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Bacterial and Viral Disease
Brit. vet. ]. ( 1 g65),
121,
398
BACTERIAL AND VIRAL DISEASE By D. G.
HOWELL
Glaxo Laboratories Limited, Greenford
Prevention as one of the main aspects of veterinary medicine is likely to assume even greater importance as intensive systems of husbandry become more common. In the interests of economics and food production, the h ealth of a h erd or flock must far outweigh considerations involving the survival of the individual animal. Generally speaking, two methods of control are available. There is control by what are sometimes called sanitary methods, often implying a slaughter policy or the banning of animal movement into or out of particular territories, and then there is, perhaps more usually, the development of r esistance in the herd or flock by artificial means. The first method is well known and often highly efficacious, particularly when applied to territories with no land frontier across which animals may inadvertently stray or even be smuggled. Methods of this kind have been successfully applied in controlling some of the major epizootics and have succeeded in keeping from the United Kingdom such scourges as foot and mouth disease, rinderpest, blue tongue and contagious bovine pleuro-pneumonia, to name a few. Island countries such as Australia and New Zealand have succeeded in keeping out a large number of "animal plagues", notably amongst them Newcastle disease and swine fever. Generally, however, recourse has to be made to the use of artificial immunizing agents, because the viral or bact erial disease is enzootic. Eradication may be impracticable on economic grounds, and a balanced situation needs to be established between the organism concerned and its host, so that actual disease is either not provoked at all or, if it is, will be of little clinical or economic significance. The situation in the United Kingdom vis-a-vis Newcastle disease is probably the best modern example of this kind. The remarkable growth of the poultry industry in Britain in the past decade coupled with a high d egree of intensification resulted in a situation whereby the form er policy of slaughter and restriction of movement could not contain the problem; consequently, as a result of a government inquiry, a policy of vaccination was applied (Report of the Committee on Fowl Pest Policy, 1962). The success of this change of policy appears to have been fully confirmed, since Newcastle disease in the United Kingdom is probably now lower in incidence than it has been for a great many years. Both the farmer and the veterinary surgeon are coming to realize the important role of the latter as the conserver of the health and wealth of the "national flock", and this is particularly so when it comes to programmes of rational prophylaxis involving the use of vaccines. The fundamental principles of creating artificial immunity are classical, and reference to standard works, such as those of Topley & Wilson (1946), Bedson, Downie, MacCallum &
PREVENTIVE MEDICINE: BACTERIAL AND VIRAL DISEASE
399
Stuart-Harris (I 950), or Gray ( 1g64) will give the veterinary surgeon an adequate background to the subject. None the less, it is necessary for the practitioner in particular to realize the limits and scope of modern vaccine prophylaxis if he is to carry out his proper role, and the intention here is merely to highlight one or two salient points about immunization. Not all infective agents, whether viral or bacterial, lend themselves to immune-engendering procedures, and acquired immunity can assume any value from zero upwards. Immunity is seldom absolute; clearly, however the higher the level of protective antibody the better the chances of real protection, though it should be evident that a large enough dose of pathogenic organisms can overwhelm the animal's immune defences. A full realization of these facts must underlie any advice the practitioner gives to his client, since
400
BRITISH VETERINARY JOURNAL, I 2 I, 9
optimal at this time; often with this particular disease the natural stimulation of immunity will ensure its effectiveness thereafter. Perhaps one of the major difficulties in the successful creation of immunity by artificial means lies in the inability of the recipient to respond adequately owing to immunological immaturity or to the presence of existing antibody. Wolfe, Amin, Mueller & Aronson (1g6o), for example, made an interesting study of the chick to show its response to antigens in general. It is generally agreed that, after about seven to ten days of age, in the absence of maternal antibody, the chick will respond actively to the administration of vaccine, because protective immunity can thereafter be demonstrated. Wolfe and his colleagues, however, showed that chickens first injected at 20 days of age, although actively responding, failed. completely to yield a typical secondary response when revaccinated at six weeks of age. Their response was in fact no greater than that of similar birds vaccinated at six weeks of age for