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Immunization to control East Coast Fever
4
Parasitology Today, vol, 3, no. I, 1987
Control of East Coast Fever East Coast Fever (ECF) (Box I) is a disease of cattle in East Africa caused by species of Theileria transmitted by the brown ear tick Rhipicephalus appendiculatus (Fig. 3). In the past, E C F has been controlled by reducing tick infestations through strict short-interval acaricide application, adherence to legislation on cattle movements and quarantine, and good livestock and pasture management. However, civil unrest, illegal cattle movement, poor management and inadequate maintenance of acaricidal dips and sprayraces, acaricide resistance and the high cost of acaricides, have all combined to make this approach to E C F control less reliable. An alternative approach is through immunization. Cattle which recoverfrom the disease have a solid immunity to homologous challenge, and this has encouraged the search for a vaccine since the early 1900s. Two methods of active immunization have been extensively explored; immunization with schizont infected cells is not yet practicable, but immunization by infection followed by drug treatment, has great potential for immediate use. In this debate, Tom Dolan discusses the immunization approach to E C F control, while Roger Tatchell explains how tick control can contribute to this strategy.
I m m u n i z a t i o n to Control East Coast Fever T.T. Dolan International Laboratory for Researchon Animal Diseases(ILRAD) P© Box 30709 Nairobi, Kenya
Early attempts to immunize cattle against ECF were based on the inoculation of spleen and lymph homogenates from confarmed ECF cases using a method devised by Theiler2. In the period 1911-1914, this approach was used to immunize 283 000 cattle in the Transkei territories of South Africa 3. In the original laboratory investigation 2, this method killed 25% of treated cat-
fie but 60% of the survivors were immune. However, no treatment was then available for animals that became clinically ill, and the inoculation procedure was difficult because of the need to use the immunizing material immediately after harvesting from sick animals. This approach was therefore abandoned. Successful in vitro cultivation of T.
© 1 9 8 7 E sevpe~Science Publishers BV., Amsterdam 0169-4758/87/$0200
Parasitotog/ Today, voL 3, no. I, 1987
5
annulata, the cause of mediterranean theileriosis, revived interest in immunization with infected lymphoid cells. Passageattenuated cultures of this parasite were effective in immunization with low numbers (about 104) of cells4. The successful cultivation of T. parva infected lymphoid cells 5 was followed by exhaustive testing of dose, passage attenuation and inactivation 6,7. The results showed that killed material did not immunize, while high numbers of living cells were required to infect cattle reliably, and the results of immunization were often unpredictable. Unlike T. annulata, T. parva schizonts transferred slowly from donor to recipient cells, and recipient animals rejected incompatible donor cells s. It is still possible that a universally acceptable cell may be discovered or produced by manipulation, or that a cell line from which more frequent transfer of parasites occurs will be found. If any of these advances are made, immunization with T. parva-infected cells could become a practical possibility. Infection and Treatment Immunization by ird~ction and treatment is usually based on the inoculation of cryopreserved suspensions of T. parva sporozoites from triturated R. appendiculatus ticks, followed by administration of tetracyclines or parvaquone to control the infection in recipient cattle 9-12. The technical advances that made tllis method practical were the cryopreservation of Theileria sporozoites as stabilates 13, and the control of the infections they induced by a limited number of tetracycline m~atments (Box 2). Theileria parva isolates from various parts of East Africa were used in a series of crossimmunity studies using infection and treatment which revealed the antigenic complexity of field challenge 11. The breakdown of immunity engendered by single isolates led to the selection of three isolates which, when combined, could be controlled by a single treatment with long-acting tetracycline at the time of ~fection and which provided a broad J.mIrlunJty 9,11,17. This combination of isolates ('the Muguga cocktail') included T. p. parva (Muguga), T. p. parva (Kiambu 5), and T. p. lawrencei (Serengeti-transformed - a parasite isolated from a buffalo in Tanzania and passaged through ticks and cattle). Since these original studies in the 1970s many trials have been undertaken and there are two approaches to the use of infection and treatment immunization in the field. One recommends the use of the 'Muguga cocktail'
or another combination produced in a central facility for national or international use. The other recommends the use of local parasite isolates taken from the region where cattle are to be immunized. The advantages of the cocktail approach are that the parasites are of known character and drug responsiveness; they provide a broad protection and can be supplied for use in areas or countries unable to produce or store their own stabilates. A disadvantage is that the cocktail may not protect against all challenge in the field, particularly against T. p. /awrence/. In areas where breakthrough infections occur, these parasites will have to be isolated and added to the cocktail, tailoring it to specific needs. The use of local parasite isolates for immunization requires effective laboratory support because each isolate or combination of isolates (from tick collections in the field, bait cattle or captive buffalo) has to be assessed for infectivity and drug responsiveness. Not all isolates can be controlled by a single treatment with long-acting tetracycline at the time of infection (Box 1). Each testing step is expensive and time consuming. In addition, the definition of what constitutes a local strain requires care, because 'local' can refer to just one farm or to a much larger region. Carrier states (persistent tick-transmissible infections) are known to occur following infection with T. p. lawrencei is and recent studies have established that this is usually the case with T. p. parva 19,2°. It is
Technical advances in cryopreservation of Theileria sporozoites, tetracycline treatment, and in vitro culture of schizont infected cells for immunization, were achieved during the UNDP FAO programme (I 96776) at the East African Veterinary Research Organization under the leadership of the late Dr M.P. Cunningham.
