DR. RACE: Can you hear me?
DR. FREAS: We can hear you, I believe.
DR. RACE: Okay. I guess I can tell you just a little bit about some of the earlier work that we did and then some more recent work that we've done looking for infectivity in sheep tissues, sometimes from a diagnostic point of view and other times trying to understand a little bit more about the pathogenesis of scrapie in sheep, and then you know, if you have questions may be you can target where you want to go with the discussion from that.
I believe you have available at least one of the papers. Do you have both papers?
DR. FREAS: We have received both papers. One paper they received this morning, and the other is in their blue folders.
DR. RACE: Okay. So the papers basically summarize what we've done in terms of looking for infectivity in various tissues, and the first paper, that was basically Bill Hadlow's paper from 1982, showed that infectivity, high levels of infectivity are present in the central nervous system, relatively high levels, but much lower than PNS tissue present in lymphoid tissues and infectivity, either very, very low or nondetectable in the other tissues that were examined. The other tissues, the negative tissues that were looked at are indicated in the note at the bottom of Table 2.
In the second paper, we looked at essentially the same kinds -- we looked only really at central nervous system lymphoid, and we added placenta because we were interested in knowing more about placenta, whether or not that might be a major source of pasture contamination.
There was some indication that it could be, but really no follow-up in recent years to really look at that. So with newer techniques, with our ability to look at PrP-RES and disease associated protein and infectivity, we thought we'd look at that again, and so that paper actually adds placenta, but it in all other respects is similar to the first paper in terms of what we found, and that is that central nervous system tissue is highly infected lymphoid tissue, and we looked only at spleen and a couple of selected lymph nodes, and also infected in about 80 percent of the animals, and placenta was infected in about 60 percent of the animals if the animals were scrapie positive, and we did not screen animals that had not been non-clinical.
So that the placenta aspect of that paper is a little bit biased in that we looked only at animals we already knew were scrapie positive when we were looking at those placentas.
In terms of transmission, I don't think our attitude has changed any from 1982. We still think that it's primarily via the oral route. We really don't know, and I don't think anybody else knows what part feces and urine, milk, colostrum, semen, tissues that might be ordinarily expected to be excreted to the environment plays other than that we've found them to be negative, but I don't know of anyone who has gone to heroic measures to try to concentrate agent that might be present in those tissues, and we actually now have techniques available where, you know, hopefully somebody might decide to do that using purification techniques where we can concentrate disease associated protein and thereby then associate infectivity and look at that.
I think there are some people that are thinking about doing those kinds of things, but for lymphoid tissue and central nervous system tissues of sheep are infected by -- very highly infected if one considers that we're crossing a species barrier and going from sheep to the bioassay animal, which what we have used is mice, and we've used these rml mice, our titration mouse, and it's a very sensitive strain. It's worked better for me than a number of inbred strains, and I've utilized those.
I think that the data is pretty accurate at least to this point.
Would there be questions or kinds of other information would you like to hear about?
CHAIRMAN BROWN: If anybody on the panel would like to ask Dr. Race a question now, please do so. Otherwise we'll have Dr. Race continue.
I don't see any hands, Dr. Race.
DR. RACE: Okay.
CHAIRMAN BROWN: Oh, wait. There's one hand in the back row. Actually he's a "back bencher." Dr. Asher.
DR. ASHER: Can you hear me here?
DR. RACE: Yes.
DR. ASHER: This is David Asher.
I'm just wondering how sensitive the rml mice are relative to sheep. Is there any estimate of that?
DR. RACE: We've never done an estimate of that. In the later study, the 1998 study where we were actually looking at placenta, where the amounts of agent looked to be fairly low, if we had a very, very low PrP-RES signal by immunoblot, we actually did find some infectivity. The two seemed to correlate pretty well and, you know, suggested to me that the mice were actually in this situation doing a fairly good job of detecting agent that might be there.
Usually if we get a PrP-RES signal, we usually get -- definitely get a titer in the mice, and that seemed to correlate even in placenta in these studies where the amount of infectivity seemed to be fairly low.