the first time. On the other hand, if the first vaccination was delayed to between six and twelve weeks of age, a typical secondary response was evoked at subsequent revaccination. The implications of this phenomenon, particularly in relation to the level of immunity existent in the bird destined to become adult, are obvious. No doubt a similar phenomenon occurs in other species. Maternal antibody is perhaps one of the most important complicating factors in successful vaccination; certainly it is of considerable practical significance in vaccination against Newcastle disease and canine distemper. As vaccination programmes become more widely adopted, the general level of immunity in a given population will progressively rise. In direct consequence more young animals will tend to be born with substantial levels of specific antibody acquired from the colostrum of their dam, and this will mean that the time at which vaccination should be undertaken needs to be extended. This applies particularly when the vaccine is live, since to exert its antigenic stimulus it has first to multiply in the recipient. If passively acquired maternal antibody prevents this happening, the ensuing antigenic stimulus will be virtually cancelled. Although more difficult to appreciate, it is none the less true that this phenomenon occurs when some dead vaccines are given as early in life as possible; this has been clearly illustrated for first vaccination of the young chick against Newcastle disease with dead vaccine (Keeble, Box & Christie, 1963; Box, 1965). Vaccines are one of the most important weapons available to preventive veterinary medicine. Although the fundamental principles of their use have long been established, continued study is necessary, both in the laboratory and the field, to ensure their most effective use against the background of the everchanging pattern of husbandry. REFERENCES
BEnsoN, S. P., DowNIE, A W., MAcCALLUM, F. 0. & STUART-HARRIS, C. H. (I950). Virus and Rickettsial Diseases. London: Edward Arnold. Box, P. G. (I965). Vet. Rec., 77, 246. GRAY, D. G. ( I964). Immunology. London : Edward Arnold.
PREVENTIVE MEDICINE: BACTERIAL AND VIRAL DISEASE
40I
KEEBLE, S. A., Box, P. G. & CHRISTIE, D. W. ( Ig63). Vet. R ec., 75, 151. REPORT OF THE CoMMITTEE ON FowL PEST Poucy (1962). London: H.M.S.O. TOPLEY, W . W. C. & WILSON, G . S. (1946). Principles of Bacteriology and Immunity, ed . Wilson & Miles. Vol. 2. London: Edward Arnold. WoLFE, H. R., AM1N, A., MuELLER, A. P. & ARONSON, F. R . ( Ig6o). Int. Arch.Allergy , 17, 106. (R eceived for publication 30 July, 1965)
121,
398
BACTERIAL AND VIRAL DISEASE By D. G.
HOWELL
Glaxo Laboratories Limited, Greenford
Prevention as one of the main aspects of veterinary medicine is likely to assume even greater importance as intensive systems of husbandry become more common. In the interests of economics and food production, the h ealth of a h erd or flock must far outweigh considerations involving the survival of the individual animal. Generally speaking, two methods of control are available. There is control by what are sometimes called sanitary methods, often implying a slaughter policy or the banning of animal movement into or out of particular territories, and then there is, perhaps more usually, the development of r esistance in the herd or flock by artificial means. The first method is well known and often highly efficacious, particularly when applied to territories with no land frontier across which animals may inadvertently stray or even be smuggled. Methods of this kind have been successfully applied in controlling some of the major epizootics and have succeeded in keeping from the United Kingdom such scourges as foot and mouth disease, rinderpest, blue tongue and contagious bovine pleuro-pneumonia, to name a few. Island countries such as Australia and New Zealand have succeeded in keeping out a large number of "animal plagues", notably amongst them Newcastle disease and swine fever. Generally, however, recourse has to be made to the use of artificial immunizing agents, because the viral or bact erial disease is enzootic. Eradication may be impracticable on economic grounds, and a balanced situation needs to be established between the organism concerned and its host, so that actual disease is either not provoked at all or, if it is, will be of little clinical or economic significance. The situation in the United Kingdom vis-a-vis Newcastle disease is probably the best modern example of this kind. The remarkable growth of the poultry industry in Britain in the past decade coupled with a high d egree of intensification resulted in a situation whereby the form er policy of slaughter and restriction of movement could not contain the problem; consequently, as a result of a government inquiry, a policy of vaccination was applied (Report of the Committee on Fowl Pest Policy, 1962). The success of this change of policy appears to have been fully confirmed, since Newcastle disease in the United Kingdom is probably now lower in incidence than it has been for a great many years. Both the farmer and the veterinary surgeon are coming to realize the important role of the latter as the conserver of the health and wealth of the "national flock", and this is particularly so when it comes to programmes of rational prophylaxis involving the use of vaccines. The fundamental principles of creating artificial immunity are classical, and reference to standard works, such as those of Topley & Wilson (1946), Bedson, Downie, MacCallum &
