Should there be a change in the definition of avian influenza for legislative control and trade purposes?

Abstract

The current OIE and EU definitions of avian influenza (AI) to which control measures or trade restrictions should apply were both drafted over 10 years ago. These were aimed at including viruses that were overtly virulent in in-vivo tests and those that had the potential to become virulent. At that time the only virus known to have mutated to virulence was the one responsible for the 1983/84 Pennsylvania epizootic. The mechanism involved has not been seen in other viruses, but the definition set a precedent for statutory control of potentially pathogenic as well as overtly virulent viruses. Evidence accumulated to date indicates that HPAI viruses arise from LPAI H5 or H7 viruses infecting chickens and turkeys sometime after spread from free-living birds. At present it can only be assumed that all H5 and H7 viruses have this potential and mutation to virulence is a random event. Therefore the longer the presence and greater the spread in poultry the more likely it is that HPAI virus will emerge. The outbreaks in Pennsylvania 1983, Mexico 1994 and Italy 1999 are demonstrations of the consequences of failing to control the spread of LPAI viruses of H5 and H7 subtypes. It therefore seems desirable to control LPAI viruses of H5 and H7 subtype in poultry to reduce the probability of a mutation to HPAI occurring. This in turn may require redefining statutory AI. There appears to be three options: 1. Retain the current definition with locally imposed restrictions to limit the spread of LPAI of H5 and H7 subtypes. 2. Define statutory AI as an infection of birds/poultry with any AI virus of H5 or H7 subtype. 3. Define statutory AI as any infection with AI virus of H5 or H7 subtype, but modify the control measures imposed for different categories of virus and/or different types of host. Both the EU Scientific Committee on Animal Health and Animal Welfare in 2000 (Scientific Committee on Animal Health and Animal Welfare 2000) and the OIE ad hoc Committee on AI in 2002 (OIE 2002) recommended that relevant legislative processes concerned with control or trade should be extended to all infections of poultry with either H5 or H7 viruses.