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Rinderpest: A resume of recent progress in East Africa
405
RINDERPEST. :
RINDERPEST:
A
,
RESUME OF RECENT
PROGRESS IN EAST AFRICA
By R. DAUBNEY, l)irectar uf Veterinary Services, Kenya Colony.
AI,THOUCU it is not strictly speaking an entirely tropical or suhtropical disease, economically rinderpest is undouhtedly the most important disease in tropical Africa and Asia, and the communication by Curasson and Delpy, 1926, of their success in applying the inactivated tissue-vaccine method of immunisation marked the opening of a new era in the control of this disease. The fact that Kakisaki (1918) in Japan and Boynton (1918) in the Philippines had been employing this method of immunisation for some time was unknown to most workers, and Curasson's publication was the first on this subject to appear in Western literature. In East Africa Hornby (1928) and Daubney (1928) published reports of studies on the rinderpest virus, the latter including records of experimental and field immunisations by the tissue-vaccine method. Since that time a good deal of work has been carried out on tissue vaccination against rinderpest in East Africa, and a new method of control of the disease has been evolved from the early laboratory experiments. It has been shown that amounts of inactivated spleen tissue as small as 1 gramme may be large enough to protect high-grade cattle against subsequent inoculation of from 2,000 to 5,000 minimal infective doses of virus, when the immunising tissue, taken from a reacting animal of high susceptibility, has a virus content of from 20,000 to 50,000 minimal infective doses per gramme, and that a higher degree of immunity is evoked if even this small dose is divided into two halves which are given at an interval of from 7 to 14 days. Following a single injection of vaccine containing 4 grammes of spleen tissue, immunity will begin to wane after about 15 months; but when a total of 6 grammes of tissue is given in two weekly doses immunity may persist for more than 2 years. When the same quantity of tissue is given in three equal weekly injections a few animals have been found to retain an almost complete immunity 4 years and :1 months later (Daubney, 1934 and unpublished results). It is our experience that hy this method the more susceptible breeds of cattle (e.g., European breeds) are more readily immunised than more resistant breeds. A further point of interest is that the immunity provoked by a course of three injections of vaccine can be reinforced and maintained at a high level by single injections of vaccine repeated each year. The figures relating to the duration of immunity apply to adult animals only; they are less applicable to yearling animals, and not at all to calves, which appear rapidly to outgrow their immunity. The last remark refers equally to calves born of susceptible mothers and to the progeny of
40f)
JUBILEE NUMBER.
immune cows, and is quite independent of any passive immunity that may have been transferred to the calf via the milk. As a method of state control vaccination has been in force over an area of some 7,000 to 8,000 square miles in Kenya during a period of nine years. The use of vaccine has been entirely voluntary, and most farmers have neglected to vaccinate their cattle until rinderpest has appeared in the district. In spite of this, and in spite of the annual use of virus in the immunisation of cattle on certain farms in the northern part of the area, there were only 49 outbreaks in the area during the 4 years 19~~3<~6, and during 11 months of 19~~7 there have been seven outbreaks only, of which six had a common origin. The area in question encloses extensive forest and mountain ranges inhabited by buffalo and other large ruminant game, but in the absence of infection in the cattle the game in the centre of the area has entirely escaped infection, although twice the disease has been introduced by buffalo with the creation of numerous foci of infection within and near the periphery of the area. It is quite evident to anyone who has watched the progress of events in this field experiment that rinderpest can be eradicated from any closely-settled area by compulsory annual vaccination and simple control of outbreaks. The vaccine-virus method of immunisation has been developed from the original experiments with slightly infective vaccines. Daubney (1933) has published records of long-term immunity tests made on animals vaccinated (a) with a single dose of infective vaccine (formalin 1/1,500) and (b) with a dose of completely inactivated vaccine followed by a dose of infective vaccine. The testing of random samples drawn at intervals of 12! to 15, ~~2 and 6~~ months showed that method (b) evoked a significantly higher degree of immunity, and that by this method animals might be protected against severe reaction ancl!or death for periods in excess of 5 years. Further experiments made in 1932 to 1935 on a group of 1,200 cattle showed that if one desires to immunise animals permanently by an injection of vaccine followed by an injection of virus, it is necessary to provoke a high percentage of reactions to the virus inoculation. The desired result can be achieved by balancing the doses of virus and vaccine, but the more certain method is to employ a fairly large dose of vaccine and a large vaccinating dose of virus. This is the practice that has largely been followed for a number of years in certain parts of Kenya Colony. It is to be emphasised that so far no entirely risk-free method of provoking a permanent immunity against rinderpest, which at the same time is 100 per cent. effective, has yet been discovered, and that it is unlikely that such a method ever will he available. If one is determined that no single animal shall escape complete immunisation one must narrow the margin of safety either by decreasing the protective dose of serum or vaccine, or by increasing the infective dose of virus quantitatively or qualitatively to the point at which some mortality will probably occur: one works in fact on a threshold level with all its attendant risks.
