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Animals and Antigens

THE PRESIDENT, THE EARL OF HALSBURY, in opening, the proceedings said it was his great pleasure to introduce Dr. William MacGregor Henderson, Doctor of Science and Fellow of the Royal College of Veterinary Surgeons, Secretary to the Agricultural Research Council, who had kindly consented to deliver the Stephen Paget Memorial Lecture for 1974. Dr. Henderson had chosen as his subject "Animals and Antigens", a subject he was well qualified to deal with. Dr. Henderson had a distinguished career in veterinary medicine and entered the Agricultural research service in 1939 as a member of the staff and subsequently Deputy Director of the Animal Virus Research Institute, Pirbright. In 1957 he was appointed Director of the Pan American Foot andMouthDiseaseCenter,Rio de Janeiro,Brazil. On his return toEnglandhe was appointed Head of the Department of Microbiology, A.R.C. Institute for Research on Animal Disease.Comptonand in 1967 he became Director of the Institute until his present appointment in 1972. 

 

Dr. Henderson has served on various Govern­ment appointed bodies, on foot and mouth disease, rabies and other animal diseases. In 1971 he headed a United Nations Development Programme mission to appraise the work ofPanAmericanZoonosesCenter.Argentinaand this year he was awarded the British Veterinary Association's Sir Weldon Dalrymple-Champneys Cup and Medal given to mark and recognise scientific or clinical work of out­standing merit.

 

Animals and Antigens

By Dr. W. M. HENDERSON, D.Sc. F.R.C.V.S., Secretary to the Agricultural Research Council

 

STEPHEN PAGET was born in 1855 and practised medicine from the 1880's until his retirement in 1917. During this period he might well have seen horses, cattle, pigs, sheep and dogs afflicted with one or other of the following major animal plagues—cattle plague, contagious bovine pleuropneumonia, anthrax, glanders, rabies, foot-and-mouth disease, swine fever, and tuberculosis.

 

I do not know to what extent the contempor­ary agricultural scene influenced Stephen Paget in his appreciation of the importance of scientific research and his recognition of the justification of using animals in experiments. Although he would have been only ten years old he could hardly have been completely oblivious of the disastrous outbreak of cattle plague in 1865 that led to the establishment of a State veterinary service inGreat Britain. It is almost certain that he was aware of the greatly improved procedures for the control of bovine pleuropneumonia which were introduced soon after he was elected to the Fellowship of the Royal College of Surgeons in 1885. These procedures included the slaughter of suspected cases and the post­mortem examination of the lungs at theRoyalVeterinaryCollege. While this was important in connexion with pleuropneumonia it also revealed that 25 per cent of the lungs from cows inLondonhad lesions of tuberculosis.

 

It was also about this time that glanders in horses was causing serious problems for London County Council and so grave was the situation with respect to canine rabies that in 1889 the Veterinary Department of the then Board of Agriculture introduced the Rabies (Muzzling of Dogs) Order. This replaced the permissive regulations hitherto in the power of the local authorities. Three years previously an outbreak of rabies amongst fallow deer inRichmondParkhad resulted in the deaths of 257 deer. Whatever note he may have taken of such incidents in animals he would most certainly have been involved in the then current controversy over Koch's claims to have used 'tuberculin' in the successful treatment of tuberculosis in man.

 

I have selected these diseases as they are now not part of our daily scene although all were commonplace in the life time of the man we are honouring this evening. Their control and their eradication became possible only because of the knowledge of their aetiology, pathology and epidemiology acquired by observation, deduc­tion, and experimentation.

 

When I received the invitation of the Society to deliver this Forty-third Stephen Paget Mem­orial Lecture, your Honorary Secretary suggested that I should speak from my veterinary experi­ence which from 1939 to 1972 was in the conduct of research or in its direction, especially on the subject of foot-and-mouth disease. Before referring specifically to examples from my experience which are pertinent to the subject of this lecture I should like to give you the back­ground to the veterinary research scene which is somewhat different from that of medical research.

