Avian Influenza: Preparing for a Pandemic

vian influenza has emerged as the primary public health concern of the 21st century. Although vari-ous strains of avian influenza have been recognized for decades, the scope, lethal-ity, and mutability of the Asian H5N1 subtype make it a likely source of the next human influenza pandemic—an event that could kill millions. H5N1 no longer is confined to waterfowl and poultry in southeast Asia and China and appears to be expanding its host and geographic ranges.1 As of late June 2006, 228 persons had been infected with H5N1, most of whom had close contact with infected birds; 130 of these persons died.2 Other mammals such as cats and pigs also have been infected. Waterfowl migra-tion helped the virus spread into Europe and Africa, and it may not be long before it appears in North America. Preparing for this new threat has replaced eradication as a global public health priority. History of Avian Influenza Avian influenza A virus is shed in the feces of healthy-appearing waterfowl (primar-ily ducks), which in turn infect chickens and other poultry with which they come in contact. Mortality rates in chickens and other birds can be devastating with highly pathogenic strains. Conditions for transmis-sion and jumping species barriers often are ideal in Asia, where poultry, ducks, pigs, and humans live in crowded conditions. Although pigs were thought to be a necessary “bridge ” species for avian viruses to infect humans, it now seems likely that the avian influenza virus is mutable enough to make the interspecies leap on its own.3 H5N1 first emerged as a human threat in Hong Kong in 1997. An outbreak in poultry infected 18 people, six of whom died; how-ever, the disease seemed to vanish after the prompt culling of all poultry in Hong Kong.4 In 2003, additional cases were reported in Hong Kong, followed by serial outbreaks of the even more pathogenic “Z strain