RINDERPEST.
407
In the search for an inexpensive infective method of immunisation attention has been centred on possible methods of producing a variant of attenuated virulence for bovines. Schein (1924, 1926) and Edwards (1929) were the first to revive interest in the method of serial goat passage as a possible means of attenuation, and Edwards' attenuated goat virus (Muktesar) has been extensively employed in the immunisation (without serum or other passive protective agent) of the more resistant Indian plains cattle. Apparently the Muktesar strain showed evidence of attenuation for bovines after as little as :~o passages, but the field work carried out in India subsequent to Edwards' laboratory experiments has until recently been insufficiently documented. In Kenya we have been able to compare the virulence of the Muktesar virus with our standard Kabete bovine strain, and with an attenuated variant of this strain produced by goat passage. Inoculated alone to grade and native cattle in Kenya the l\luktesar virus has provoked a mortality of approximately 18 per cent. Reactions and mortality are, however, easily controllable by the use of a small simultaneous dose of serum or a previous dose of vaccine. In a recent experiment 100 cattle were inoculated with vaccine (1 gramme of tissue) and 14 days later with 2 C.c. of a 2 per cent. suspension of infected goat spleen tissue. Eighty-five of the animals reacted to the goat-virus inoculation, and although there was a noticeable loss of condition, no mortality occurred. These animals completely resisted a further virus inoculation-this time with 2 C.c. of fullv virulent 10 per cent. bovine spleen suspension given 2 months later~ Nine of the animals failed to react to the goat virus, three animals failed to react to either of the virus inoculations, and three reacted to the goat virus and subsequently exhibited doubtful reactions to· the bovine virus. There was no mortality. Our general experience is that animals that have reacted to the Indian goat-virus-whether partially protected by passive means or not-do not again react to our standard Kabete virus; but in a recent test of 22 animals double-inoculated with goat virus and serum, five animals that had reacted to the double-inoculation reacted (three of them clinically) to a test dose of bovine virus given 27 days later. It is just possible that these unexpected reactions may in reality be exacerbations of unusually prolonged reactions from the first inoculation, and that they are caused by the second inoculation of virus having been administered too soon. The chief apparent disadvantage of goat virus is the variability of goat spleens in respect of virus content; some of the spleens contain more than 50,000 minimal infective doses for cattle per gramme, while others may fail to induce reactions in 100 per cent. of inoculated animals when a dose of 0·02 gramme is employed. This difficulty can be countered by the pooling of several spleens and the testing of a bulked batch of suspension, provided a satisfactory method of preservation can be worked out, and we propose to attempt the storage of frozen desiccated tissue. The spleen of an ox, killed at the height of its
408
JUBILEE NUMBER.
