Avian influenza viruses with the same subtype of H, which stands for hemagglutinin , and the same subtype of N, which stands for neuraminidase can be slightly different and can cause either a very severe disease that kills most birds that become infected with it or may just cause mild disease or no disease at all.
LPAI or HPAI is determined according to laboratory tests of pathogenicity. The World Organization for Animal Health (OIE), establishes that an avian influenza virus with the subtype of H5 or H7 with an intravenous pathogenicity index of greater than 1.2 in 6 week-old chicks is an HPAI virus. Alternatively, an avian influenza virus that causes 75% mortality in 4-8 week-old is also considered to be an HPAI virus.
LPAI viruses are those of the H5 or H7 subtype that are not HPAI. Nevertheless, LPAI can mutate into HPAI viruses.
Note that the OIE calls HPAI, HPNAI and LPAI , LPNAI. The added N stands for “notifiable” meaning that if these avian influenza viruses are detected, they should be reported to the OIE as well as corresponding local animal and human health authorities.
See also the USDA Fact Sheet Avian Influenza Low Pathogenic H5N1 vs. High Pathogenic H5N1 updated September 28, 2006.
- Center for Infectious Disease Research and Policy, University of Minnesota, Avian Influenza (Bird Flu): Agricultural and Wildlife Considerations. See text: http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/biofacts/avflu.html#_Agent_1. Updated August 8, 2006.
- World Organization for Animal Health (OIE), Terrestrial Animal Health Code, 2005. Chapter 2.7.12, Avian Influenza. See text: http://www.oie.int/eng/normes/mcode/en_chapitre_2.7.12.htm