Mad Cow Disease – TSEs

by Vincent Racaniello, PhD

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    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.

    About the Lecture

    The lecture Mad Cow Disease – TSEs by Vincent Racaniello, PhD is from the course Prions.

    Included Quiz Questions

    1. They are not inactivated by cooking.
    2. They are not digested by proteinase K.
    3. They have many alpha chains.
    4. They form GIP linkage with beta chains of striated muscles.
    5. They are primarily found in renal cells.
    1. Healthy cows were being fed meat from infected animals.
    2. Healthy cows were receiving blood transfusions from infected animals.
    3. Healthy humans were receiving transplants from infected animals.
    4. Healthy animals were receiving growth hormone injections.
    5. Infected animals were being slaughtered at an early age.
    1. The incubation for this prion disease was five years.
    2. The two conditions were unrelated.
    3. The infected meat was frozen and sold only at a much later time.
    4. The infected cow meat was ingested by an intermediate host and then passed on to humans.
    1. Humans
    2. Cows
    3. Sheep
    4. Rats
    5. Bulls
    1. 2%
    2. 99%
    3. 50%
    4. 20%

    Author of lecture Mad Cow Disease – TSEs

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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