6
Parasitology Today, vot. 3, no. I, 1987
understanding of the molecular biology of parasites and of the immune responses of cattle, encourages the hope that immunogens on theilerial sporozoites, intralymphocytic schizonts, or infected cell membranes may be identified and prod u c e d synthetically. But experience so far suggests that infection has to be established in the host to provide protection. Perhaps by better antigen presentation, the use of adjuvants, or by manipulation of the quality or duration of the immune response, it may be possible to use these immunogens effectively.
Fig. 2. The distribution of ECF in East Africa matches that of its vector
Rhipicephalus appendiculatus.
~'~ R appendtculatus
Field Application probable that most cattle immunized by infection and treatment will become cartiers. This should not complicate immunization with local isolates because no new parasites are being introduced into an area. However, combinations of strains from elsewhere - such as the Muguga cocktail which is known to produce carriers11 may introduce new strains. Those who favour the more pragmatic cocktail approach play down the importance of carrier states following immunization11,21. The influence of carrier states on stable epidemiological environments has yet to be demonstrated but the proponents of 'local' isolate immunization are unwilling to take this risk. Important political problems might arise following the international use of parasites that induce a carrier state. _
Future Developments
Acknowledgements: I thank Dr P. Conrad for helpful criticism of this manuscript and to Ms D, Churl for typing. This is ILRAD publication no. 473
We rely mainly on parasite behavioural characteristics and cross-immunity studies for parasite definition, both for epidemiological studies and immunization. A solution to the problem of defining Theileria species and strains requires the development of critical laboratory tests which can be substantiated by cross-immunity studies. Monoclonal antibody profdes provide some measure of identity, and isoenzyme patterns have a limited but useful application9. Expansion of these methods of characterization together with the development of DNA probes, cytotoxicity assays and protein analysis by two-dimensional gel electrophoresis, may provide the tools to define the antigenic nature of the parasites from the field and allow the selection of appropriate parasite stocks for immunization in any locality. There are great hopes for 'dead' vaccines in the future. The recent expansion in our
While these newer 'vaccines' are being developed, why have we not started to use immunization by infection and treatment? The technique has been available for more than 10 years and works 9. Reasons are not easy to discern. Governments have been cautious because of a suspicion that immunized cattle may be less productive, and because of the uncertainty about carrier states mentioned above22. It is also possible that tick control programmes, which control other tick-borne diseases as well as ECF, might be less strictly applied if the threat of ECF was removed. At two recent meetings held to discuss ECF immunization, particularly by infection and treatment, these problems were thoroughly discussed9,23. The productivity of immunized cattle has been measured in all recent or current trials and specific studies are being conducted to examine the effect of immunization on fertility. The proposal to establish a standardized data recording system and a network to record and analyse data from immunization trials in different countries reflects a genuine effort to assess the method critically. Specific experiments are also planned to study the effect of the introduction of recovered carrier cattle into a paddock infested with uninfected ticks, to monitor the infection in ticks and the transmission to susceptible cattle if they occur. The implementation of infection and treatment or any other immunization method against ECF must go hand in hand with a new attitude to tick control, which will allow enough tick-cattle, contact to boost and perhaps expand the ECF immunity. It must also incorporate appropriate immunization, or awareness of risk, for other tick-borne diseases. We have passed the point where we can rely upon total tick control for the protection of cattle against ECF and other tick-borne diseases.
