DR. SUTTON: My intent is to explain the past history of scrapie in the U.S., what steps have been taken from a regulatory point of view to control it, where we're at now with the program, and what's currently going on with the certification program.
It's not going to cooperate. Maybe it's on the remote here.
I just want to acknowledge that the epidemiological information came from Dr. Nora Wineland out at the Centers for Epidemiology and Animal Health.
As you're all aware, we've been knowing about scrapie for over 300 years, and it originally occurred from Europe to Canada to the U.S. with the first case being reported in 1947.
The United States Department of Agriculture initiated an eradication program in 1952, at which time a state of emergency was declared. A total flock depopulation with indemnity was the manner of control.
In 1957, source flocks -- these are the flocks from which scrapie infected sheep were born -- were also included in the program, and animals that traced out of exposed flocks were also slaughtered.
Oops. Went the wrong way. This is my first time using this. So I'm having an entertaining time.
In 1965, the option for bloodline slaughter was introduced along with a two-year quarantine for non-bloodline animals. The theory at the time being was that it might be a genetic disease, and the majority of animals that were found to be positive were the offspring of previously infected animals.
In 1975, that option was eliminated as science became better known that it was an infectious, widely transmitted disease.
In 1978, the indemnity was increased in hopes that this would facilitate the program and also a surveillance requirement was added.
In 1983, bloodline option was once again reinstated at the request of the sheep industry.
In 1952 everyone came to the realization that the eradication program as it had existed since 1952 just wasn't getting the job done, and negotiated rulemaking process was started between the sheep industry, renderers, USDA, state animal health officials, and other interested parties.
And what came out of that was the voluntary scrapie flock certification program, which I'll describe in detail in a few minutes.
The one thing to remember is at the time that the voluntary program came into effect, that didn't make the regulatory control program go away. We still have an active program. There are interstate restrictions on the movement of sheep that are infected with scrapie or that come from scrapie infected or scrapie source flocks, source flocks being defined as any flock from which two scrapie infected animals were born under the age of 54 months.
The other thing to be aware of is that we are currently revising the rules that apply to scrapie to make a more stringent and powerful control program. The proposed rule is currently undergoing the clearance process.
For those of you who are not inside government, that means I can't discuss the details of what's in there.
Currently, in order to get off the federal infected or source flock list, the flock has to undergo what's called a herd plan or flock plan, and this basically requires the removal of high risk animals, cleaning and disinfection, being on surveillance program in most cases, and identifying all animals in the flock.
Since the beginning of scrapie in the U.S., we've had 943 infected flocks identified, 1,503 confirmed scrapie cases, and of course, these are only the reported cases that were confirmed at NVSL. We've had seven cases in goats. Of the cases in goats, many of them trace back to exposure to sheep, and the remainder could not be determined whether any exposure to sheep had occurred.
In the U.S., the average age of an infected animal at death is 47 months, the majority of the animals, of course, female due to the high number of female animals in any flock. Male animals typically die at a slightly younger age, 42.9 months; females at a little higher age, 47.8 months.
We have not been able to detect any seasonality with the disease. The disease is distributed throughout the U.S., and a wide range of breeds are affected, including all of these listed: Suffolk, Hampshire, Cheviot, Southdown, Shropshire, Rambouillet, North County Cheviot, Dorset, Finnsheep, Corriedale, Merion, Montadale, Columbia, Cotswold, Border Leicester, and Textel. So there are very few breeds that have not been affected.
The vast majority have occurred in the Suffolk breed and the black faced breeds, but as you can see, the white faced breeds are well represented.
In a NAHMS study survey conducted in 1996, this was an owner questionnaire and involved voluntary reporting. Owners reported anywhere from a 0.3 percent of infected flocks up to 2.6 percent. When this was averaged out over the whole United States, it came out to 1.2 percent.
Basically the people were asked: have you seen a case of scrapie in your flock in the last five years?
These are the scrapie confirmed cases for fiscal year '98. We had 63. These are the cases to date, for fiscal year '99. As you can see, we're having comparable numbers this year as last.
These are our scrapie source flocks. We currently have six. These are the scrapie infected flocks. These are the flocks that have not completed a flock plan in order to get off of the infected list. We currently have 65. As you can see, they're well distributed.
