Vaccination for avian influenza in Asia

Abstract

Continuing outbreaks of highly pathogenic avian influenza (HPAI) in a number of Asian countries, caused by an influenza type A virus of H5N1 subtype represent a serious risk for animal and public health worldwide. Although vaccination programmes have been recommended recently, there is field evidence that vaccination alone will not achieve eradication, and if not used appropriately it may result in the infection becoming endemic. At the recent FAO summit meeting in Rome (3–4 February 2004), vaccination, using the differentiating infected from vaccinated animals (“DIVA”) strategy, was recommended when there is risk of major spread and depopulation is not feasible or desirable [1]. The main reasons for this recommendation are that in the current situation stamping out of infected animals may result in the removal of a major source of food for rural communities and damage the commercial viability of the local poultry industry. Attempts at controlling AI infections of poultry with H5 and H7 subtype viruses by homologous and recombinant vaccines without monitoring field exposure and the application of stringent control measures on field exposed farms (regardless of the state of vaccination) have been unsuccessful in eradicating the infections in Mexico (H5N2) and Pakistan (H7N3) [2]. From experimental data it is known, for both low pathogenic avian influenza (LPAI) and HPAI, that vaccination protects against clinical signs and mortality, reduces virus shedding and increases resistance to infection [2,3]. However, the virus is still able to replicate in clinically healthy vaccinated birds and this is probably why vaccination alone has been unsuccessful in achieving eradication [2]. In 2000, a novel DIVA strategy that enables the differentiation of vaccinated/uninfected from vaccinated/infected animals was developed in Italy and remains to date the only successful field experience of this strategy. Between 2000 and 2003 its application successfully eradicated two epi-