Pandemic influenza is a zoonosis, as it requires introduction of avian-like gene segments in the human population

Human influenza viruses manage to cause epidemics almost every year. The ci rculating viruses change their surface glycoproteins by accumulating mutati ons (antigenic drift) which results in variant viruses of the same subtype that are able to evade the immune pressure in the population. Every now and then, a completely new subtype of influenza A virus is introduced in the h uman population, which can result in an influenza pandemic. Pandemic human influenza viruses have been emerging for many centuries. Based on the genet ic information of influenza viruses that have been isolated in this century , introduction of genes of the avian influenza virus reservoir obviously is required. Interspecies transmission, via another mammalian host and reasso rtment of avian and human influenza viruses are potential mechanisms for su ch an introduction. A summary of the cases in which influenza viruses containing avian-like gen e segments were introduced into the human population is presented. In three cases, such infections resulted in conjunctivitis. Influenza-like illness and even pneumonia was reported in some other infections. Finally, a mortal ity rate of 33% was observed in the avian influenza A (H5N1) viruses that i nfected 18 people in Hong Kong in 1997. Although some of these viruses fulf illed some criteria of pandemic influenza viruses, they lacked the ability to rapidly spread through the human population. (C) 2000 Elsevier Science B .V. All rights reserved.