There are a number of strategies that can be used to reduce the risk of sourcing materials from scrapie infected sheep. One which Lisa Ferguson will discuss later is sourcing from scrapie from scrapie free countries. The other is to source from scrapie free flocks in infected countries. The other is to test individual animals.
The problem that we have with this at this time is that currently the only validated test is on a dead animal, and many tissues are not acquired from dead animals.
There are two live animal tests under development by ARS. One is the third eyelid test, and the other is the capillary electrophoresis test. Neither test has been fully validated nor approved by USDA, and we anticipate it will be a while before that happens.
However, ARS has indicated to me, and I have seen literature from a commercial company, indicating that they intend to make the third eyelid test commercially available in the very near future, possibly even this month.
The other thing to consider is the genotype of the animals involved. Of course, by selecting for genetically resistant animals, you do not know whether or not based on current science whether they contain the PrP agent.
For those of you who are not familiar with the third eyelid test, I threw this slide in just so you could see what it involves. It's a topical anesthetic on the eye, pulling back the third lid and snipping out a small portion of lymphoid tissue.
Okay. One option, as I mentioned is the source from free flocks within affected countries. USDA runs a voluntary scrapie flock certification program, which is based on the monitoring and self-reporting of flock owners. Possible additional criteria that could be added on top of requiring participation in this program would be to require the examination of tissues of all animals dying over 18 months of age. This is only currently required in the voluntary program for those flocks for which there is a suspicion that there might be scrapie, i.e., a trace or exposed flock.
The other option might be to require sentinel animals that are genetically susceptible to scrapie to be kept in the flock and to be examined upon death.
The other potential that's been suggested by some ARS researchers is to require that the entire flock be QQ in order to more rapidly detect the presence of infection in the flock.
Once a live animal test is available, that, of course, would be a strong option, but we at this time do you feel that the live animal test that will soon be commercially available has been adequately validated, at least for USDA program purposes.
The other possible consideration is the past history of the flock with regard to scrapie, both of the animals themselves and of the premises on which they're being reared.
The other consideration is whether or not the flock should be required to remain totally closed as opposed to allowing the movement of animals as is currently permitted in the program.
And the other option is to consider the feeding practices for those animals, which is not a requirement currently of the program.
Now, I'm going to discuss some of the specific details of the voluntary program as it currently exists. These are the numbers of participating flocks. As you can see, we've got a large number throughout various areas of the U.S. We currently have 27 scrapie certified free flocks.
There are two potential categories in which a person can enroll in the voluntary program. One is the complete monitored category, and that's the only category that allows a flock to progress to certified free status.
The other category, the selective monitor category, is primarily intended for commercial producers who produce slaughter lambs. The intent is to provide them with a way of monitoring for scrapie based on examination of cull ewes prior to slaughter and with any animals that look clinically suspicious being sent in for diagnostic testing.
The general provisions of the voluntary program are that the owner agrees to report any scrapie suspects. They agree that should a suspect be found, to allow that animal to be euthanized and those tissues collected and submitted for diagnosis.
They also agree to identify all animals over a year of age with a relatively permanent form of ID, such as a tatoo and electronic implant or a tamper resistant ear tag.
They also must identify any animal being sold for other than slaughter purposes no matter what its age.
They are required to keep records, and those records are inspected on a yearly basis, which includes all identification that's found on the animal, the sex, age, and breed of the animal, the date of birth or the date of acquisition and which flock it originated from, sire and dam identification information, progeny identification and sex, and the disposition of that animal, and should it have died, the presumed cause of death.
They are also required to provide us with authorization to contact any breed association with which they might have registered animals should it become necessary to trace out any animals from the flock.
They also are required to notify us within 30 days of any acquisitions that are not in compliance with the program or that would lower their status.
Each of these flocks is inspected by a federal veterinarian or a state veterinarian every 11 to 13 months. The main thing that is done at those inspections is to review the identification on all of the animals, and then to check that identification against the inventory records and determine the disposition and acquisition of all animals.
Also, the animals are observed for any potential signs of scrapie, and any animals that are suspicious are identified and rechecked as needed, and if it becomes clear that they are highly suspicious, then those animals are euthanized, and the tissues are submitted for diagnosis.
