Avian influenza viruses and pandemic influenza

Abstract

Human influenza viruses can hardly be labeled as reemerging pathogens because they cause annual human epidemics of symptomatic disease, affecting approximately 20% of children and 5% of adults worldwide, and have probably done so since ancient times. Around the year 400 B.C., Hippocrates recorded epidemic catarrhs associated with seasonal periods, which may well have been attributable to influenza viruses. Periodically, however, completely novel antigenic subtypes of influenza viruses were introduced in the human population, causing large-scale global outbreaks with high death tolls. These pandemic strains can certainly be regarded as (re)emerging pathogens. The Athens plague described by Hippocrates contemporary Thucydides is believed by some to constitute the first account of such a devastating influenza epidemic. Since the 16th century, many large-scale outbreaks of influenza-like illnesses have been described in Europe. In 1580, one of such outbreaks spread from Europe into Africa and Asia, possibly making it the first recorded influenza pandemic. The most devastating influenza pandemic in modern recorded history, known as the Spanish flu, occurred in 19181919, killing up to 100 million people worldwide. Other less destructive pandemics during the past century occurred in 1957 and 1968. Avian influenza A viruses are key to the emergence of human influenza pandemics. The virus strains implicated in the 20th centurys influenza pandemics originated directly from avian influenza viruses, either through genetic reassortment between human and avian influenza strains (1957, 1968) or possibly through adaptation of purely avian strains to humans (1918). It was long thought that the host range of avian influenza viruses precluded direct transmission to humans and that the emergence of pandemic strains required genetic reassortment between avian and human strains. However, occurrences of direct bird-to-human transmission of avian influenza viruses have increasingly been reported in recent years, culminating in the ongoing outbreaks of influenza A (H5N1) among poultry and wild birds in several Asian, European and African countries with continuing instances of human infections. These unprecedented developments have resulted in increasing global concerns about the (re)emergence of pandemic influenza A strains and the role of avian influenza viruses in this.