INTERVIEW - Mad Cow DNA May Offer Clues to Latest US Case
Date: 20-Jun-05
Country: USA
Author: Sophie Walker
Danny Matthews, head of the research and surveillance program for transmissible spongiform encephalopathy at the Veterinary Laboratories Agency in Weybridge, said British scientists have done similar DNA analyses on other animals.
The internationally recognized laboratory in England will carry out a series of tests on the suspect US animal, which has already undergone several with conflicting results.
While the British lab is conducting its tests, US Agriculture Department scientists will perform DNA sequencing of the prion protein from the animal's brain.
The second possible US case of mad cow disease surfaced on June 10, when it was revealed that an older animal had tested positive for the brain-wasting disease.
The question of whether mad cow disease or bovine spongiform encephalopathy (BSE) can arise spontaneously is an important one. Scientists have long believed BSE is transmitted through contaminated feed, which led to most countries banning the use of cattle remains in cattle feed.
Mad cow disease is caused by abnormal or misfolded prions in the brain.
"We have been doing ... some sequencing on the prion PRP gene, the gene that codes for producing prion protein. The hypothesis is that an abnormality in that gene could generate a spontaneous disease," Matthews said in a telephone interview.
"If anything is being done in the US on that line I suspect that is what they are looking for, to see if there's any mutation ... that might account for what they've seen."
A scientist with USDA's animal disease lab in Ames, Iowa, told Reuters this week the unusual test results could point to a new BSE strain some scientists believe occurs spontaneously.
The aging beef animal was slaughtered in November. Because it was a "downer" unable to walk, its carcass was banned from the human food supply and incinerated. If the new tests confirm infection it would be the second US case of mad cow disease.
WHAT IS 'ATYPICAL?'
Matthews said it was far from proven that atypical BSE cases mean the disease occurred spontaneously.
"Some countries have claimed that they have found something that's different. All they are seeing on the Western blot test is that the banding pattern is slightly different to a normal BSE," he said. "That is insufficient to say this is not BSE."
Many of the atypical samples found so far were in older cattle. But he cautioned that was not conclusive evidence of genetic mutation causing a spontaneous case of the infection.
Since the first confirmed US case of mad cow disease was found in December 2003, scientists in France, Italy and Japan say they have discovered different strains of BSE from the outbreak that swept European herds in the 1980s.
"The Italians published over a year ago and inoculated cattle and mice with their samples but still haven't got any evidence of transmission. France and Japan have inoculated but there are no results published so far," Matthews said.
"It may be BSE, it may be another strain of BSE or it may be something quite different which isn't even transmissible and could well be spontaneous -- but we don't know," he said.
UK TESTING FINISHED NEXT WEEK
Matthews confirmed that the brain tissue samples from the US animal had arrived at Weybridge. Test results were likely to be ready by the end of next week, he said.
The suspect animal has already undergone a series of tests. A rapid screening test on Nov. 15 returned inconclusive results. Sophisticated immunohistochemistry (IHC) tests cleared the animal of any infection, but a third round of testing using a Western blot procedure showed a "weak positive".
Weybridge will do an IHC test plus three kinds of Western Blot tests on the samples. They will use "methods of slightly different analytical sensitivity that give us the greatest number of opportunities to interpret what we see," he said.
US beef industry leaders say scientists sh
