00:00
Let’s now turn to two diseases of animals
that are apparently a threat to our human
food supply. The first is called bovine spongiform
encephalopathy or mad cow disease. I’m sure
you've heard of it. This was a disease that
appeared in the 80s in the UK, and we talked
about this in our introductory prion lecture.
You can go back there and check that out.
00:27
What happened in the UK was they feed cows
meat, protein to make them grow quickly, so
they can slaughter them and sell it, instead
of giving them corn or grass which takes longer.
00:40
Not a lot of grass and corn areas for cows
to graze in the UK, so they feed them meat.
00:45
Where does the meat come from? They take other
animals and they grind it up, feed it to cows.
00:49
Those animals include other cows who die,
sheep, and it turns out that some of those
animals have TSEs and we don't know it, because
they're dying and we are not diagnosing why.
00:59
And in the early 80s, they changed the way
this meat was prepared. And apparently that
allowed scrapie prions to get into the cow
food supply, and that caused this outbreak
of bovine spongiform encephalopathy. So it's
basically a form of cannibalism, we are feeding
cows themselves, as well as sheep, with a
TSE and the cows developed TSEs. So it was
a big outbreak of TSE in the UK, bovine spongiform
encephalopathy rather, among British cattle.
01:31
Hundreds of thousands of cows were slaughtered
to get rid of it and many millions in fact
acquired the infection. The problem was, this
didn't go detected until some of that beef
got into the human food supply. As you know
we eat cows, some of us do, in the form of
steaks and hamburgers, and even if you have
a well-done meat, you know if you listen to
a parasite lecture and you know you have to
cook your meat well. Well it doesn’t do
anything if there's a prion in it because
prions cannot be inactivated by cooking. In
fact they can’t be inactivated by a lot
of things, which is another reason why they
are very scary. So here is a graph of the
outbreak of BSE in cows, those are the orange
colored bars, and you can see the number of
cases, which is listed on the left, goes up
starting after 1987, peaks in about 92, starts
to go down again. So at some point we said:
“Uh oh, we are making these cows get BSE,
let's change the feed preparation”, they
changed it, they got rid of prions, you can
see gradually the number of cases in cows
go down. We thought we had this solved,
then all of a sudden in 1994, we start seeing
a new kind of Creutzfeldt-Jakob disease in
people, those are the green bars going up
and then going back down again. It's a new
disease, because it has a different incubation
time, it is much shorter than classical Creutzfeldt-Jakob.
It happens in younger people, and they die
much quicker, and the epidemiology said this
is because of BSE, and there was a peak and
then finally it went down because we stopped
feeding cows scrapie prions and eventually
we got those out of the food supply and so
the human cases declined. There is a big lag
because there is a long incubation period
here. So a variant Creutzfeldt-Jakob arose
because of feeding cows scrapie proteins and
then they in turn transmitted it to humans.
03:25
So that's a big problem of course, and we’re
still worried about it.
03:28
In cows, new cases of BSE still occur, very
rare, but they occur. They are probably sporadic,
we no longer feed cows scrapie prions as far
as we know, so a cow now and then, it is extremely
rare, but now and then will develop a prion
disease, and it is probably the sporadic form
where the PrPc simply misfolds and you get
disease. The problem with cows is that, we
slaughter them pretty early on in life, about
two years or so, and usually they don't develop
the disease until five years, so they may
be incubating prions but we don't know it.
04:05
So that's why it's a problem. Now we would
like to protect our food supply from bovine
prions, The problem is of all the cattle we
slaughter worldwide, in the US and Canada,
we have a lot of statistics here in Europe,
less than 2% are actually tested. We don't
look at the meat in any way that would reveal
if there were any problems with those animals,
and in fact you have to look at the brain
tissue to really know. And obviously it would
be good to have a diagnostic test to do this.
The Western blot test that I showed you before
is really too laborious to look at every cow
that’s slaughtered in the world, this is
simply not going to work. It's going to put
the price of beef through the ceiling, which
may not be a bad idea in the end. I do like
meat myself but maybe it's not a good practice.
04:54
So here is a graph which shows you how rare
BSE is in cattle since 1993 or so towards
the end of the BSE outbreak in cattle. And
these are colored according to where the cows
were, when they were diagnosed with BSE, so
you see they are not too many. Each of the
little blocks whether it's yellow or blue
or light blue, is just one case. So you can
see in the US, which is orange, there is just
a handful of cases, 1, 2, 3, 4 cases on this
graph. One of them was imported from Canada
and another one was born after the feed
was changed, so that's probably a spontaneous
case. Canada is listed in blue of different
colors depending on when it's before or after
the feed ban. So whenever there is a case
diagnosed, this is usually pretty fortunate
because we don't look at that many cows. It’s
because a cow all of a sudden in the barn
gets sick, and then there's all kinds of regulations
thrown into place and people get worried for
a while that similar cows are elsewhere are
getting into the food supply. So clearly we
need to have a good way of diagnosing the
presence of prions. So we need to have a diagnostic
test. We need to have drugs also, if we found
a cow with prions in it, it would be nice
to have a drug to block it. And of course
in people, if we can diagnose prion disease
in people, it would be nice to have drugs
to cure the infection. So people are working
on this and a couple of diagnostic tests have
been developed. I told you one which is laborious
and not likely to be of much use. But there
is a second one which is pretty cool and I
want to share that with you.