There are two potential statuses within the complete monitored program. One is enrolled, which simply means that the producer is participating, and the other is certify which means that they have been continuously in compliance with all of the standards for a period of five years.
Female animal acquisitions and commingling requirements. In other words, how can a person get a female animal from another flock? Basically a certified flock can only acquire a female animal from another certified flock. Enrolled flocks may acquire female animals from any other enrolled flock which has a similar or older status date than they do, in other words, have the same risk level or less.
The male animal acquisition requirements are less stringent for two reasons. One is because we believe the male animal to be at very low risk of transmitting scrapie, and the other is that sheep owners need to bring in new genetics into their flocks or they can't have a productive and viable flock.
In order to acquire a male animal, the animal must be officially identified, must be shown on the flock inventory, and he must not have been exposed to scrapie, cannot be a scrapie positive animal. He can't be a scrapie affected animal, can't be a scrapie suspect, can't be exhibiting any clinical signs, have been designated high risk or have originated from a source or infected trace or exposed flock if his actual status can't be determined. Also, he cannot currently reside in an infected or source flock.
For certified flocks, there's also been a small tightening in the new version, which is the yellow book that you've all received in which it states that a male animal must be acquired from an enrolled flock, and that animal must have resided in that flock for at least a year or have been borne there. Previously that was not a requirement.
Likewise, for female animals in certified flocks, they can only commingle with female animals from other certified flocks, and they may not reside in the kidding or lambing facilities of anything other than another certified flock or their own flock.
We did have a problem with that, and that's why we've made that change.
Commingling of male animals. It is permissible under the current program for rams from certified flocks to be permitted to be leased to nonparticipating flocks for the purpose of breeding, but they may not be exposed to female animals at or near lambing or for 60 days after lambing. They may not be housed in lambing facilities, and they may not reside in any other flock except the certified flock, except for the purpose of breeding.
And if they are used in a flock that is known to be infected, source traced, or exposed, then they cannot return to the certified flock without them losing their certified status.
Should there be an occurrence of scrapie in a certified or enrolled flock, that flock is removed from the program.
Other actions that can potentially affect the status or status date of an enrolled flock is the use of semen and embryos. For semen, basically the standard is the same for the acquisition of male animals. They may use semen from any flock unless that animal is himself scrapie positive or unless that animal is a high risk animal or is of unknown, undeterminable status based on his previously having been in an infected or source flock.
For embryos, the standards are basically equivalent for those for female animals in that the embryo must have the same status as the flock. In the case of certified flocks, the embryo would have to come from a certified donor.
For imported animals, basically the animal would either have to come from a scrapie free country or the animal would have to have been participating in a program equivalent or more stringent than the American program as evaluated by USDA and be an equivalent level in the program in order to be imported into a participating flock.
That's all I have. Any questions?
CHAIRMAN BROWN: Thank you.
We will entertain questions for Dr. Sutton, as for the next two speakers, without waiting until the end.
DR. PRUSINER: I'm curious about your ideas about the genotyping of these sheep and if you use, for instance, an RR 171 sheep, and you think then that the scrapie agent, freon, whatever, virus goes underground.
CHAIRMAN BROWN: "Whatever" is the preferred term here. Okay?
DR. PRUSINER: Right. We got that one yesterday, right?
DR. PRUSINER: That goes underground and you can't find it then. That's what you said, I think.
DR. SUTTON: What I said was the research is insufficient at this time for us to know whether the scrapie agent is present in those sheep or not.
DR. PRUSINER: Okay. I don't know how to proceed here. Let me just take two -- I'll make this very, very brief.
I mean, I think that you really ought to consider an alternate view of all of this. These animals that these basic residues like arginines create dominant negatives in these animals. The same thing is true in humans, and I think that's a very plausible explanation.
It's not proven, but it looks very good now, and it's not acknowledged at all in this book. so it's for the record.
DR. SUTTON: One of the primary reasons that USDA has not yet accepted genotyping as a means of scrapie control for official purposes is because there is one case report in the literature of an RR being clinically affected with scrapie, and there are four case reports of QRs being clinically affected with scrapie, and our experience -- I probably shouldn't mention this -- with the New Jersey pilot slaughter project suggests, although that information is unpublished and not complete yet, that the genotype may not prevent PrP scrapie RES from being found on immunohistochemistry.