 

BACKGROUND TO VETERINARY RESEARCH

 

I have read with great interest the Memorial Lecture delivered in 1972 by Lord Halsbury, the President of your Society, on the subject "Ethics and the exploitation of animals". In this he debated "Man's exploitation of animals as an alteration in the balance of Nature undertaken by him on his own authority and for his own advantage". Lord Halsbury was dealing more specifically with animal experimentation as part of such exploitation and he concluded that, from the standpoint of Ethics and Moral Philosophy, animal experimentation was justifiable, at least within the controls which we have established in this country. I am not competent to consider the subject of the ethics of Man's use of animals from the same intellectual point of view. I confess to recognising, in addition, a more utili­tarian approach in which the rearing of animals by Man is accepted whether it be for his food, his clothing, his other domestic necessities, his labour and his leisure. As a member of the veterinary profession. I am involved in a respon­sibility to allay the suffering of animals whether due to injury or disease, to combat animal disease, to collaborate with the medical profes­sion in the wide field of public health. My specific involvement is, to a great extent, with the search to improve systems of animal production as a component of the agricultural industry.

 

I have said on many occasions in many coun­tries that the greatest handicap to the efficiency of animal production is animal disease. I have been concerned throughout my career as a veterinary research worker to find ways in which to reduce the losses caused by disease. Having accepted animal production as being solely for the benefit of man I have not found it illogical to accept animal experimentation as a requirement to learn better how to combat animal disease. This does not mean that this approach is without regard for animal suffering, without regard for the ethics of the case and without regard for the responsibility which everyone of us has who deliberately subjects an animal to any type of experimental or assay procedure. This regard for moral values is specific in degree to each of us, but it would be unrealistic for many of us to avoid introducing a utilitarian background. Anyone who has attempted to obtain support for programmes of animal disease control or even to have research results applied on the farm will know what I mean. It is exceptional to be successful without having to demonstrate that there are economic benefits.

 

In speaking to you I am using my experience in the field of animal disease. There are so many differences in this veterinary scene to those of medicine and of biological research in general that it is necessary to describe some of them. It is only against this background that much of veterinary research should be judged.

 

Of the animals that Man has domesticated, those which he has put to his own use to the greatest extent are controlled in a way which probably few realise outside the very small pro­portion of our population that is engaged in agriculture. No animal is born, no animal mates, no animal conceives, no animal lives and no animal moves even from field to field other than by Man's decision. These are facts of great importance in veterinary public health and in veterinary research. Three points are especially significant. Firstly, veterinary research has an advantage compared with medical research in that the subject studied can itself be used as the animal for experiments. Secondly, as the rearing of farm livestock leads eventually to their pre­determined slaughter, a slaughter policy for the eradication of disease is an acceptable possibility. Thirdly, the fact that no animal moves from place to place except by man's decision has important consequences with regard to the epidemiology of disease and this fact is, of course, made use of in disease control.

 

FOOT-AND-MOUTH DISEASE RESEARCH

My first task as a young veterinary research worker some 35 years ago was to isolate the viruses causing outbreaks of foot-and-mouth disease in this country and to determine their immunological type. The methods employed, although simple, were time-consuming, insensi­tive, not especially reliable, costly, and hazardous with regard to disease security. One was entirely dependent on the use of susceptible animals;

guinea pies for routine work or cattle in special cases. The rapid, sensitive, quantitative, in vitro methods available today were still some years away. But even with the most modern virus typing tests it is only the antigen antibody reaction that is in vitro. The antibody can be produced only in living animals, but by the use of an inactivated or attenuated pathogen, these need not necessarily be subjected to the disease in question.

 

In those early days of the steam-age of virology, the virus assay of infective material or the titration of virus infectivity was to me, at least, so unacceptably crude that the techniques certainly did not warrant these descriptive titles which I have just used. It was again necessary to infect animals, this time by the inoculation of a series of increasing dilutions of the virus prepar­ation. Such was the lack of appreciation of quantitative methods, however, that the magni­tude of the possible error often meant that they had been sacrificed in vain. This problem occupied much of my attention with, of course, the realisation that greater reliability could be achieved only by increasing the number of animals used in each test: a paradoxical situa­tion from the point of view of the animals' welfare. For reasons dictated by the different responses of various hosts to the virus of foot-and-mouth disease. I was able to develop a technique which gave greatly increased accuracy without demanding any increase in the number of animals. The host specificity of the virus, as known then, made it necessary to use cattle in titrations of virus isolated from outbreaks of the disease in that species. The presence of virus in a preparation was detectable by the inoculation of susceptible cattle. For an acceptable degree of accuracy it was necessary to obtain 10 observ­ations, either positive or negative, for each of four dilutions of the infective material. The required number of observations could have been obtained by using one animal for each response, that is 40 cattle in each test. The economic impracticability of such an experimental design led to the development of a technique by which the required 40 observations were obtained from 2 cattle by the multiple inoculation of their tongues. This is a feasible procedure because the virus produces a limited local response in the superficial layer of the epithelium of the tongue within 24 to 48 hours of a discrete inoculation. The motivation to seek this solution was overridingly economic. Today, other methods are available in which cultures of cells are used instead of live animals.

