W. IVAN MORRISON
Ivan Morrison is deputy director of the Institute of Animal Health, Immunology and Pathology, Compton, England (e-mail: email@example.com).
Supplies of livestock products in developing countries must increase to meet growing demand from burgeoning populations and rapid urbanization. Because of competition for land use, the necessary growth in livestock output will have to come in great part from improvement in the efficiency of production systems. Disease is one of the major factors contributing to poor livestock productivity in developing countries. This is particularly true for Sub-Saharan Africa, where animal losses due to disease are estimated to be US$4 billion annually, approximately a quarter of the total value of livestock production. Tsetse fly-transmitted trypanosomosis and tick-borne diseases are me most important livestock disease problems in this region. Therapeutic agents are available for some of these diseases, but problems remain. Chemotherapy, for example, is impractical as a primary means of disease control, because costs are high and intensive application can create drug-resistant organisms. Controlling arthropod vectors to prevent diseases, particularly tick-borne diseases, has proved difficult to sustain because of cost, the need for well-developed infrastructure, and the emergence of resistance to the chemicals used. Vaccination offers a potentially more effective and sustainable method of disease control.
OPPORTUNITIES PRESENTED BY NEW BIOTECHNOLOGY AND IMMUNOLOGY
Vaccines developed using traditional approaches have had a major impact on the control of foot-and-mouth disease, rinderpest, and other epidemic viral diseases that affect livestock. But there are many other important diseases, notably parasitic diseases, for which attempts to develop vaccines have been unsuccessful. Rapid advances in biotechnology and immunology over the last two decades have created new opportunities to develop vaccines for parasitic diseases. Initial optimism in the early 1980s that vaccine products would quickly emerge from applications of recombinant DNA technology has not been fully realized. Subsequent experience has demonstrated that, unlike traditional approaches to vaccine development, effective exploitation of recombinant DNA technology requires knowledge of the target pathogens and the immune responses they induce, and an understanding of how immune responses can be manipulated. Since the early 1980s a series of fundamental discoveries in immunology have led to a detailed understanding of how the immune system processes and recognizes pathogenic organisms, and the different ways that immune responses control infections. This new knowledge is directly relevant to all stages of vaccine development, from identification of the genes or proteins that need to be incorporated into a vaccine, to the design of a vaccine delivery system that will induce a particular type of immune response. These advances, coupled with further developments in the application of DNA technology, now provide a strong conceptual framework for the rational development of new vaccines.
USE OF BIOTECHNOLOGY TO DEVELOP CANDIDATE VACCINES
Two main approaches are being pursued to develop vaccines using recombinant DNA technology. The first of these involves the deletion of genes that determine virulence of the pathogen, thus producing attenuated organisms (nonpathogens) that can be used as live vaccines. With current technology, this strategy is more appropriate for viral and bacterial diseases than for parasites. Attenuated live vaccines have been developed for the herpes viruses that cause pseudorabies in pigs and infectious bovine rhinotracheitis in cattle. A number of candidate Salmonella vaccines have also been produced.
The second strategy is to identify protein subunits of pathogens that can stimulate immunity. This is the preferred approach to many of the more complex pathogens. It requires knowledge of the immune responses that mediate immunity. This knowledge helps identify the relevant target proteins. The strategy can be illustrated by the approach the International Livestock Research Institute (ILRI) (incorporating the former International Laboratory for Research on Animal Diseases) took to develop a vaccine against Theileria parva, the parasite that causes East Coast Fever in cattle in Africa. Studies of immune responses to the parasite have revealed antibody responses to the tick-derived infective stage of the parasite, as well as cell-mediated immune responses against the parasite stages that reside within cattle cells. A parasite protein recognized by the antibody response and the corresponding parasite gene have been identified. Protein expressed from this gene, when used to vaccinate cattle under experimental conditions, has been shown to protest a proportion of animals against parasites. Identification of the parasite proteins recognized by the cell-mediated immune responses presents a greater challenge, but a number of recently developed methodologies for this purpose are now being applied to the problem. It is worth emphasizing that these novel approaches to develop a vaccine for East Coast Fever would not have been possible without the strategic research that had been devoted to understanding the immunology of the disease.
An additional novel strategy developed to vaccinate against blood-sucking parasites involves the use of components of the gut wall of the parasites that are not usually exposed to the hosts immune system. Antibodies induced by the vaccine are ingested by the tick during feeding, causing destruction of the gut wall and death of the parasite. This strategy has been used successfully to develop a vaccine against the one-host tick Boophilus microplus. Recent rapid advances in pathogen-genome sequencing promise to be of enormous benefit for developing attenuated pathogens and for identifying proteins suitable for use as vaccines. Complete genome sequences are now available for a growing list of human bacterial pathogens. Completion of the sequences of the human malaria parasite, Plasmodium falciparum, is expected within a year. These developments will undoubtedly have an impact on vaccine development strategies.