All of these make us suspicious that there may be an infectious agent in these animals.
DR. DETWILER: Can I further comment on that since that's what I think Jeff Almond was referring to about this little pilot study? But all of the questions that it brings up is that we have found the different genotypes with PrP-RES, but again, if they were positive across the board in these, then it would make you feel better.
And there were animals that were clinically asymptomatic, that were positive in the brain, in the lymphoreticular tissues, but then you had ones mostly in these other genotypes where you might have the tonsil or lymph node or the brain. Then you had an IHC positive, but not the Western blot, but we can even wonder about those that its collection techniques -- that you don't test the same part of that tissue with the different methods, you know. So it's not the exact same thing you're testing.
But they just leave us with more questions than they do really answers.
CHAIRMAN BROWN: Stan?
DR. PRUSINER: Yes, I think you want to make -- did you want to talk first? Go ahead.
I just wanted to say that I don't think that the protease resistance of PrP is an absolute indicator of infectivity. It's a surrogate market, and when it's there, it's useful. When it's not there, it becomes problematic.
So you can't use this as an absolute. I mean, we have multiple transgenic models now, and we have some human diseases like GSS 102 where proteinase K resistance is really not a good marker. So you have to be careful of this.
That's why we developed this new assay where we're looking at a buried epitope and then a form of PrP that has a high beta sheep content.
So I think you have to be very, very careful of how you interpret this.
DR. DETWILER: That's why we haven't published those, but do you want to test some tissues for us?
DR. ALMOND: I'm not quite sure where we're going in this other than to say it is difficult to interpret. The studies of Nora Hunter suggest that different or certain genotypes of sheep are much less susceptible to certain strains of scrapie, but if you change the strain of scrapie, the pattern becomes rather different.
One of the most interesting observations that she's made in recent years is that certain genotype -- and I can't remember if it's 136 or 171 -- but in the U.K. environment, those sheep inevitably, almost inevitably develop scrapie, and yet she's found that same genotype in New Zealand, and they're completely scrapie free, of course.
And then there are other observations as well that relate to this. Moira Bruce, for example, with Richard Kimberlin did some transitions of CJD to mice. The mice didn't develop any illness in their lifetime, but when they were examined in old age, they had some spongiform change in their brain, suggesting that they were, in fact, incubating a spongiform encephalopathy, and the implication of that could be that this was a disease which was taking more than the life span of the animal to actually develop itself as a clinical disease, but potentially there could be a source of infection to other animals with adaptive passage that, therefore, actually represent the reservoir.
And I did raise the question in my presentation about whether there is silent infection. So there could be a reservoir of prions which you simply don't see because in that particular genotype of animal the incubation time is longer than the life span of that animal.
CHAIRMAN BROWN: Kiki?
DR. HELLMAN: I just have a question. Kiki Hellman, FDA.
I know very little about sheep, but I would imagine that somewhere there must be a lineage history of the different breeds of sheep that we have today, where they derive from, where they're found. That might help us.
And now, of course, since you are introducing different breeds, you get hybrids and so on, but that might help us in perhaps the evolution of scrapie or the agent. It's just a thought.
DR. SUTTON: Actually a part of the presentation that Nora Wineland lent to me did actually address that and showed where the various breeds came from, and I don't think one could make an argument that it was directly due to these animals being mixed and that follow along breed lines.
I think it's pretty well accepted that it was due to lateral transmission.
CHAIRMAN BROWN: Dean?
DR. CLIVER: Yes, as another non-sheep person, it would be helpful in some of these summaries that you presented had you used prevalence data rather than incidence data. Like we saw an enormous number of Suffolk that had succumbed, but we have no idea of what rate that represents against the population of Suffolk in the United States.
DR. SUTTON: Right.
DR. CLIVER: So that annual prevalence, state prevalence, things like that, we who don't look at these figures very often get a better sense of what's going on if they're presented as prevalence rather than --
DR. SUTTON: The breed data that I showed was for the entire 1,503 sheep for which we had data from the history of, for instance, the start of scrapie in the U.S.
