Genetic engineering holds out great promise for the development of improved poultry vaccines. Engineered microbes containing foreign genes from several pathogens provide the basis for multivalent vaccines. The vaccine vector should be non-pathogenic and capable of incorporating and expressing foreigngenes. Subunit proteins expressed by non-avian vectors, such as baculoviruses, can be used to evaluate the nature of their antigenic and protective properties and could find use as subunit or virus-like particle based vaccines. Live recombinant viruses can also be used as vaccines. While RNA viruses such as picornaviruses have been used as vectors, DNA viruses are more easily engineered. Avian poxviruses expressing the Newcastle disease virus (NDV) haemagglutin neuraminidase (HN) and fusion (F) proteins protect against Newcastle disease. Because of their ease of administration through the feed or water, their ability to establish mucosal immunity, and recent advances in genome manipulation, avian adenoviruses are now being developed as recombinant avian vaccines. A recombinant fowl adenovirus expresses a green fluorescent protein reporter gene cloned into a non essential TR-2 region of viral DNA and is also immunogenic. Current efforts are directed at incorporating genes, such as HN from NDV, from avian pathogens into this vector. Genetic engineering can also improve diagnostics. For example, recombinant HN can be used in enzyme-linked immunosorbent assay (ELISA) based NDV diagnostic kits. Baculovirus expressed NDV nucleoprotein (NP) used in a differential ELISA can differentiate between birds immunised with a fowl pox virus expressing NP and NDV infected birds. Polymerase chain reaction (PCR) also has an impact on avian health. PCR can be used in diagnostics and, because of its extreme specificity, in epidemiological studies. Rapid and accurate identification and emerging or re-emerging microbial diseases are some challenges that can be more readily addressed by the application of molecular approaches including genetic engineering. Similarly, the development of multivalent and multi-pathogen vaccines through genetic engineering is certainly a realistic objective. Such vaccines would provide multiple protection in a cost effective manner.