Parasitology Today, vol, 3, no. I, 1987
Control of East Coast Fever East Coast Fever (ECF) (Box I) is a disease of cattle in East Africa caused by species of Theileria transmitted by the brown ear tick Rhipicephalus appendiculatus (Fig. 3). In the past, E C F has been controlled by reducing tick infestations through strict short-interval acaricide application, adherence to legislation on cattle movements and quarantine, and good livestock and pasture management. However, civil unrest, illegal cattle movement, poor management and inadequate maintenance of acaricidal dips and sprayraces, acaricide resistance and the high cost of acaricides, have all combined to make this approach to E C F control less reliable. An alternative approach is through immunization. Cattle which recoverfrom the disease have a solid immunity to homologous challenge, and this has encouraged the search for a vaccine since the early 1900s. Two methods of active immunization have been extensively explored; immunization with schizont infected cells is not yet practicable, but immunization by infection followed by drug treatment, has great potential for immediate use. In this debate, Tom Dolan discusses the immunization approach to E C F control, while Roger Tatchell explains how tick control can contribute to this strategy.
I m m u n i z a t i o n to Control East Coast Fever T.T. Dolan International Laboratory for Researchon Animal Diseases(ILRAD) P© Box 30709 Nairobi, Kenya
Early attempts to immunize cattle against ECF were based on the inoculation of spleen and lymph homogenates from confarmed ECF cases using a method devised by Theiler2. In the period 1911-1914, this approach was used to immunize 283 000 cattle in the Transkei territories of South Africa 3. In the original laboratory investigation 2, this method killed 25% of treated cat-
fie but 60% of the survivors were immune. However, no treatment was then available for animals that became clinically ill, and the inoculation procedure was difficult because of the need to use the immunizing material immediately after harvesting from sick animals. This approach was therefore abandoned. Successful in vitro cultivation of T.
© 1 9 8 7 E sevpe~Science Publishers BV., Amsterdam 0169-4758/87/$0200
Parasitotog/ Today, voL 3, no. I, 1987
5
annulata, the cause of mediterranean theileriosis, revived interest in immunization with infected lymphoid cells. Passageattenuated cultures of this parasite were effective in immunization with low numbers (about 104) of cells4. The successful cultivation of T. parva infected lymphoid cells 5 was followed by exhaustive testing of dose, passage attenuation and inactivation 6,7. The results showed that killed material did not immunize, while high numbers of living cells were required to infect cattle reliably, and the results of immunization were often unpredictable. Unlike T. annulata, T. parva schizonts transferred slowly from donor to recipient cells, and recipient animals rejected incompatible donor cells s. It is still possible that a universally acceptable cell may be discovered or produced by manipulation, or that a cell line from which more frequent transfer of parasites occurs will be found. If any of these advances are made, immunization with T. parva-infected cells could become a practical possibility. Infection and Treatment Immunization by ird~ction and treatment is usually based on the inoculation of cryopreserved suspensions of T. parva sporozoites from triturated R. appendiculatus ticks, followed by administration of tetracyclines or parvaquone to control the infection in recipient cattle 9-12. The technical advances that made tllis method practical were the cryopreservation of Theileria sporozoites as stabilates 13, and the control of the infections they induced by a limited number of tetracycline m~atments (Box 2). Theileria parva isolates from various parts of East Africa were used in a series of crossimmunity studies using infection and treatment which revealed the antigenic complexity of field challenge 11. The breakdown of immunity engendered by single isolates led to the selection of three isolates which, when combined, could be controlled by a single treatment with long-acting tetracycline at the time of ~fection and which provided a broad J.mIrlunJty 9,11,17. This combination of isolates ('the Muguga cocktail') included T. p. parva (Muguga), T. p. parva (Kiambu 5), and T. p. lawrencei (Serengeti-transformed - a parasite isolated from a buffalo in Tanzania and passaged through ticks and cattle). Since these original studies in the 1970s many trials have been undertaken and there are two approaches to the use of infection and treatment immunization in the field. One recommends the use of the 'Muguga cocktail'
or another combination produced in a central facility for national or international use. The other recommends the use of local parasite isolates taken from the region where cattle are to be immunized. The advantages of the cocktail approach are that the parasites are of known character and drug responsiveness; they provide a broad protection and can be supplied for use in areas or countries unable to produce or store their own stabilates. A disadvantage is that the cocktail may not protect against all challenge in the field, particularly against T. p. /awrence/. In areas where breakthrough infections occur, these parasites will have to be isolated and added to the cocktail, tailoring it to specific needs. The use of local parasite isolates for immunization requires effective laboratory support because each isolate or combination of isolates (from tick collections in the field, bait cattle or captive buffalo) has to be assessed for infectivity and drug responsiveness. Not all isolates can be controlled by a single treatment with long-acting tetracycline at the time of infection (Box 1). Each testing step is expensive and time consuming. In addition, the definition of what constitutes a local strain requires care, because 'local' can refer to just one farm or to a much larger region. Carrier states (persistent tick-transmissible infections) are known to occur following infection with T. p. lawrencei is and recent studies have established that this is usually the case with T. p. parva 19,2°. It is
Technical advances in cryopreservation of Theileria sporozoites, tetracycline treatment, and in vitro culture of schizont infected cells for immunization, were achieved during the UNDP FAO programme (I 96776) at the East African Veterinary Research Organization under the leadership of the late Dr M.P. Cunningham.