CHAIRMAN BROWN: Dr. -- do you have another question?
Dr. Almond, did you have a question or a comment for Dr. Race?
DR. ALMOND: It was just a comment on that last point. The experiments carried out by MAFF looked at the relative susceptibility of cattle and R3 mice to BSE by IC inoculation, and the difference in sensitivity is about 1,000-fold. Cattle are 1,000 times more sensitive. So the species barrier there, if you like to put it in those terms, is 1,000.
Orally it's not clear whether that 1,000 difference will be maintained when one compares oral transmission of cow brain to cows versus cow brain to mice.
DR. RACE: Yes, I think generally, you know, we also would agree with that. I think, you know, most of our PrP-RES infectivity correlations where we've used tissue culture cells or mouse hamster systems, we think that the infectivity assay is about 1,000 times, 1,000-fold better as well.
I was a little bit surprised by the study with the placenta, you know, that it turned out to be as sensitive as it did.
If you take the amount of infectivity in the placenta, if you look at it, the incubation periods are generally longer, and in two of the animals we found nothing. In two other animals we only killed one out of eight or nine assay mice. The others were a little bit stronger, and so we're really on the borderline, I think, on about four of those animals as far as placenta goes.
You know, as far as infecting animals, I think that whatever the source of the infectivity is, whether it's placenta or some tissue, to me it seems like it might be a very low grade exposure over a prolonged period of time that actually accounts for infectivity. So to really rule out some of the tissues that have not been positive in the past is going to require a little bit more in terms of trying to concentrate agent that might potentially be there.
The tissues that are positive are ones that, you know, it's a little bit difficult to envision getting fairly large amounts of infectivity into the environment unless it's prolonged, very low grade kinds of exposures or contamination.
CHAIRMAN BROWN: Stan.
DR. PRUSINER: Rick.
CHAIRMAN BROWN: This is Stan Prusiner.
DR. PRUSINER: How are you?
DR. RACE: Good. How are you doing?
DR. PRUSINER: Fine. Just a comment and then maybe you would respond to it. It seems to me that you really can't do much in the way of quantitative estimates of infectivity from sheep into mice when the highest the brain samples only begin to bring the mice down at 500 days of age because then after that there's really not a lot of time to do an endpoint titration, do all of the serial dilutions before the mice really begin to die off for other reasons.
DR. RACE: Yes, that's true. I mean, if you have the really most concentrated samples, you're still dealing with a long incubation period. So it is difficult to make those valuations.
Yes, as far as quantitative differences, it's pretty subjective. I mean, I think we all know that.
CHAIRMAN BROWN: There are no more questions, Dr. Race. Did you have other aspects of the topic that you wanted to transmit?
DR. RACE: Only if there is some other information that you think I could be of any help.
CHAIRMAN BROWN: Evidently the Committee is satisfied, and we thank you very much. It's nice to hear your voice again.
DR. RACE: Okay. Thank you.
CHAIRMAN BROWN: Bye-bye.
DR. RACE: Bye.
CHAIRMAN BROWN: I think we'll now, since we had an abbreviated break earlier, we'll take a five-minute leg stretcher, and so in about just five minutes we'll start again.
(Whereupon, the foregoing matter went off the record at 10:45 a.m. and went back on the record at 10:52 a.m.)
CHAIRMAN BROWN: We will dismiss a question period. I think we've had opportunity to ask questions of virtually everybody, and we will proceed with -- oh, I'm sorry. Dr. Sutton is not here?
DR. FREAS: She just got here.
CHAIRMAN BROWN: Okay. We will now have three successive presentations focused on this country, and the first will be from Dr. Diane Sutton in the USDA. The second will be in my draft as an unidentified FDA speaker, and that turns out to be our old friend, Dr. Honstead, and the third will be from Lisa Ferguson.
The first presentation is entitled "Potential for Human and Animal Exposures to Animal TSE Agents in the USA." Dr. Sutton.