reaction to Muktesar goat virus, contained more than 50,000 minimal infective doses of virus, and the virus appeared to be unchanged so far as its virulence for bovines was concerned: here is another promising source of material for vaccinations. A recent series of passages of the stock Kabete bovine strain of virus in goats has given interesting results. The virus was rapidly adapted to the goat and soon reached the stage of causing 100 per cent. mortality in this species. Taken back to cattle at the 12th, 18th, 24th, 31st, 41st and 48th passages it produced death in 9, 8, 8, 7, 8 and 9 days respectively. From the 69th to the 80th passages four inoculated cattle died in 16, 14, 12 and 13 days. At the 85th passage the first recovery occurred, and from then on to the 150th passage 15 animals recovered and only two died. There was a gradual suppression of the more serious symptoms from the 85th passage onwards-mouth lesions became transient and finally disappeared, conjunctivitis was next greatly reduced, the nasal discharge became less profuse and anorexia was observed for 2 or :3 days only. The last symptom to disappear was the diarrhcea, but the reduction in severity of the rinderpest reaction was accompanied by an increase in the tendency of animals to break down to coccidiosis in the later stages of the reaction. In the later passages pharyngitis is absent, gastritis greatly reduced in severity, and what lesions are present in the alimentary tract are in the main confined to the large intestine. In view of our ability to produce variants of the virus by prolonged serial passage in goats, it seems highly probable that similar variants may also be produced by serial passage in tissue culture, particularly if the embryonic tissues of species normally insusceptible or only slightly susceptible to the infection are employed. In two series of observations at Kabete the virus has been carried through three, four and five passages in bovine, caprine and rabbit tissues, and there is little doubt that in the near future it will be successfully maintained in cultures over a long series of passages. It is in this direction that we should look for a cheap infective form of vaccine which will enable governments to effect eradication of rinderpest from less closely administered countries by universal inoculation of stock over a period of years. The virus of rinderpest does not appear to be pantropic and all attempts to produce a neurotropic variant have failed. Rabbits, mice, guinea-pigs and goats have been used in these attempts, and an extensive series of inoculations has been made in the last-named species employing the technique of Mackenzie, Findlay and Stern, in which the inoculated animal is passively protected against visceral multiplication of the virus by an injection of hyperimmune serum. The virus has only rarely survived the few days occupied by the first intracerebral passage and has never been detected at the second passage.
RINDERPEST.
409
REFERENCES.
Boynton, W. H. (1918). Philippine J. Sci., 39. Curasson, G., and Delpy, L. (1926). Bull. Soc. Central Vet., 297. Daubney, R. (1928). J. Compo Path. & Ther., 41, 228. (1929). Bull. No.8, Dept. Agric. Nairobi, Kenya, 1934, 1935, 1936. -~. (1933-6). Ann. Reps. Dept. Agric., Kenya. - - . (1936). Rep. Conf. on Co-ordination Vety. Research, Kabete, 24. Hornby, H. E. (1928). J. Compo Path. fJ! Ther., 41,17. Kakisaki, C. (1918). Kitasato Arch. Exp. iVIed., 139. Mackenzie, R. D., Findlay, G. M., and Stern, R. O. (1936). Brit. J. Exp. Path., 17, 352.
RINDERPEST. :
RINDERPEST:
A
,
RESUME OF RECENT
PROGRESS IN EAST AFRICA
By R. DAUBNEY, l)irectar uf Veterinary Services, Kenya Colony.
AI,THOUCU it is not strictly speaking an entirely tropical or suhtropical disease, economically rinderpest is undouhtedly the most important disease in tropical Africa and Asia, and the communication by Curasson and Delpy, 1926, of their success in applying the inactivated tissue-vaccine method of immunisation marked the opening of a new era in the control of this disease. The fact that Kakisaki (1918) in Japan and Boynton (1918) in the Philippines had been employing this method of immunisation for some time was unknown to most workers, and Curasson's publication was the first on this subject to appear in Western literature. In East Africa Hornby (1928) and Daubney (1928) published reports of studies on the rinderpest virus, the latter including records of experimental and field immunisations by the tissue-vaccine method. Since that time a good deal of work has been carried out on tissue vaccination against rinderpest in East Africa, and a new method of control of the disease has been evolved from the early laboratory experiments. It has been shown that amounts of inactivated spleen tissue as small as 1 gramme may be large enough to protect high-grade cattle against subsequent inoculation of from 2,000 to 5,000 minimal infective doses of virus, when the immunising tissue, taken from a reacting animal of high susceptibility, has a virus content of from 20,000 to 50,000 minimal infective doses per gramme, and that a higher degree of immunity is evoked if even this small dose is divided into two halves which are given at an interval of from 7 to 14 days. Following a single injection of vaccine containing 4 grammes of spleen tissue, immunity will begin to wane after about 15 months; but when a total of 6 grammes of tissue is given in two weekly doses immunity may persist for more than 2 years. When the same quantity of tissue is given in three equal weekly injections a few animals have been found to retain an almost complete immunity 4 years and :1 months later (Daubney, 1934 and unpublished results). It is our experience that hy this method the more susceptible breeds of cattle (e.g., European breeds) are more readily immunised than more resistant breeds. A further point of interest is that the immunity provoked by a course of three injections of vaccine can be reinforced and maintained at a high level by single injections of vaccine repeated each year. The figures relating to the duration of immunity apply to adult animals only; they are less applicable to yearling animals, and not at all to calves, which appear rapidly to outgrow their immunity. The last remark refers equally to calves born of susceptible mothers and to the progeny of
40f)
JUBILEE NUMBER.
