The use of vaccination as an option for the control of avian influenza

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Abstract
Recent epidemics of highly contagious animal diseases included in list A of the Office International des Épizooties, such as foot-and-mouth disease, classical swine fever and avian influenza (AI), have led to the implementation of stamping-out policies resulting in the depopulation of millions of animals. The enforcement of a control strategy based on culling animals that are infected, suspected of being infected or suspected of being contaminated, which is based only on the application of sanitary restrictions on farms, may not be sufficient to avoid the spread of infection, particularly in areas that have high animal densities, thus resulting in mass depopulation. In the European Union, the directive that imposes the enforcement of a stamping-out policy (92/40/EC) for AI was adopted in 1992 but was drafted in the 1980s. The poultry industry has undergone substantial changes in the past 20 years, mainly resulting in shorter production cycles and in higher animal densities per territorial unit. Due to these organizational changes, infectious diseases are significantly more difficult to control because of the greater number of susceptible animals reared per given unit of time and due to the difficulties in applying adequate biosecurity measures. The slaughter and destruction of great numbers of animals is also questionable from an ethical point of view, particularly when human health implications are negligible. For this reason, mass depopulation has raised serious concerns for the general public and has recently led to very high costs and economic losses for national and federal governments, stakeholders and, ultimately, for consumers. In the past, the use of vaccines in such emergencies has been limited by the impossibility of differentiating vaccinated/infected from vaccinated/non-infected animals. The major concern was that through trade or movement of apparently uninfected animals or products, the disease could spread further or might be exported to other countries. For this reason, export bans have been imposed on countries enforcing a vaccination policy. This review considers the possible strategies for the control of avian influenza infections, bearing in mind the new proposed definition of AI, including the advantages and disadvantages of using conventional inactivated (homologous and heterologous) vaccines and recombinant vaccines. Reference is made to the different control strategies, including the restriction measures to be applied in case of the enforcement of a vaccination policy. In addition, the implications of a vaccination policy on trade are discussed. It is concluded that if vaccination is accepted as an option for the control of AI, vaccine banks, including companion diagnostic tests, must be established and made available for immediate use.