6
Parasitology Today, vot. 3, no. I, 1987
understanding of the molecular biology of parasites and of the immune responses of cattle, encourages the hope that immunogens on theilerial sporozoites, intralymphocytic schizonts, or infected cell membranes may be identified and prod u c e d synthetically. But experience so far suggests that infection has to be established in the host to provide protection. Perhaps by better antigen presentation, the use of adjuvants, or by manipulation of the quality or duration of the immune response, it may be possible to use these immunogens effectively.
Fig. 2. The distribution of ECF in East Africa matches that of its vector
Rhipicephalus appendiculatus.
~'~ R appendtculatus
Field Application probable that most cattle immunized by infection and treatment will become cartiers. This should not complicate immunization with local isolates because no new parasites are being introduced into an area. However, combinations of strains from elsewhere - such as the Muguga cocktail which is known to produce carriers11 may introduce new strains. Those who favour the more pragmatic cocktail approach play down the importance of carrier states following immunization11,21. The influence of carrier states on stable epidemiological environments has yet to be demonstrated but the proponents of 'local' isolate immunization are unwilling to take this risk. Important political problems might arise following the international use of parasites that induce a carrier state. _
Future Developments
Acknowledgements: I thank Dr P. Conrad for helpful criticism of this manuscript and to Ms D, Churl for typing. This is ILRAD publication no. 473
We rely mainly on parasite behavioural characteristics and cross-immunity studies for parasite definition, both for epidemiological studies and immunization. A solution to the problem of defining Theileria species and strains requires the development of critical laboratory tests which can be substantiated by cross-immunity studies. Monoclonal antibody profdes provide some measure of identity, and isoenzyme patterns have a limited but useful application9. Expansion of these methods of characterization together with the development of DNA probes, cytotoxicity assays and protein analysis by two-dimensional gel electrophoresis, may provide the tools to define the antigenic nature of the parasites from the field and allow the selection of appropriate parasite stocks for immunization in any locality. There are great hopes for 'dead' vaccines in the future. The recent expansion in our
While these newer 'vaccines' are being developed, why have we not started to use immunization by infection and treatment? The technique has been available for more than 10 years and works 9. Reasons are not easy to discern. Governments have been cautious because of a suspicion that immunized cattle may be less productive, and because of the uncertainty about carrier states mentioned above22. It is also possible that tick control programmes, which control other tick-borne diseases as well as ECF, might be less strictly applied if the threat of ECF was removed. At two recent meetings held to discuss ECF immunization, particularly by infection and treatment, these problems were thoroughly discussed9,23. The productivity of immunized cattle has been measured in all recent or current trials and specific studies are being conducted to examine the effect of immunization on fertility. The proposal to establish a standardized data recording system and a network to record and analyse data from immunization trials in different countries reflects a genuine effort to assess the method critically. Specific experiments are also planned to study the effect of the introduction of recovered carrier cattle into a paddock infested with uninfected ticks, to monitor the infection in ticks and the transmission to susceptible cattle if they occur. The implementation of infection and treatment or any other immunization method against ECF must go hand in hand with a new attitude to tick control, which will allow enough tick-cattle, contact to boost and perhaps expand the ECF immunity. It must also incorporate appropriate immunization, or awareness of risk, for other tick-borne diseases. We have passed the point where we can rely upon total tick control for the protection of cattle against ECF and other tick-borne diseases.