FAO: H5N2 in Taiwan Sheng China

Click on image:




From FAO Document:
Excerpt:
UNDERSTANDING AVIAN INFLUENZA
2.1. Mutation from LPAI virus to HPAI virus
Excerpt:
An HPAI virus has been generated experimentally by repeat passage of a LPAI virus through chickens by air sac and intracerebral inoculation (Ito et al, 2001) but the exact triggers for this change under natural conditions are not known. In some earlier outbreaks of HPAI, it was evident that the change from a LPAI virus to an HPAI virus followed introduction of LPAI virus to large flocks of commercial poultry. This change apparently occurred within a matter of days in some outbreaks (as was the case of the 2004 Canadian outbreak [Bowes et al, 2004]). On the other hand, in some Central American countries, low pathogenicity H5N2 strains have circulated in poultry for a number of years without developing into highly pathogenic strains. Even in Mexico, where mutation of a LPAI H5N2 virus to an HPAI virus occurred in 1994 and this HPAI virus strain was subsequently eliminated, H5N2 LPAI viruses continue to circulate (Villarreal, 2006) but have not reverted to high pathogenicity.
http://www.fao.org/avianflu/documents/key_ai/key_book_ch2.htm