- "... 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
- 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