while others suggest that the infectious agent is a virino - a bit of abnormal genetic material that somehow gets the animal's system to make a protective coating to prevent it from being destroyed.
Whatever the infectious agent is, it does not provoke any immune response or inflammatory reaction in the infected animal; the animal's system does not fight the disease as it does most other diseases.
BSE was not found until 1986. It had been common practice to include parts from slaughtered animals in feed for other animals to boost protein content. Scientists speculate that parts from sheep infected with scrapie - a related disease that afflicts sheep and goats - were used in cattle feed. The theory suggest that parts from those cattle went into cattle feed as well after they were slaughtered, compounding the problem. Because BSE has a long incubation period, the contaminated feed was widely distributed and consumed by the time cattle began dying and scientists were able to make the connection.
Scientists theorize that BSE appeared when it did because of changes in the rendering industry. Rendering is the process of removing and using parts from slaughtered animals that are not going to be used as food for humans. Until the late 1970s, the process used solvents and extremely high temperatures. To protect workers in rendering plants, those methods were changed, which may have allowed the BSE agent to survive rendering.
In cattle that died from BSE, the agent has been found only in brain tissue, spinal cord, and retina. When scientists experimentally infected cattle, they found the agent in the intestine, bone marrow, and ganglia (nerves). It has not been found in the muscles, which is the part of the animal that goes into unprocessed meats like steaks and roasts. BSE has not been found in milk, either. Ground meats and some cuts of muscle meats with nerves running through them could be suspect, however.
Normal sterilization methods do not kill the agent. Nor does any method exist to detect it in live animals. Currently the only way to diagnose BSE definitely is to examine its brain tissue under a microscope after death.
BSE apparently spreads only when one animal (or possibly a person) eats contaminated tissue from an infected animal. It does not spread by touch, air, manure, urine, semen or milk.