Summary
Vaccines are one of the principle mechanisms utilised to reduce the impact of infectious diseases in livestock industries. Several factors determine the effectiveness of a vaccine. One is the vaccine formulation which is determined during the development phase of the vaccine, and apart from the end cost of the vaccine does not impact on production costs. Another is the delivery mechanism which is probably the most significant factor when considering the end user of the vaccine, namely the producer.
The majority of current vaccines are delivered by injection. While injection is a very effective delivery mechanism it has significant drawbacks. Animals must be mustered and restrained to ensure safety of both the animals and workers involved. This adds to the labour costs. Furthermore, injectable vaccines generally require multiple doses, thus further increasing the associated labour costs. Perhaps the most significant disadvantage of injectable vaccines is the potential for lesion formation at the injection site. At the time of slaughter these lesions must be trimmed from the carcass and discarded. The economic losses due to this trimming were estimated to be $8.95 (CAN) per head of cattle for the Canadian beef industry in 1997.
A better option is to take vaccines to the animal, rather than taking the animal to the vaccine. This can be achieved by oral delivery of vaccines in block or lick formulations, or the addition of vaccine material to foodstuffs. The vaccine to be delivered may consist of either transgenic plant-derived material or viral vector vaccines. The concept of plant-derived vaccination is now well accepted and the scientific literature contains many examples where plant-derived vaccines have conferred viral resistance in animals. However, these have been in either laboratory animals (mice and rats) or other non-ruminant animals.
The delivery of plant-derived vaccines to ruminants presents difficulties due to the structure of the digestive system. For monogastrics, most research groups have targeted the intestinal lymphoid tissue. For ruminants, a potential target for oral vaccines is the oro-pharyngeal tissue. Typically, orally delivered vaccines do not provoke large immune responses and multiple does are required. This is not a problem for vaccines delivered in a feed formulation.
Plants also offer considerable advantages as a vaccine production system.Plants are a cost efficient production system requiring only soil, water and light to grow. While this is an oversimplification, compared to the high costs of constructing a manufacturing facility required for production systems based on mammalian cell culture, plants are highly attractive. Further as plants are not susceptible to mammalian pathogens the risk of vaccine contamination and associated components by adventitious infectious agents, such as non-cytopathic pestivirus or prions, is eliminated.
The application of oral vaccination with plant material also provides a cost effective delivery mechanism in difficult to access areas. Further, studies looking at the stability of human vaccine components in freeze-dried plant material have demonstrated prolonged stability at environmental temperatures thus eliminating the requirement of cold-chain storage. To develop an effective vaccine that can be administered orally to ruminants the antigen needs to survive passage through the rumen to stimulate the immunological tissues of the small intestine. In order to facilitate this in this study, viral structures which are naturally resistant to the rumen environment were tested. Segments of the bovine parvovirus were expressed and these proteins auto-assembled into virus-like-particles (VLP). The VLP do not carry any part of the viral genome and as a result are neither infectious nor pathogenic. Typically the proteins which form VLP are immunostimulatory and act as natural adjuvants to enhance the responses of the immune system.
This project has demonstrated that transgenic plants can be utilised to produce antigens from economically important pathogens of cattle. Further, preliminary sheep feeding studies demonstrated that parvovirus VLP was able to elicit specific immunological responses in some animals. These promising results were not supported in more extensive animal testing. The primary reason for this is thought to be due to inconsistent expression of the antigen between transgenic plant lines resulting in suboptimal levels of antigen being fed to the animals. In spite of this the studies presented have advanced the development of an effective oral vaccination strategy for ruminants by demonstrated the feasibility of the process. Future studies are required where the expression of the parvovirus VLP is optimised to maximise expression in transgenic plants, and also to engineer the VLP to carry antigens from economically important pathogens.
The activity of these hybrid VLPs would then need to be assessed in functional disease models, including challenge models, to demonstrate protection from infection. The successful application of oral vaccination in red meat industries is a long term objective (>10 years). However, in three years this project has been able to demonstrate that oral vaccination of ruminants is achievable with a model antigen (parvovirus VLP) under controlled experimental conditions. To be a routinely available technology, further studies are required to demonstrate effective vaccination in an economically important disease model. This would then need further validation in pen and field trials.
Consideration must also be taken into account that approval would be needed for the production of the transgenic plants on a much larger scale. In this study sufficient material could be produced in a controlled environment (glasshouse) however if a vaccine was being produced for use in the beef industry as a whole or even a sector, such as feedlots, then a much larger scale transgenic plant production system would be required. Currently the production of transgenic plants on this type of scale is subject to strict regulation.
All red-meat producers stand to benefit from the findings of this study. While demonstrating protection from infection was beyond the scope of this study, the study provided evidence that this is achievable. Fully validated oral vaccination will be of particular use in extensive meat production systems by providing easier compliance with vaccination regimes leading to better disease control. Intensive producers (such as feedlots) will also benefit as this sector currently utilises injectable vaccines and as a result is at risk of losses associated with lesion formation. In addition, excessive handling of animals has been associated with production loses.
