Avian influenza virus (H5N1): a threat to human health

Abstract

Pandemic influenza virus has its origins in avian influenza viruses. The highly pathogenic avian influenza virus subtype H5N1 is already panzootic in poultry, with attendant economic consequences. It continues to cross species barriers to infect humans and other mammals, often with fatal outcomes. Therefore, H5N1 virus has rightly received attention as a potential pandemic threat. However, it is noted that the pandemics of 1957 and 1968 did not arise from highly pathogenic influenza viruses, and the next pandemic may well arise from a low-pathogenicity virus. The rationale for particular concern about an H5N1 pandemic is not its inevitability but its potential severity. An H5N1 pandemic is an event of low probability but one of high human health impact and poses a predicament for public health. Here, we review the ecology and evolution of highly pathogenic avian influenza H5N1 viruses, assess the pandemic risk, and address aspects of human H5N1 disease in relation to its epidemiology, clinical presentation, pathogenesis, diagnosis, and management.

FIG. 1.

Ecology of influenza A viruses and interspecies transmission.

FIG. 2.

Genetic reassortment and emergence of influenza A virus (H5N1) in Asia from 1999 to 2005. The eight gene segments (represented by horizontal bars) starting from top downwards are PB2, PB1, PA, HA, NP, NA, M, and NS. Each color represents a distinct virus lineage. Red represents the A/Goose/Guangong/1/96-like lineage. Distinct genotypes (i.e., gene constellations) are denoted by a letter. The origin of the 1997 virus in Hong Kong is not represented here but was a separate reassortment event with only the HA being derived from A/Goose/Guangdong/1/96 and the other seven gene segments being derived from H9N2 and H6N1 avian influenza viruses found in quail. (Adapted from reference by permission from Macmillan Publishers Ltd.)

FIG. 3.

Ecology, maintenance, and spread of H5N1 viruses (based on data from references , , , , , , , , , and 265).

FIG. 4.

Mechanisms of pathogenesis of human H5N1 disease.