Avian Influenza and Influenza Pandemics

By Lazzari, Stefano; Stohr, Klaus | Bulletin of the World Health Organization, April 2004 | Go to article overview

Avian Influenza and Influenza Pandemics


Lazzari, Stefano, Stohr, Klaus, Bulletin of the World Health Organization


Well before the 31 influenza outbreaks recorded since the first pandemic was described in 1580, pandemic-like events were reported as early as the fifth century BC by the Greek physician Hippocrates. In the last century, the "Spanish flu" pandemic of 1918-19 killed 20-40 million people, while the "Asian flu" pandemic in 1957 and "Hong Kong flu" in 1968 each caused an estimated 1-4 million deaths.

Pandemics originate with the emergence of a new subtype of influenza virus able to cause disease, replicate in humans, and spread efficiently from one person to another. An avian influenza virus can improve its transmissibility in humans by adaptive mutation or genetic reassortment (the mixing of animal and human influenza viruses). Recent studies confirm that the 1918-19 pandemic probably originated from the reassortment of avian and human viruses (1).

Reports of human infections and deaths caused by avian influenza virus A(H5N1) in Hong Kong Special Administrative Region in 1997 (2) and 2003, and in Viet Nam and Thailand in 2004 are stark reminders of the threat of pandemic influenza. Outbreaks of avian influenza are increasingly frequent, probably as a result of intensive agricultural practices, high virus transmissibility and the presence of natural reservoirs in migratory birds. The ability of three of the 15 known subtypes of avian influenza viruses (H5N1, H7N7 and H9N2) occasionally to infect human beings makes them one of the most likely candidates to become the next pandemic virus.

The current influenza outbreaks in poultry are unprecedented in their scale, geographical spread and adverse economic effects among affected populations. Containment will requite the mobilization of significant resources over several months, leading to continued exposure of many individuals to H5N1. In addition, resistance in current strains to one of the two classes of available antiviral drugs has been demonstrated in vitro. Because of its high pathogenicity in humans, there are growing concerns that if H5N1 were to become transmissible in humans, the heavy toll of the 1918-19 "Spanish flu" pandemic could be repeated (3).

Most experts believe that future influenza pandemics are inevitable and may be imminent (4). Nobody can be sure when a pandemic will happen, how quickly it will spread, and what morbidity, mortality and economic impact it will cause. Forecasting models predict a major disease burden, with 25-30% of the population filling ill and potentially enormous economic costs worldwide, especially in the poorest countries where resources for surveillance and health care are limited and population health and nutritional status are poor.

A comprehensive public health approach to influenza pandemics must include at least four key activity areas:

Limiting the circulation of avian and other animal influenza viruses. The main strategy for preventing the emergence of a pandemic virus is to reduce the opportunities for human exposure to animal viruses with demonstrated human pathogenicity. Human infections with avian influenza viruses and mutations leading to efficient human-to-human transmission are both rare events. However, large outbreaks in poultry magnify the possibility of human infection. …

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