 

The development of such assay procedures was required as part of the search for increasing­ly effective vaccines. In foot-and-mouth disease research with this objective the principal prob­lem has always been that of producing sufficient quantities of the virus which is used as the antigen to stimulate the immunological response in the vaccinated animals. In the early years of this work the only known means of obtaining the virus was by inoculating susceptible cattle from which the infective material was then collected at the height of the disease. The modern alterna­tive to this practice is. of course, to produce the virus in a tissue or cell culture system which although it may necessitate the slaughter of animals for the provision of the appropriate tissue, does not involve subjecting the animal to the disease. It is interesting to note in passing that the virus of foot-and-mouth disease in addition to being the first animal virus to be discovered (by Loeffler and Frosch inGermanyin 1897) was one of the first to be cultivated outside the living animal. This was achieved by Hecke inGermany(1930) and by the Maitlands in this country (1931). It was not until anti­biotics became freely available in the late 1940's that commercial-scale virus culture could be developed. The tissue that was used was the superficial epithelial layers of the bovine tongue which were obtained following the normal slaughter of cattle for beef. This technique was shown by Frenkel inHollandto be capable of large-scale exploitation provided antibiotics were used to control the bacterial and fungal contam­ination that was unavoidable in material of this origin. The use of this ''Frenkel" technique was progressively adopted during the 1950's. It was of some satisfaction to me that during the 9 years that I spent at the Pan-American Foot-and-Mouth Disease Center, the use of this technique completely supplanted the inoculation of cattle for the production of virus for the preparation of foot-and-mouth disease vaccine throughoutSouth America. This was partly due to the technical assistance programme of the Center and partly due to the commercial initiative of European and South American manufacturers of biological products. The number of laboratories still employing this technique is becoming fewer as during the 1960 s a stable cell line began to be used instead. This is the cell line developed from baby hamster kidneys by MacPherson and Stoker in 1962, adapted to large-scale suspension culture by Capstick, Telling, Chapman and Stewart and later developed into a commercial operation by the Wellcome Foundation Ltd. It was only when the inoculation of animals for the production of antigen could be replaced' by large-scale culture of virus that controlling foot-and-mouth disease became a possibility in the countries in which it is endemic. Such has been the success of campaigns based on vaccines prepared from culture virus that the prevalence of foot-and-mouth disease on the Continent of Europe has fallen dramatically during the last ten years. Also, serious attempts are being made to control the disease in other parts of the world. InSouth America, for example, the Inter-American Development Bank has invested some S80 million during the last ten years in support of control programmes. In this country we have benefited greatly, helped also by being more restrictive with regard to imports, and we are now in the seventh year of freedom from this catastrophic disease.

 

There remains, however, one unsatisfactory feature, namely, that the essential tests for vaccine potency still require that animals be subjected to the disease. How to avoid this is the subject of research in many countries, but no satisfactory substitute to using animals in one way or another for the testing of vaccines has yet been developed.

 

GENETIC CONTROL OF RESISTANCE TO DISEASE

I have referred earlier to animal disease as presenting the greatest handicap to the efficiency of animal production. It would also be true to say that diseases of plants can seriously restrict efforts to obtain higher yields from our crops. One of the principle ways in which this problem is solved is by the plant breeder introducing disease resistance in the development of new varieties. This procedure is feasable because resistance or susceptibility to many of the pathogens that attack plants is genetically controlled. The selec­tion intensity is increased artificially by rearing hundreds, perhaps thousands, of plants and detecting the resistant specimens by exposing all to the pathogen in question. Does this approach have any application in the long term control of animal disease? The principal difficulty to answering this question is our lack of knowledge about the genetic control of disease resistance or susceptibility in animals or, at least, in farm livestock. One is dealing with an interacting host/pathogen relationship in which degrees of resistance or susceptibility are recognised within the naturally susceptible animal species. I have already referred to the host specificity of a virus but this is something that can readily be altered by selection or modification of the pathogen without regard to the host. There are two diseases, however, in which the susceptibility or resistance of the host would appear to be under a greater degree of genetic control than is usual. These are scrapie of sheep and leukosis of poultry.

