New Scientist
New form of mad cow disease found
Fri Feb 20 00:58:39 2004
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http://www.newscientist.com/news/news.jsp?id=ns99994689


New form of mad cow disease found

Two cows have been discovered with a form of BSE that looks very different from the usual kind, Italian scientists have reported.

It resembles one form of the human prion disease, sporadic CJD, raising the possibility that this human disease is acquired from cattle. Unlike vCJD, which is caused by eating BSE-infected beef, sporadic CJD is thought to occur spontaneously.

However, other scientists caution that many more than two cows will have to be found before it can be concluded that a new form of disease has been discovered. It may be that the two cows simply caught BSE via a different method of infection.

In transmissible spongiform encephalopathies (TSEs), of which BSE is one type, an insoluble form of a protein called a prion accumulates in the brain - with fatal consequences. In different TSEs, these accumulations look different.

In cows with BSE, the prion deposits lead to spongy holes, primarily in the brainstem and lower brain. But Salvatore Monaco, of the University of Verona, reports that two cows that tested positive for BSE at normal slaughter in an Italian abattoir in 2003 had completely different patterns of deposition.


Layered arrangement


Instead of the brainstem, there were prion deposits in higher areas of the brain, the thalamus and the olfactory bulb. The deposits were also arranged in layers called amyloid, which is found in vCJD, but rarely in cattle.

Furthermore, the prion protein was less glycosylated - modified by having sugars attached - than the usual BSE protein, and was lighter when tested by electrophoresis.

It could be the same BSE prion as usual, but deposited differently in the brain because it was inhaled rather than swallowed, Monaco suggests, as the prion was also found in olfactory tissue. However, it might also be a separate TSE.

If it is, it could be fairly widespread, as earlier studies found amyloid in one in 20 cows with BSE. Monaco notes similarities between this form of cattle TSE, which he calls Bovine Amyloid Spongiform Encephalopathy (BASE), and one of the six types of sporadic CJD, M/V2, which has similar glycosylation and particle size.


Abattoir testing


The two cows with BASE were detected by a BSE test made by the Swiss company Prionics. Markus Moser, at Prionics, says that even if BASE is a distinct form of BSE, it is not likely to escape being detected by the tests used in European abattoirs.

This should mean it does not pose an added risk to people eating beef in places such as Europe, where older beef cattle must pass a BSE test. But if it is a second cattle TSE, it could arise spontaneously in places with no history of BSE infection. This would pose a potential risk, especially as those places allow cattle brain in food.

To prove BASE is a second TSE, Moser says the same pattern of chemical properties and deposition will have to be found in far more than two cattle. Then the prion must be injected into the brains of mice and shown to cause a different pattern of damage than the usual BSE prion. It must also maintain its distinctive properties when put back in cattle.

The strain is now being tested in mice, but the tests can take years. If it does have the same effect as M/V2 CJD prions in mice, it would raise the possibility that M/V2 CJD is not spontaneous after all, but could be caused by BASE in cows.

In 2001, French scientists were shocked when a strain of scrapie from French sheep looked identical to another form of human sporadic CJD in mouse tests. Scientists at the time said this meant an infectious cause for sporadic CJD could "no longer be ruled out", but further studies have yet to be concluded.

Journal reference: Proceedings of the National Academy of Sciences

Debora MacKenzie

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