In addition to good biosecurity practices, control of Newcastle disease (ND) primarily consists in preventive vaccination of flocks and culling of infected and at risk of being infected birds (protection zone). Current vaccine strategies can be effective in controlling serious illness and death in infected birds, but virus replication and shedding may still occur, albeit at a reduced level.

At the present time, most vaccination programmes for ND include the use of lentogenic strains either inactivated (killed) or attenuated (live) in order to induce a good protective immunity while producing minimal adverse effects in birds.

Professor Thierry Van den Berg

Both types of vaccine have their advantages and disadvantages but the continuous outbreaks of fatal ND in commercial poultry flocks in many part of the world indicate that routine vaccination in the field often fails to induce sufficiently high levels of immunity to control ND. Especially, in region where ND is enzootic and where there is high pressure from the field, the efficacy of very early immunisation is hampered by the interference of an important passive immunity. In addition, current vaccination strategies can be effective in preventing serious illness and death of infected birds but do not stop sub-clinical infection, e.g. egg drop, and circulation of virus.

In this context, the aim of our work at CODA-CERVA was to further characterise the chicken's immune responses elicited by different live and a recombinant rHVT-ND vaccines, for which efficacy was largely proved previously by challenge experiments. In these earlier studies, there was a good agreement between protection and the systemic humoral response after vaccination as measured by classical tests (HI or ELISA assays) but no investigation about cell-mediated and local/mucosal immune responses was performed. Professor Van den Berg's team evaluated the specific response of T-cells as well as the specific antibody present in serum samples, in lachrymal, pulmonary and biliary secretions and in culture supernatants of duodenal tissues from birds immunised by the oculo-nasal route. They also investigated the main site of replication (tropism) for each strain, in order to increase our understanding of the protective immune response against the virus.

Finally, the researchers considered the presence of MDA in the serum and the local secretions in order to measure their interference with vaccination. The next goal of our studies was to determine if a vaccination strategy including in ovo vaccination with rHVT-ND combined with a vaccination at day-old with an apathogenic enterotropic live ND vaccine co-administrated or not with chitosan as adjuvant, could increase protection and reduce viral shedding, and presumably, the spreading and the transmission of the virus. Therefore, a very virulent viscerotropic NDV field strain (Chimalhuacan, Mexico) was used for challenge of conventional chickens possessing MDA. This strain induced 100 per cent mortality within six days when inoculated at five weeks of age, when MDA had waned, thus validating the challenge.

In summary, Professor Van den Berg said his investigations have revealed several interesting features:

• Firstly, the development of different immunological techniques indicated differences in cell-mediated and local immunity according to the tropism of the vaccine as well as different immunological mechanisms induced by the rHVT-ND and the live ND vaccines.
  
  
• Secondly, the combination of the rHVT-ND with live day-old ND vaccination and the chitosan adjuvant provides the individual advantages of its three components: the definite advantage of overcoming the MDA in neonatal chickens by the in ovo vaccination with a rHVT-ND, the active immunity induced after vaccination with the live enterotropic ND vaccine and the adjuvant effect of chitosan on NDV-specific CMI.
  
  
• Finally, in addition to the combined vaccination against Marek's disease, this rHVT-ND/live ND/chitosan vaccination program points out a more efficient vaccination in poultry that can be applied at the hatchery, warranting a controlled vaccine uptake and earlier induction of immunity.

Further investigations, regarding the duration of immunity and the induced respiratory immunity, are, however, necessary.

Further Reading

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You can view other papers presented at the Vector Vaccines Symposium by clicking here.

Further Reading

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Find out more information on Newcastle disease by clicking here.