Poultry Vaccines: a 15-year perspective

Given the considerable increase in our understanding of the avian
immune system and the pathogenesis of avian micro-organisms over the
last few decades, plus the technological advances in molecular biology,
genomics, proteomics, bioinformatics and vaccine delivery systems,
there is rightly a high expectation for new generations of poultry
vaccines.

Our knowledge is increasing rapidly, bringing great optimism

High Expectations With greater understanding of avian immune systems and the pathogens afflicting the birds there is a high expectation for new generations of poultry vaccines.

Given the considerable increase in our understanding of the avian immune system and the pathogenesis of avian micro-organisms over the last few decades, plus the technological advances in molecular biology, genomics, proteomics, bioinformatics and vaccine delivery systems, there is rightly a high expectation for new generations of poultry vaccines. However, our expectations should be moderated to some extent, as the pace of change is likely to be slower than hoped for.

There are many reasons for this. Poultry vaccines have more complex requirements in comparison to vaccines for other species. Of course new poultry vaccines should be safe and efficacious, but they should also be quick and easy to apply, be in the correct combinations plus, whenever possible, be broad spectrum to reduce the number of vaccinations and/or handling of the birds. All of this should be achieved within the constraints of a strict economic framework whilst the regulatory, political, environmental and disease pressures are constantly shifting. In addition to this, the fact that most of the current vaccines work extremely well compounds the problem. For these reasons we should adopt a balanced, cautious optimism for future advances.

So what have we learned that can assist us in the next 15 years? Of particular note is the advance in our knowledge of avian immunology including the innate immune response, the role of chemokines/cytokines, antigen processing and presentation, cell mediated and mucosal immunity. From studies of the innate immune response we now better understand the way in which immune responses are initiated, the manner in which pathogens are recognized being crucially important.

This knowledge has to some extent shifted the emphasis on adjuvant research to target the innate immune response. By developing adjuvants that initiate the immune response more optimally we may be able to lower antigen doses, to give fewer doses, to obtain faster onset of immunity whilst maintaining a long duration of immunity. From the chemokines/cytokines field we have learned that chickens express many of the regulatory proteins that mammals do. Much effort is ongoing to augment immune responses or even alter a bird’s capacity to respond to vaccines by delivering these proteins as vaccine constituents. Avian dendritic cells are now being characterized and this research suggests that these cells, like their mammalian counterparts, are the key antigen-presenting cells in the initiation of a robust immune response. Targeting dendritic cells with vaccines is now a realistic target for many research groups.

Finally increasing knowledge of the cell-mediated and mucosal immune responses may help us analyze and target these vital components in order to optimize vaccine efficacy. No technology has promised more for a new generation of poultry vaccines than molecular biology and finally we are beginning to see the fruits of this work.

Fowlpox and turkey herpesvirus recombinant vaccines targeting five different diseases plus Salmonella and E. coli deletion mutants have regulatory approval in the United States. In the future we can expect different types of vectors to become available, especially now that we can manipulate RNA viruses as readily as DNA viruses. Increased knowledge should allow more marker vaccines and the commercial optimization of various antigen expression systems should help improve vaccine design plus play a role in manufacturing logistics, reducing our reliance on the use of egg-based production systems.

Significant advances have also been made in our knowledge of avian micro-organisms. It is not surprising to find that many of these have evolved mechanisms to either avoid or regulate the host immune response. This knowledge should enable us to counter the strategies of such organisms and novel antigen discovery technologies such as reverse vaccinology will assist this field. However, here we should acknowledge that our information is far from complete and, of course, the organisms are constantly evolving.

Although the poultry industry already has the most varied application routes of all species, there remains scope for further changes and refinements. Vaccine delivery devices are still being improved and there are plenty of opportunities for more vaccines to be delivered via in-ovo vaccination. Advances in vaccine and drug delivery technologies may also filter into the poultry vaccine field, broadening our opportunities.

In summary, our knowledge of science and technology that can influence poultry vaccines is increasing rapidly. This brings great optimism for the future although the complex requirements of a successful poultry vaccine suggests that only a fraction of research ideas will make it through to the market.