Abstract
Highly pathogenic avian influenza (HPAI) H5N1 are considered to be a major threat for both the poultry industry and public health, and Indonesia is one of the HPAI H5N1 endemic country with the highest incidence of human cases worldwide. The control measures of HPAI, like stamping-out were insufficiently effective in
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Indonesia, therefore, vaccination was included as an additional tool in the control of HPAI. Vaccination has been applied in large-scale since 2004 in Indonesia, however the outbreaks were continued to occurred. The main goal of this thesis are provide more insight of vaccine effectivenesswith respect to transmission and causes of vaccine failure. To this end, the knowledge that was gained from the studies might be used to improve the current vaccination programmes in Indonesia or other HPAI endemic countries. Backyard flocks which is consist of native chickens has been suggested as low responder upon vaccination. In Chapter 2, we demonstrated that native chickens can be vaccinated effectively and these birds respond similary to infection and vaccination as layer type chicken do. In Chapter 3, the efficacy of vaccination in broilers was determined. Our results indicate that vaccination is not effective in broilers, as early vaccination does not induce a good immune response probably due to maternally derived antibody and if vaccination is applied later, the birds may be protected the moment they are slaughtered. In Chapter 4, we conducted a transmission experiment among vaccinated layers, and our result showed a single vaccination applied under field condition induced clinical protection, but seemed to be insufficient to induce protection against virus transmission, indicating that silent spread of virus in commercial flocks may occur. An alternative method of vaccine application are by aerosol/ spray vaccination, this method were considered less harmful for birds. Up to now, this is not feasible for AI vaccines, mainly because live influenza virus vaccines are considered not to be safe neither for poultry nor for humans, because its zoonotic potential. Administration of inactivated vaccines by spray with the low pathogenic avian influenza (LPAI) H9N2 strain was not successful, as it did not induce detectable levels of antibodies or protection against challenge with H9N2 strain (Chapter 5). Due to antigenic variaton, the influenza vaccines request frequent updating. Antigenic relatedness can be assessed by measuring serological cross-reactivity using haemagglutination inhibition (HI) tests. However, it is not known how this relatedness, determined in vitro, reflects vaccine efficacy in vivo with respect to reduction of virus transmission upon challenge of vaccinated birds. In Chapter 6 we conducted transmission experiment to quantify protection of two vaccines, derived from two strain of highly pathogenic H5N1 AI virus against challenge with the homologous or heterologous strain. Our study showed asymmetrical cross-protection between two highly pathogenic H5N1 virus strains, which implies that extrapolation of in vitro data to clinical protection and reduction of virus transmission might not be straightforward.
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