Suffolk sheep are the largest pure bred sheep that we have as a breed, that we have in the U.S. currently. What number they represent out of that entire group, what the relative percent is, I'm not absolutely sure.
DR. CLIVER: Well, I wasn't expecting you to be able to deliver that from the hip, so to speak, but just as you're compiling data for this kind of a group, it's really helpful to give it on a prevalence basis.
DR. ROHWER: You very nicely have described the scrapie control program, but didn't say anything about how effective it was, and I see a big disincentive in this program to reporting scrapie if it should appear in one of these flocks that people have expended a great deal of money and effort to establish, and then what if they do get -- in introducing new stock, they do introduce scrapie into a flock like this? Can they really afford to report that? I mean with a decade or more of hard work to establish what they've got already.
And do you have any experience yet -- I mean, is the program established enough to know how successful it has been? Have people had problems like this, or how is it working?
Also, what percentage of the sheep industry has enrolled in this program?
DR. SUTTON: Okay. The program stated in 1992. We currently have 419 enrolled producers, which is a dramatic increase over last year. We've had a 75 percent increase.
Of those, 27 have reached certified status, which means they've been enrolled for at least five years, and that would make them compliant with the program for that duration of time.
Of the flocks that have reached certified status, we haven't had one go down with scrapie yet, which doesn't mean it won't potentially happen.
We have had flocks that were at the lower levels of the program that had infection discovered and were moved back, either taken totally out of the program or went out of the program and then reapplied.
CHAIRMAN BROWN: What proportion of sheep -- I may have missed it, for which I excuse myself -- what proportion of sheep in this country are enrolled in the program?
DR. SUTTON: Okay. We have 7.2 million sheep in this country and approximately 68,800 flocks. Of those, approximately 17,000 are what are called seed stock producers. Nearly all of the enrolled producers are seed stock producers. So we have approximately just under two percent of our seed stock producers enrolled at this time.
CHAIRMAN BROWN: Seed stock being breeders?
DR. SUTTON: Correct.
CHAIRMAN BROWN: Two percent of the breeding population or the flocks used primarily or exclusively as breeders are enrolled in the program.
DR. SUTTON: Correct.
CHAIRMAN BROWN: Okay. Ermias.
DR. BELAY: I just wanted to clarify one issue. The ban on importation of live animals from countries where BSE has been identified that the USDA put in place in 1989, does it include sheep and goat in addition to cattle?
DR. SUTTON: Yes.
CHAIRMAN BROWN: Ray?
DR. ROOS: I guess there was some brief discussion about susceptibility of different breeds, and I guess what I heard is that one polymorphism was discussed as far as in the PrP and how effective it was as far as determining susceptibility.
Now, am I correct in thinking that there must be a lot of other genetic determinants outside that PrP, polymorphism, in other words? Some breeds that have identical sequence of PrP nevertheless have very variable susceptibility or isn't that known?
DR. SUTTON: You're out of my area of expertise, but there are at least three codon areas on the PrP that are related to susceptibility, 136, 154, and 171.
DR. DETWILER: I can add a little bit more on that. It does depend what breed. There are some breeds that are called the 136 breeds, and the other breeds they call the 171 breeds. The Suffolks are 171, where that's the main dependency.
And it does seem like most breeds follow that 171 with the arginine homozygotes of being only one reported case of clinical scrapie, and then you have breeds like in Britain that would not follow the U.S. pattern. They're the 136 breeds that valine homozygotes are what they call their positive line and alanine homozygotes, but in the U.S. almost all of our sheep are alanine at 136. So that does not in this country appear to play a role.
DR. ROOS: But is there anything outside the PrP gene polymorphisms that look important?
DR. DETWILER: Not to my knowledge, but I don't know.
DR. ALMOND: Actually there is in mice. Stan will help me with which is which, but you know, the S7P7, as they used to be called by the Edinburgh Group; I think it's VMs and 357 blacks that are both P, and they have different incubation times, and if you do crosses between those, you get intermediate incubation times.
I am aware that there are experiments in progress which are trying to map the determinants of the differences in incubation time in mice strains where the PrP gene sequence is identical.
DR. PRUSINER: I think these differences are very small though, Jeff. I don't think they're large.
