SIGHTINGS


 
Mad Cow Hidden Horror -
Infection Rate May
Be 100 Times Higher
By Debora MacKenzie
6-11-98
 
 
"... some cows may be carrying a silent infection that could be even more dangerous to people than overt BSE: "It may be that there is rather more infectivity in muscle (meat) or other tissues in those animals and that is why they do not have a brain disease."
 

 
Hundreds of thousands of apparently healthy cattle could be infected with BSE, new Swiss data suggest. For every case of mad cow disease in Switzerland, more than 100 animals may be "silently" carrying the infection.
 
If this pattern holds true in Britain, the number of British cattle now carrying the disease as of last year will have exceeded 450 000.
 
Last year, Switzerland started slaughtering herds in which a case of BSE had been confirmed. The Swiss Federal Veterinary Office then began looking for signs of BSE in the brains of these apparently healthy cattle. These tests have just been repeated using a sensitive diagnostic test developed by Prionics, a company in Zürich.
 
 
In the Prionics test, brain tissue is homogenised, then treated with an enzyme to break down proteins apart from the rogue form of PrP, the protein that becomes misshapen in BSE. The mix is then put on a gel in an electric field, which separates the remaining protein fragments. Malformed PrP is detected using an antibody that binds tightly to the protein. Previous tests of this type, called Western blots, have taken three days. "We can have a result in 12 hours, before a carcass leaves the slaughterhouse," says Markus Moser of Prionics.
 
 
Only one or two cases of BSE typically occur in each affected herd in all countries in which the disease has shown up. "The official theory is that only the sick cows ate a lump of infectious feed," says Moser. "But other cattle may be infected, and just haven't shown symptoms."
 
The Prionics test has confirmed this. Of 1761 healthy cows slaughtered in the culling programme, eight tested positive for BSE. Six of these were also picked up by the veterinary office using other tests.
 
Eight infected cows out of 1761 gives a rate of "silent" infection of 4.5 per thousand animals--more than 100 times Switzerland's 1997 rate of clinical BSE.
 
 
The Swiss government and Prionics will start testing the brains of 3000 randomly selected cattle at more than 20 slaughterhouses later this year, to see if the same rate of infection holds for herds in which BSE has not yet been recorded. Two abattoirs say they will screen all slaughtered cattle.
 
Epidemiological data suggest that a similar pattern may emerge. If so, says Bruno Oesch, head of Prionics, "then 1800 subclinical cases may have ended up on the table" in Switzerland last year.
 
 
No one has tested for subclinical BSE infection in Britain. But if British herds contain more than 100 infected animals for every one with obvious symptoms, the number of subclinical cases in 1997 would have been around 460,000.
 
 
The Ministry of Agriculture, Fisheries and Food (MAFF) says that only older cows are likely to pose any risk of infecting people. And since 1996, all British cattle older than 30 months have been destroyed. "This removes the possibility of any animal harbouring infectivity from entering the food chain," claims a MAFF spokesman.
 
 
But some of the government's scientific advisers remain worried about the risks posed by subclinical infection. John Collinge of Imperial College London, a member of the Spongiform Encephalopathy Advisory Committee, last week told the official BSE Inquiry of his fears that some cows may be carrying a silent infection that could be even more dangerous to people than overt BSE: "It may be that there is rather more infectivity in muscle or other tissues in those animals and that is why they do not have a brain disease."
 
 
Collinge has tried to get MAFF to look for subclinical infection using sensitive tests like the one developed by Prionics, but with no success. "I have raised that several times," he told the inquiry.
 
 
MAFF is now considering plans to study subclinical BSE, but could provide no details.
 
From New Scientist, 13 June 1998


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