Use of strategic vaccination for the control of avian influenza in Pakistan.

The first outbreak of highly pathogenic avian influenza (HPAI) virus caused by subtype H7N3 appeared in Pakistan in 1995. A homologous aqueous-based vaccine prepared from the field isolate employed as ring vaccination around the epicentre of the outbreak helped in controlling the disease. Later, in 1998, an outbreak of avian influenza virus (AIV) subtype H9N2, particularly affecting the broilers and broiler-breeders, was dealt with using the same vaccination approach. However, this time, the virus could not be successfully contained in the affected areas. Later on, low pathogenic avian influenza (LPAI) virus subtype H7N3 re-emerged in the broiler-breeder flocks in different poultry estates of the northern area of the country during 2000. This prompted the introduction of a vaccine strategy, whereby both aqueous and oil emulsion vaccines were routinely employed in and around the affected areas. This helped in the control of disease in this region. However, no AI monitoring and control strategy could be launched on a sustainable basis in these areas. In November 2003, new outbreaks of HPAI subtype H7N3 occurred specifically in commercial layers in the southern part of the country, which were never vaccinated against AI in the past. In many cases, subtype H9N2 was also recovered from the diseased flocks, with or without the presence of subtype H7N3. The disease caused heavy losses in the south. An emergency plan to control this outbreak was developed with the help of FAO in this regard. Under this plan, a nationwide AIV monitoring and vaccination strategy was developed. This paper discusses the role of vaccine type and vaccination schedule in the control of HPAI in this country.