PREVENTIVE MEDICINE: BACTERIAL AND VIRAL DISEASE
399
Stuart-Harris (I 950), or Gray ( 1g64) will give the veterinary surgeon an adequate background to the subject. None the less, it is necessary for the practitioner in particular to realize the limits and scope of modern vaccine prophylaxis if he is to carry out his proper role, and the intention here is merely to highlight one or two salient points about immunization. Not all infective agents, whether viral or bacterial, lend themselves to immune-engendering procedures, and acquired immunity can assume any value from zero upwards. Immunity is seldom absolute; clearly, however the higher the level of protective antibody the better the chances of real protection, though it should be evident that a large enough dose of pathogenic organisms can overwhelm the animal's immune defences. A full realization of these facts must underlie any advice the practitioner gives to his client, since
400
BRITISH VETERINARY JOURNAL, I 2 I, 9
optimal at this time; often with this particular disease the natural stimulation of immunity will ensure its effectiveness thereafter. Perhaps one of the major difficulties in the successful creation of immunity by artificial means lies in the inability of the recipient to respond adequately owing to immunological immaturity or to the presence of existing antibody. Wolfe, Amin, Mueller & Aronson (1g6o), for example, made an interesting study of the chick to show its response to antigens in general. It is generally agreed that, after about seven to ten days of age, in the absence of maternal antibody, the chick will respond actively to the administration of vaccine, because protective immunity can thereafter be demonstrated. Wolfe and his colleagues, however, showed that chickens first injected at 20 days of age, although actively responding, failed. completely to yield a typical secondary response when revaccinated at six weeks of age. Their response was in fact no greater than that of similar birds vaccinated at six weeks of age for the first time. On the other hand, if the first vaccination was delayed to between six and twelve weeks of age, a typical secondary response was evoked at subsequent revaccination. The implications of this phenomenon, particularly in relation to the level of immunity existent in the bird destined to become adult, are obvious. No doubt a similar phenomenon occurs in other species. Maternal antibody is perhaps one of the most important complicating factors in successful vaccination; certainly it is of considerable practical significance in vaccination against Newcastle disease and canine distemper. As vaccination programmes become more widely adopted, the general level of immunity in a given population will progressively rise. In direct consequence more young animals will tend to be born with substantial levels of specific antibody acquired from the colostrum of their dam, and this will mean that the time at which vaccination should be undertaken needs to be extended. This applies particularly when the vaccine is live, since to exert its antigenic stimulus it has first to multiply in the recipient. If passively acquired maternal antibody prevents this happening, the ensuing antigenic stimulus will be virtually cancelled. Although more difficult to appreciate, it is none the less true that this phenomenon occurs when some dead vaccines are given as early in life as possible; this has been clearly illustrated for first vaccination of the young chick against Newcastle disease with dead vaccine (Keeble, Box & Christie, 1963; Box, 1965). Vaccines are one of the most important weapons available to preventive veterinary medicine. Although the fundamental principles of their use have long been established, continued study is necessary, both in the laboratory and the field, to ensure their most effective use against the background of the everchanging pattern of husbandry. REFERENCES
BEnsoN, S. P., DowNIE, A W., MAcCALLUM, F. 0. & STUART-HARRIS, C. H. (I950). Virus and Rickettsial Diseases. London: Edward Arnold. Box, P. G. (I965). Vet. Rec., 77, 246. GRAY, D. G. ( I964). Immunology. London : Edward Arnold.
PREVENTIVE MEDICINE: BACTERIAL AND VIRAL DISEASE
40I
KEEBLE, S. A., Box, P. G. & CHRISTIE, D. W. ( Ig63). Vet. R ec., 75, 151. REPORT OF THE CoMMITTEE ON FowL PEST Poucy (1962). London: H.M.S.O. TOPLEY, W . W. C. & WILSON, G . S. (1946). Principles of Bacteriology and Immunity, ed . Wilson & Miles. Vol. 2. London: Edward Arnold. WoLFE, H. R., AM1N, A., MuELLER, A. P. & ARONSON, F. R . ( Ig6o). Int. Arch.Allergy , 17, 106. (R eceived for publication 30 July, 1965)