Swiss BSE infoFrom New Scientist (U.K) June 13 1998
BSE's hidden horror
By Debora MacKenzie
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 carrying the disease 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 Zurich.
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.