Avian influenza viruses are zoonotic agents recognized as a continuing threat to both veterinary and human public health. During the past 6 years, infection of humans with avian influenza viruses of three subtypes (H5, H7, and H9) has been detected on multiple occasions (1, 2). In 1997, H5N1 avian influenza viruses transmitted from birds to humans in Hong Kong caused the deaths of 6 of 18 infected persons (3, 4). The virus was eradicated by the slaughter of all poultry in Hong Kong, but new genotypes of H5N1 virus continued to emerge in poultry in Hong Kong in 2000 and 2001 (5, 6), and in 2003, antigenically and biologically novel H5N1 influenza virus killed one of two infected humans (7).

Subtype-H9N2 avian influenza virus was isolated from two sick children in Hong Kong in 1999 (8) and from six patients in mainland China (9); all recovered from the infection. The H9N2 variant isolated from humans on the mainland also was isolated from pigs in southern China in 1999 (10, 11), but there was no serologic evidence that a stable virus lineage had become established in pigs or humans. The H9N2 viruses have continued to circulate in poultry throughout Europe and Asia, and are now considered to be enzootic throughout the entire region.

The H7N7 avian influenza virus that caused highly pathogenic avian influenza on 225 poultry farms in Holland in 2003 was associated with conjunctivitis in 347 humans (12). Infection with H7N7 influenza virus was confirmed in 87 of these cases; one person, a veterinarian, died of the infection. There was also human-to-human transmission of the virus and serologic evidence of H7N7 infection of pigs (13). This outbreak was controlled by culling and quarantine of infected poultry; to prevent the emergence and spread of human朼vian virus reassortants, the poultry workers were given human influenza vaccine and antineuraminidase drugs.

The transmission of H5N1 "bird flu" to humans in 1997 first established the ability of avian influenza viruses to be transmitted to humans despite their preferential binding to avian sialic acid receptors (i.e., those with a terminal 2,3Gal linkage) (14). Before 1997, there had been isolated reports of human infection with H7N7 influenza virus (usually causing conjunctivitis) (12), but there was no convincing evidence of repeated transmission of avian viruses to humans. How did avian influenza viruses come to acquire a progressively greater capacity to infect mammals? Here, we characterize a series of 21 H5N1 influenza viruses isolated from apparently healthy domestic ducks in coastal provinces and cities of southern China from 1999 through 2002. These isolates were highly lethal to chickens and demonstrated the progressive acquisition of pathogenicity to mice (a mammalian host). They were genomically heterogenous, having acquired multiple gene segments and deletions in their nonstructural (NS) and neuraminidase (NA) genes. We propose a hypothetical mechanism to explain the selection of H5N1 viruses with increasing pathogenicity to mice.