Focus on Food

On Wednesday, the USDA said that an elderly cow slaughtered in Texas in the spring did not have mad cow disease based on tests conducted by the agency's laboratory in Ames, Iowa, and the Central Reference Laboratory in Weybridge, England

Two cases of mad cow disease have been confirmed in the United States. In 2003, a cow born in Canada was discovered in Washington State in December 2003. The second was in November in a Texas-born cow.

This latest cow required testing at the world-class UK lab after initial tests proved inconclusive.

Bio-Rad, the California-based molecular biology toolmaker, holds a huge share of the market for BSE tests globally.

The company provides an ELISA test, a process in which brain material is homogenized, with normal prion protein being destroyed by a protease, and any remaining abnormal prion protein bound to the surface of a clear microtiter well and immunologically detected. The signal is detected by a spectrophotometer and results are output in a quantitative format. This testing procedure takes four hours to complete.

The USDA recently said that if Bio-Rad's screen test should indicate a positive, then it will require a follow-up "Western" blot.

In this proceedure, brain material is homogenized, and normal prion protein is destroyed by a protease, and the sample is electrophoretically separated in a gel, whereby remaining abnormal prion protein is immunologically detected. For example, antibodies to the prion protein are bound and then detected by a second antibody linked to an enzyme; and the bound enzyme reacts with a substrate resulting in staining or light emission from the membrane. This process requires one to two days to complete.

All of today's tests are performed post mortem, and the research challenge is to be able to detect the disease in live animals.

Barry Shlachter of Knight Ridder newspapers has written an informative article on BSE in the US, which is worth reading.