NEW VACCINE DELIVERY SYSTEMS
Live, attenuated vaccines stimulate immune responses similar to those induced by the parent pathogen and usually provide long-lasting immunity. Vaccines using killed organisms require incorporation of adjuvants (agents that enhance immunity-giving characteristics), and the immune responses they induce are usually more limited and of shorter duration than those induced by live vaccines. Co-administration with adjuvants is also a standard method used with subunit proteins but may be ineffective in some cases. Advances in biotechnology have provided a number of alternative vaccine delivery systems for subunit proteins that overcome these shortcomings and offer some of the advantages provided by live vaccines. Two of the most promising approaches are the use of attenuated organisms as live vectors and vaccination with DNA.
Live-vectored vaccines incorporate a gene encoding a subunit protein into the genome of an attenuated organism, which itself may be in use as an attenuated vaccine. The protein is then produced when the organism replicates in the animal. A vaccine containing a rabies virus gene has been used to protect foxes against rabies. An attenuated strain of sheep and goat pox virus containing rinderpest virus genes has been shown to protect cattle against rinderpest. Although this system offers little advantage over the conventional rinderpest vaccine, it illustrates the potential of the vector for delivery of other proteins.
The use of DNA for vaccination is based on the discovery that injection of genes in the form of plasmid DNA can stimulate immune responses to the respective gene products. This occurs as a result of the genes being taken up and expressed by cells in the animal following injection. Stimulation of immune responses and partial protection have been reported for a number of pathogen genes in livestock species, but none of these has yet led to a fully effective vaccine.
The live-vector and DNA vaccination systems could be manipulated further to enhance the immunity-conferring characteristics of the gene products. Experimental studies have demonstrated that these systems have enormous potential for developing vaccines that induce appropriate and enduring immune responses.
PROSPECTS FOR VACCINES AGAINST TICK-BORNE DISEASES
The tick-borne parasitic and bacterial diseases (theileriosis, heartwater, babesiosis, and anaplasmosis) that affect cattle in tropical and subtropical regions constitute a major focus for vaccine development because of their substantial impact on livestock production. Early observations showed that animals that recovered from these diseases subsequently remained immune. These findings encouraged the view that vaccination should be possible. Indeed, various protocols for vaccinating with live organisms (either with attenuated organisms or by infection and treatment) were shown to be effective for theileriosis and babesiosis, but their use in developing countries was limited because of the complex infrastructure required to produce and distribute live parasites. Although new vaccines have not yet been produced for these diseases, encouraging progress has been made in identifying new candidate vaccines. The recent development of an efficient culture system for Cowdria ruminantium, the bacterium that causes heartwater, has led to immunization experiments with inactivated bacteria that have yielded promising results. A protein from the infective stage of the Theileria parva parasite has also been shown to have protective properties, and advances in understanding of the immunology of this parasite have led to the development of screening procedures to identify proteins recognized by protective cell-mediated immune responses. Proteins from both stages of the parasite will probably need to be used to produce a robust vaccine against East Coast Fever. Similar studies of the immune responses of cattle to the organisms causing babesiosis and anaplasmosis have resulted in the identification of a number of proteins, some of which give protection under experimental conditions.
There is good reason to believe that vaccines will be produced against some or all of the major animal diseases, given the necessary scientific and financial resources. However, the complexity of the problems that are being addressed should not be underestimated. The opportunities presented by advances in biotechnology can only be exploited effectively if there is a thorough understanding of the biology of the target pathogens and the diseases they produce. Such an approach requires substantial investment in strategic research. For understandable reasons, current funding policy in the developing countries strongly emphasizes tackling the problems that will yield practical benefit in the short term. In determining future policy, policymakers and funding bodies must not lose sight of the substantial benefits that can be gained in the longer term by investing in strategic research on vaccine development.
For further information see N. Mowat and M. Rweyemamu, eds., Vaccine Manual: The Production and Quality Control of Veterinary Vaccines for Use in Developing Countries, FAO Animal Production and Health Series No. 35 (Rome: Food and Agriculture Organization of the United Nations, 1997); D. J. McKeever and W. I. Morrison, Novel Vaccines Against Theileria parva: Prospects for Sustainability, International Journal of Parasitology 28 (1998): 693-706; and Parasitology Today 15 (No. 7, 1999), special issue on vaccines for tick-borne diseases.