DR. ALMOND: Well, the issue -- well, I know they're small. They're still measurable actually in 100 days type level or 80 days. The point is they're entirely reproducible, and when you make the hybrids, you get something in between.
When you do R3 mice or VM mice, you have a tight cluster around the endpoint, and when you do C57 blacks you have a distinguishable, but again a tight cluster. They are reproducible. They are not related to PrP gene sequence because that is identical in the animals.
CHAIRMAN BROWN: Would it be a fair summary of this entire discussion to say that the genotyping of sheep unfortunately turns out not to be a straightforward, simple matter, and that it is in flux, and that eventually it may get shaken out, and one will find one or more really crucial points on this or another gene which will clarify matters?
But at the moment, sheep genotyping and its relationship to susceptibility is in evolution.
DR. ALMOND: I think one would also add that unlike the situation in cattle, and of course, we've looked at a lot of cattle of different breeds in the U.K., the number of polymorphisms in cattle is very small. You have the five versus six off the peptide repeat, and that's about it.
Whereas, of course, in the sheep there are all sorts of polymorphisms scattered across the different breeds, and it's much more difficult then to interpret scrapie susceptibility.
It may be saying something about the co-evolution of TSE in sheep, and therefore, there's been selection pressures on certain prion genotypes in sheep which hasn't existed in cattle. I don't know, but it is more complicated in sheep.
CHAIRMAN BROWN: Bob, yours is the last comment.
DR. ROHWER: Oh, if I only get one comment, then I'll get off genetics. I want to go back to the question that Dr. Belay asked and make sure that we're absolutely sure on this.
It seemed to me that you left open a loophole at the end of your presentation there for importation of animals from BSE affected countries if they met certain standards or something like that, and I'd like it made perfectly clear whether that can happen and whether that provision, if there is a provision, extends to the wider provision that was implemented and that was laid down in 1997 for BSE to all of Europe, et cetera.
DR. SUTTON: For imported animals, that restriction applies. These would be imported animals from countries that are not known to be affected with BSE and are not believed to be at risk of BSE, that are not currently excluded, or at least it can better answer that.
DR. ROHWER: I'm not sure what distinction you're making.
DR. FERGUSON: Okay. Let me clarify a bit. Our initial import restrictions that we started in 1989, we applied to countries that had diagnosed and identified BSE in native animals, and those restrictions were for all ruminants. It was not just cattle. It was for all ruminants, including sheep, goats, cervidae, llamas, whatever. Ruminants from BSE affected countries were not allowed in.
Those same restrictions, the end of '97, January of '98, publication of the interim rule, when we extended those restrictions across all of Europe, again, those same restrictions applied. It was all ruminants that could not come in.
Now, the comment that Dr. Sutton made in her program about allowing imported animals into the voluntary program, that would come from countries that were not restricted due to BSE, and a big chunk here would be let's take Canada as an example. So, you know, those types of animals could come from that type of a country, and they could go into a flock enrolled in the program only if they came from a flock in that other country enrolled in a similar or equivalent type program.
Now, there could be imports going into nonenrolled flocks. That would not be coming from flocks in that other country in a similar type program.
Does that help or have I confused things even further?
DR. ROHWER: I think that helps, but maybe a better example would be Mexico.
DR. FERGUSON: Mexico is a touchy subject, and I was going to get into a little bit of this in my talk. So now hopefully I can leave it out.
CHAIRMAN BROWN: I'd prefer you left it in and stop now.
DR. FERGUSON: Okay, okay.
CHAIRMAN BROWN: As long as it is going to be included.
DR. FERGUSON: It will.
CHAIRMAN BROWN: So thank you very much, Diane.
Our next speaker is Dr. John Honstead, who reminded me that I was remiss in not introducing this Committee to a very distinguished gentleman. I use that word in its literal sense, who for at least three decades was Mr. Scrapie for the USDA, and that's Dr. Jim Hourrigan.
Would you stand up, Jim, and let people see who scrapie was all about?
CHAIRMAN BROWN: Jim has heard these same questions about 50 times, and the same answers keep coming back. It's very difficult to get precise, happy, satisfactory answers to virtually every question we ask on the topic.