This is a partial transcript of Special Report with Brit Hume, Jan. 6, that has been edited for clarity.

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BRIT HUME, HOST: The Agriculture Department (search) said today that that cow, out in Washington State that was diagnosed with Mad Cow disease (search) last month, came from Canada. The department also said it will slaughter another 450 cattle from that quarantined herd in which the sick one was found. At least 37 countries have now banned the import of U.S. beef. But is any of this really necessary?

For answers, we turn to David Ropeick, of the Harvard Center for Risk Analysis, who is co-author for "Risk, A Practical Guide For Deciding What`s Really Safe And Dangerous In The World Around Us." He joins us from our Boston Bureau.

Welcome to you, sir.

DAVID ROPEICK, HARVARD CTR. FOR RISK ANALYSIS: Brit, thanks for having me

HUME: First of all, what is the significance of the discovery or the determination that this cow, this sick cow, was born in Canada?

ROPEICK: Well, it increases the likelihood that cow and the Canadian cow that they found in May were fed from the same contaminated batch of feed. Which would, if is true, would be good news. Because instead of two bad, batches of feed contaminating separate animals, there'd be only one.

HUME: You say batches of feed are the culprit. Explain why that is.

ROPEICK: They think that it spreads -- yes -- they think that the disease spreads when proteins from sick animals after the animal is butchered, the leftover parts, if you will, they are getting too gross, are rendered into protein fed back to healthy animals.

The infectious material, therefore, can transmit from the dead, sick animal to the healthy ones. That's why there's a feed ban in the U.S. and Canada that will largely reduce or even almost eliminate this risk.

But if a bad batch of feed got through, contaminated material might have gotten from a previously dead, sick animal into healthy ones. If there's only one batch of feed out there instead of two doing that, that's better news.

HUME: Now, going back to the Canadian cow. There was one cow so far in Canada found. So -- that was last summer. No other sick cows have been found in Canada since, correct?

ROPEICK: Correct. And it's -- it's likely -- we did a study a few years ago at our center. I was not party of the study; I'm familiar with it. That said because of this feed ban, where you can't feed cattle protein back to healthy animals, it chokes off, even if you get one bad batch in a sick animal or two. And maybe even, if tragically, we have one human case, it's still going to stay isolated because the feed ban chokes it off before it can spread.

HUME: Now, this -- I take it that this is a disease that takes a while to get going. So if you are a baby calf in Canada and you get fed, as you obviously will, it's not surprising that it wouldn't turn up until some years later in the United States, right?

ROPEICK: That's correct. It incubates for as much as three to six years in animals. Although there's one in Japan that took just one year, but sketchy evidence on that. What`s really important here is, it usually only shows up in dairy cattle, because I learned a lot about this in the last few weeks. It turns out that beef cattle are slaughtered before they get old enough to show the disease, before the infectious prance in them have grown into enough concentration to get sick.

So this is really an issue with dairy cattle who live four to six years, have a lot of calves and stuff. But yes, it takes a while. There might be another cow or two or three out there. But it's going to be a really limited risk.

HUME: Now, this is spread through the feed. It is not, I take, it, spread through the meat. Now, let me ask you this question. Let us suppose you and I could tonight, get some of the beef from the sick cow and we decided to grill some steaks. Could we safely eat it?

ROPEICK: Yes.

HUME: Now, that's a shocker, because we got 37 countries that have banned the import of U.S. beef. We've got all kinds of criticism, critics saying that the Agriculture Department has not done nearly enough much. The Agriculture Department has now ordered the slaughter of another 450 cows that happen to be part of the same herd. Is that necessary?

ROPEICK: Yes. I'll tell you why it is. It's not based on the science of transmission of the disease, as we know it. In the British experience, where they have hundreds of thousands of sick cows, didn't know what was going on, we're learning a lot. They ground up the beef, the muscle, the steaks, of infected cows and inject it right into the brains of healthy cows and couldn't make them sick. So the consensus in the scientific community is that won't work.

But if we're afraid of terrorism, of SARS (search), of West Nile (search), of Mad Cow, the fear is a risk too. And so for the government to be over-precautionary to re-establish trust in the government so that we calm down, is an important step, not for economic trade, for the 90 percent of the beef industry that depends on consumption in the United States. The tragedy in the U.K. was 140 lives lost, billions of dollars, jobs lost, families destroyed by the fear.

HUME: How did those people in the U.K., 140 who got this disease, how did they get it?

ROPEICK: They think they got it from either the brain or spinal cord directly. That's where the, the infectious agents builds...

HUME: Well, we know that...

ROPEICK: ... up into a concentration enough to eat it. Or those materials might have been captured into ground beef. There's a real remote likelihood that that was the probable route. That's the best they guess.

HUME: But Americans -- are Americans ever exposed in their diet or food available to them to the infected parts, the brain, the spinal cord, any of those areas, which contain the disease?

ROPEICK: There have in the past been cultures that like brain and spinal cord, absolutely. And there's a process called Advanced Meat Recovery, where after the animal is slaughtered and butchered and they carved up all of it they can by hand, they kind of use a pressure system to capture the little bits of meat off the carcass. If the spinal cord isn't first removed, that pressure system can capture bits of spinal cord. If you have a lot of sick animals, once or twice you might capture a little bit of infectious material and put it into ground beef. If you don't have a lot of sick animals, the chances of that are really low.

HUME: Because this stuff simply doesn't transmit through the meat, correct?

ROPEICK: It doesn't -- meat, dairy, blood they found, evidence of the preance, these infectious agents in those tissues, not enough apparently to make other animals sick.

HUME: David Ropeick, thank you so much.

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