Avian influenza outbreaks caused by the H5N1 subtype are presently of the greatest concern for human health.
In assessing risks to human health, it is important to know exactly which avian virus subtypes are causing the outbreaks in birds. For example, the outbreak of avian influenza reported in Taiwan was caused by the H5N2 subtype, which is not highly pathogenic (not very harmful) in birds and has never been known to cause illness in humans. The outbreak announced in Pakistan was caused by H7 and H9 subtypes, and not by H5N1.
However, urgent control of all outbreaks of avian influenza in birds – even when caused by a subtype of low pathogenicity – is very important. Certain avian influenza virus subtypes, initially of low pathogenicity, can rapidly mutate (within 6 to 9 months) into a highly pathogenic subtype, if allowed to circulate in poultry populations.