 

Scrapie of Sheep

Differences in susceptibility to scrapie between breeds of sheep have long been claimed, and the sheep farmer with scrapie in his flock has long recognised that there are differences in suscepti­bility between families within breeds. This folk­lore of the disease has been submitted to scientific investigation. A paper in press reports the results of selective breeding experiments which my former colleagues and I of the Agricultural Research Council's Institute for Research on Animal Diseases conducted, which demonstrate unequivocally the genetic control in sheep of susceptibility to scrapie. There is a high probability that the progeny of susceptible or resistant parents will themselves be susceptible or resistant respectively. Elimination of suscep­tible sheep from the breeding stock can be used to establish a predominantly resistant flock. The disadvantage of this approach is that the appearance of clinical signs is the only way known, so far, for the detection of susceptible sheep. The flock would have to be inoculated with the transmissible agent of scrapie, which would inevitably subject the susceptible individ­uals to the disease. Because of the economic benefits to be derived from the elimination of scrapie from the flocks of the more susceptible breeds, research is being directed towards finding other methods of detecting the susceptible animal. If this work is successful there would be less grounds for objection to this use in animals of the techniques of the plant breeder.

 

Avian leukosis

Susceptibility of poultry to avian leukosis is controlled by a single dominant gene and advan­tage can be taken of this to establish resistant flocks. The matings required to build up such flocks are determined by testing the progeny for resistance. This can be done without subjecting the birds to the disease. A related virus to that causing avian leukosis produces pocks on the membranes that envelop the chick embryo within the hen's egg. If the very simple technique of placing this virus on the membrane does not result in the development of these pocks, it can be concluded that the chicks hatched from the' other eggs of that particular mating will be resistant to the disease.

 

THE FUTURE

The control and eradication of the major plagues of farm livestock from theUnited Kingdomhas depended upon the elimination of infection usually by slaughter of all susceptible stock infected or exposed to infection. Severe restrictions on importation of animals, products of animal origin or other suspect materials are used to prevent reintroduction of infection. In my opinion, there is no acceptable alternative to this policy when dealing with the highly infec­tious diseases of animals which have the gravest economic consequences. This is true for this or any other country but a preliminary phase must frequently be accepted when a high prevalence must be progressively reduced by sanitary and prophylactic measures until it is economically acceptable to eradicate by slaughter.

 

During the last 20 years there has been a pro­gressive use of tissue or cell culture systems which has made the production of viral antigens for vaccine production less dependent upon the use of living animals. There is, as yet, no escape in many instances from using animals in the assay of vaccine potency, but 1 believe that with a greater knowledge of the biochemistry of patho­genesis and the biochemistry of immunology this dependence upon the susceptible animal will decrease. 1 also believe that the new techniques of genetic recombination will have much to offer if we can survive the hazards that are being pre­dicted if we tamper with these fundamentals of biological activity.

 

1 fear I may have given the impression that in veterinary research consideration is given to the animal in terms which are solely economic. This is true to a certain extent, but it is certainly not the full story. Cost benefit criteria are not the only factors to be considered in relation to the improvement of animal health. From the ethical, moral and animal welfare point of view every effort must be made to eliminate or control disease once the means for doing so are available.

 

VOTE OF THANKS

The President thanked Dr. Henderson for his excellent and informative Lecture, and called upon Dr. William Lane-Petter.UniversityofCambridgeand a Member of the Council of the Research Defence Society, to propose a vote of thanks to Dr. Henderson.

Dr. Lane-Petter said that nearly two years ago Dr. Henderson left the field of active research to become Secretary to the Agricultural Research Council, the first Veterinary Surgeon to hold this appointment. For nearly 33 years Dr. Henderson had been engaged in research on animal disease and many striking advances had taken place during this period, to which he had made his own in­valuable contributions. He had shown how ani­mal experiments had played an essential part in eradicating disease in animals. A humane thread was apparent throughout the whole lecture, and he drew attention to the reduction in numbers and the replacement of experimental animals used in medical and veterinary research. Dr. Lane-Petter hoped that many people would read the lecture when it was published in Con­quest because it had been in the great trad­ition of Stephen Paget Memorial Lectures. It recalled Stephen Paget s endeavours to educate the public about the fight against disease in both men and animals.


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