Vaccines for List A poultry diseases: emphasis on avian influenza

Various vaccine technologies have been shown experimentally to be effective for immunization against avian influenza (AI) virus and include conventional inactivated oil-based whole AI virus, vectored virus, subunit protein and DNA vaccines. Vaccine-induced protection is based upon antibodies produced against the surface glycoproteins, principally the haemagglutinin, but also the neuraminidase. This protection is specific only for individual subtypes of haemagglutinin (H1-15) and neuraminidase (N1-9) proteins. AI vaccines protect chickens and turkeys from clinical signs and death, and reduce respiratory and intestinal replication of a challenge virus containing homologous haemagglutinin protein. Many of the vaccines are effective if given as a single injection and provide protection for greater than 20 weeks. Protection has been demonstrated against both low and high doses of challenge virus. Furthermore, subtype H5 AI vaccine has been shown to provide protection against heterologous H5 strains with 89.4% or greater haemagglutinin deduced amino acid sequence similarity and isolated over 38 years. Currently, inactivated whole AI virus vaccines and a fowl pox-vectored vaccine with AI H5 haemagglutinin gene insert are used commercially in various countries of the world. These vaccines have some disadvantages associated with the labour requirements for parenteral administration. However, an experimental recombinant Newcastle disease virus vaccine with an AI haemagglutinin gene insert shows some promise as a low cost, mass administered aerosol vaccine. A critical issue for the use of vaccines in the field is the need to differentiate vaccinated birds from those infected with the field virus. Differentiation is necessary for outbreak surveillance and trade. The use of AI vaccines varies with individual countries and for different AI virus subtypes.