immune cows, and is quite independent of any passive immunity that may have been transferred to the calf via the milk. As a method of state control vaccination has been in force over an area of some 7,000 to 8,000 square miles in Kenya during a period of nine years. The use of vaccine has been entirely voluntary, and most farmers have neglected to vaccinate their cattle until rinderpest has appeared in the district. In spite of this, and in spite of the annual use of virus in the immunisation of cattle on certain farms in the northern part of the area, there were only 49 outbreaks in the area during the 4 years 19~~3<~6, and during 11 months of 19~~7 there have been seven outbreaks only, of which six had a common origin. The area in question encloses extensive forest and mountain ranges inhabited by buffalo and other large ruminant game, but in the absence of infection in the cattle the game in the centre of the area has entirely escaped infection, although twice the disease has been introduced by buffalo with the creation of numerous foci of infection within and near the periphery of the area. It is quite evident to anyone who has watched the progress of events in this field experiment that rinderpest can be eradicated from any closely-settled area by compulsory annual vaccination and simple control of outbreaks. The vaccine-virus method of immunisation has been developed from the original experiments with slightly infective vaccines. Daubney (1933) has published records of long-term immunity tests made on animals vaccinated (a) with a single dose of infective vaccine (formalin 1/1,500) and (b) with a dose of completely inactivated vaccine followed by a dose of infective vaccine. The testing of random samples drawn at intervals of 12! to 15, ~~2 and 6~~ months showed that method (b) evoked a significantly higher degree of immunity, and that by this method animals might be protected against severe reaction ancl!or death for periods in excess of 5 years. Further experiments made in 1932 to 1935 on a group of 1,200 cattle showed that if one desires to immunise animals permanently by an injection of vaccine followed by an injection of virus, it is necessary to provoke a high percentage of reactions to the virus inoculation. The desired result can be achieved by balancing the doses of virus and vaccine, but the more certain method is to employ a fairly large dose of vaccine and a large vaccinating dose of virus. This is the practice that has largely been followed for a number of years in certain parts of Kenya Colony. It is to be emphasised that so far no entirely risk-free method of provoking a permanent immunity against rinderpest, which at the same time is 100 per cent. effective, has yet been discovered, and that it is unlikely that such a method ever will he available. If one is determined that no single animal shall escape complete immunisation one must narrow the margin of safety either by decreasing the protective dose of serum or vaccine, or by increasing the infective dose of virus quantitatively or qualitatively to the point at which some mortality will probably occur: one works in fact on a threshold level with all its attendant risks.
RINDERPEST.
407
In the search for an inexpensive infective method of immunisation attention has been centred on possible methods of producing a variant of attenuated virulence for bovines. Schein (1924, 1926) and Edwards (1929) were the first to revive interest in the method of serial goat passage as a possible means of attenuation, and Edwards' attenuated goat virus (Muktesar) has been extensively employed in the immunisation (without serum or other passive protective agent) of the more resistant Indian plains cattle. Apparently the Muktesar strain showed evidence of attenuation for bovines after as little as :~o passages, but the field work carried out in India subsequent to Edwards' laboratory experiments has until recently been insufficiently documented. In Kenya we have been able to compare the virulence of the Muktesar virus with our standard Kabete bovine strain, and with an attenuated variant of this strain produced by goat passage. Inoculated alone to grade and native cattle in Kenya the l\luktesar virus has provoked a mortality of approximately 18 per cent. Reactions and mortality are, however, easily controllable by the use of a small simultaneous dose of serum or a previous dose of vaccine. In a recent experiment 100 cattle were inoculated with vaccine (1 gramme of tissue) and 14 days later with 2 C.c. of a 2 per cent. suspension of infected goat spleen tissue. Eighty-five of the animals reacted to the goat-virus inoculation, and although there was a noticeable loss of condition, no mortality occurred. These animals completely resisted a further virus inoculation-this time with 2 C.c. of fullv virulent 10 per cent. bovine spleen suspension given 2 months later~ Nine of the animals failed to react to the goat virus, three animals failed to react to either of the virus inoculations, and three reacted to the goat virus and subsequently exhibited doubtful reactions to· the bovine virus. There was no mortality. Our general experience is that animals that have reacted to the Indian goat-virus-whether partially protected by passive means or not-do not again react to our standard Kabete virus; but in a recent test of 22 animals double-inoculated with goat virus and serum, five animals that had reacted to the double-inoculation reacted (three of them clinically) to a test dose of bovine virus given 27 days later. It is just possible that these unexpected reactions may in reality be exacerbations of unusually prolonged reactions from the first inoculation, and that they are caused by the second inoculation of virus having been administered too soon. The chief apparent disadvantage of goat virus is the variability of goat spleens in respect of virus content; some of the spleens contain more than 50,000 minimal infective doses for cattle per gramme, while others may fail to induce reactions in 100 per cent. of inoculated animals when a dose of 0·02 gramme is employed. This difficulty can be countered by the pooling of several spleens and the testing of a bulked batch of suspension, provided a satisfactory method of preservation can be worked out, and we propose to attempt the storage of frozen desiccated tissue. The spleen of an ox, killed at the height of its
408
JUBILEE NUMBER.
