FACTBOX – US Department of Agriculture uses Three Mad Cow Tests on Suspect Animal
Date: 14-Jun-05
Country: USA
The USDA has used at least three different tests on the current suspect animal with varying results.
Due to the uncertainty, the department has decided to send brain samples to a reference laboratory in Weybridge, England, for confirmatory testing. USDA will also conduct further testing, which will take several days to complete.
The only US-confirmed case of mad cow disease, or bovine spongiform encephalopathy, was found in December 2003 in Washington state dairy cow.
RAPID TESTS
The USDA reported on Nov. 18 that the suspect animal tested "inconclusive" in two rapid screening tests.
The quick tests, referred to as ELISA, take only a few hours to identify whether abnormal prion proteins are present in an animal's brain tissue sample. Misfolded prions are believed to be the cause of BSE, which slowly disintegrates neural tissue and is always fatal.
To test an animal carcass using the rapid test, scientists snip off a small amount of the brain stem for testing.
A reagent mixture consisting of digestion enzymes is used to isolate the BSE-specific prion protein in the sample. Antibodies are then added to the sample to detect the prions.
A chemical is then added to the sample that enables it to emit light, and the light is measured by a computer. In general, a negative sample is white and a positive is yellow.
IHC TEST
The USDA reported on Nov. 23 that the suspect animal tested negative in two IHC tests.
The immunohistochemistry test, or IHC, involves a staining technique to determine whether the brain sample has the BSE agent.
After placing antibodies on a suspect brain sample, the test causes a chemical reaction that can detect the abnormal form of the prion protein found in BSE.
The test, considered the "gold standard" by the USDA, requires two to three days to complete.
WESTERN BLOT TEST
The USDA's Inspector General earlier this week recommended that the department retest the suspect animal using the Western blot test.
The USDA reported on Thursday that the sample returned a "weak positive" for mad cow disease and would need additional testing.
The test, arguably more sophisticated than the IHC test, homogenizes a suspect brain sample and treats it with a protease enzyme that destroys normal prion proteins, but not the abnormal protein.
The sample is then run through a gel-type separation using specific antibodies that will give you bands. The USDA said the molecular weight of those bands are used to determine the outcome of the test.
These tests typically take one to two days to complete.