Human safety of veterinary vaccines

A number of veterinary vaccines contain components that are pathogenic in humans and which could conceivably cause disease, although the vast majority are specific to the animals they infect. An example of a disease that infects both animals and humans is rabies, and rabies vaccines are commercially and widely available. However, the problem is overcome by inactivation of the virus to produce a product that is safe for both the vaccinated animal and for those who administer it. However, as Table 14.1 indicates, some products are available that may cause disease in humans if used improperly or should accidental injection or a needle stick injury occur. Clearly, the results of human contamination and infection are microbiological and not toxicological. Nevertheless, it is worth examining these briefly for the sake of completeness and to emphasise that user safety is not the sole domain of pharmaceutical products. 

ISCOMs are stable (non-covalent) complexes composed of a mixture of Quil A, cholesterol and (an amphipathic) antigen. ISCOMs stimulate both humoral and cellular immune responses and have been used in the production of some veterinary vaccines. Their use in humans, however, has not been licensed so far, mainly due to safety concerns relating to the Quil A component. 

The development of veterinary products for cattle, pigs, sheep, poultry and other food producing animals also includes residue testing of all new pharmaceutical products, including adjuvants or other excipients in vaccines. Residue safety studies address the potential risk to humans due to the consumption of food from treated animals. Accordingly, the test substances in these toxicity studies are applied orally. Residue depletion studies (pharmacokinetic studies) in the target species must be carried out to define the occurence, concentration and elimination of the substance and its metabolites in edible tissues, milk and eggs. 

In conclusion, the future for gene electrotransfer for clinical use is bright. Based on many clinical trials in human and veterinary medicine that showed positive results and safety of the approaches, especially two fields, prophylactic vaccination for infectious disease and curative vaccination combined with standard treatments for cancer will benefit in the future. Further technical developments of electrodes and generators of electric pulses on one hand and further optimization of plasmids DNA regarding the controlled expression and safety on the other hand will bring gene electrotransfer into wider use. 

Many compounds with adjuvant activity are presently known (shown in Table 12.2) but only a few are applied routinely in human and veterinary vaccines. The application of the novel adjuvants are limited for several reasons, such as disappointing efficacy in the target animal species, insufficient safety, problems with large-scale preparations, and limited stability of the final formulations. Many studies have reported on O/W and W/O emulsions used as adjuvants and delivery systems for immunization (Hilgers et al., 1994a, b, 1999). The adjuvanticity of the W/O/W makes this type of multiple emulsion a suitable delivery system for immunization with prolonged release, and its interesting preparation is presented below. 

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