reaction to Muktesar goat virus, contained more than 50,000 minimal infective doses of virus, and the virus appeared to be unchanged so far as its virulence for bovines was concerned: here is another promising source of material for vaccinations. A recent series of passages of the stock Kabete bovine strain of virus in goats has given interesting results. The virus was rapidly adapted to the goat and soon reached the stage of causing 100 per cent. mortality in this species. Taken back to cattle at the 12th, 18th, 24th, 31st, 41st and 48th passages it produced death in 9, 8, 8, 7, 8 and 9 days respectively. From the 69th to the 80th passages four inoculated cattle died in 16, 14, 12 and 13 days. At the 85th passage the first recovery occurred, and from then on to the 150th passage 15 animals recovered and only two died. There was a gradual suppression of the more serious symptoms from the 85th passage onwards-mouth lesions became transient and finally disappeared, conjunctivitis was next greatly reduced, the nasal discharge became less profuse and anorexia was observed for 2 or :3 days only. The last symptom to disappear was the diarrhcea, but the reduction in severity of the rinderpest reaction was accompanied by an increase in the tendency of animals to break down to coccidiosis in the later stages of the reaction. In the later passages pharyngitis is absent, gastritis greatly reduced in severity, and what lesions are present in the alimentary tract are in the main confined to the large intestine. In view of our ability to produce variants of the virus by prolonged serial passage in goats, it seems highly probable that similar variants may also be produced by serial passage in tissue culture, particularly if the embryonic tissues of species normally insusceptible or only slightly susceptible to the infection are employed. In two series of observations at Kabete the virus has been carried through three, four and five passages in bovine, caprine and rabbit tissues, and there is little doubt that in the near future it will be successfully maintained in cultures over a long series of passages. It is in this direction that we should look for a cheap infective form of vaccine which will enable governments to effect eradication of rinderpest from less closely administered countries by universal inoculation of stock over a period of years. The virus of rinderpest does not appear to be pantropic and all attempts to produce a neurotropic variant have failed. Rabbits, mice, guinea-pigs and goats have been used in these attempts, and an extensive series of inoculations has been made in the last-named species employing the technique of Mackenzie, Findlay and Stern, in which the inoculated animal is passively protected against visceral multiplication of the virus by an injection of hyperimmune serum. The virus has only rarely survived the few days occupied by the first intracerebral passage and has never been detected at the second passage.
RINDERPEST.
409
REFERENCES.
Boynton, W. H. (1918). Philippine J. Sci., 39. Curasson, G., and Delpy, L. (1926). Bull. Soc. Central Vet., 297. Daubney, R. (1928). J. Compo Path. & Ther., 41, 228. (1929). Bull. No.8, Dept. Agric. Nairobi, Kenya, 1934, 1935, 1936. -~. (1933-6). Ann. Reps. Dept. Agric., Kenya. - - . (1936). Rep. Conf. on Co-ordination Vety. Research, Kabete, 24. Hornby, H. E. (1928). J. Compo Path. fJ! Ther., 41,17. Kakisaki, C. (1918). Kitasato Arch. Exp. iVIed., 139. Mackenzie, R. D., Findlay, G. M., and Stern, R. O. (1936). Brit. J. Exp. Path., 17, 352.