The U.S.D.A. should test every cow that goes to slaughter. The cost is not prohibitive. Fear is the problem. The current testing program for mad cow disease is intended to produce, at best, a snapshot of the likelihood of the disease. The program rests on assumptions that reflect, as assumptions tend to do, only what we know already, and we do not know nearly enough about mad cow disease.
The fear is that broad testing may reveal a higher rate of infection and destroy consumer confidence, with a devastating impact on the cattle market. Which leaves us where we are now: relying on what we don't know to make us feel safe.