Avian Flu

Avian influenza, also known as avian flu, bird flu, or fowl plague, is an animal disease of viral etiology that ranges from a mild or even asymptomatic infection to an acute, rapidly fatal disease of chickens, turkeys, guinea fowls, and other domestic poultry, as well as wild birds, migratory waterfowl, and other avian species. Inasmuch as the avian influenza viruses can be occasionally transmitted to humans, avian flu is a zoonotic disease. Due to its potential to cause worldwide epidemics in humans (i.e., pandemics), and the current A/H5N1 avian flu outbreak—that is, an epizootic or epornithic (the nonhuman equivalent of an epidemic in bird populations) in some parts of the world, particularly Southeast Asia—avian influenza has been identified as a major public health concern worldwide. Indeed, under the revised International Health Regulations, any novel (i.e., different from currently circulating human flu H1 and H3 viruses) human influenza A virus infection must be reported immediately to the World Health Organization (WHO). In fact, the chief and foremost strategy in addressing the current pandemic threat entails diminishing the pandemic likelihood by controlling highly pathogenic influenza viruses in animals, expressly the epizootic caused by A/H5N1 virus in poultry, through improved detection, surveillance, and control by way of strengthening veterinary public health structures and competencies.

The natural reservoirs of influenza A viruses—the etiologic agents of avian flu—are the aquatic birds of the world, particularly ducks, in which the viruses appear to be in evolutionary stasis or equilibrium with their natural host, causing no disease. All known influenza A subtypes exist in the aquatic bird reservoir (i.e., the 16 hemagglutinin and 9 neuraminidase surface glycoprotein subtypes); due to this fact, influenza is reckoned as not an eradicable disease. In wild ducks, flu viruses replicate preferentially in the cells linings of the intestinal tract, cause no disease signs, and are excreted in high concentrations in the feces. As much as 30% of the large number of susceptible young ducks hatched each year can shed flu [Page 66]virus before fall migration for as long as 30 days. Avian flu viruses have been isolated from fecal material and lake water, and it has been shown that viruses retained infectivity in fecal material for as long as 30 days at 48C. Waterfowl, therefore, may have a very efficient mode of virus transmission: by fecal material in the water supply. Flu viruses of avian origin have been implicated in outbreaks of influenza in mammals, such as seals, whales, pigs, mink, and horses, as well as in domestic poultry. Ducks and wading birds may play a unique and very important role in the natural history of influenza.

As striking as the apparent genetic stability of avian flu viruses in aquatic reservoirs is another conspicuous characteristic: the continued evolution and extent of genetic variation of their mammalian strains. The gene pool of influenza A viruses in aquatic birds provides all the genetic diversity required for the emergence of annual epidemics and occasional pandemics of disease in humans, lower animals, and birds. In humans, pigs, and horses, influenza A viruses show both antigenic drift and genetic shift—that is, point mutations and gene reassortment, respectively—two mechanisms of molecular changes in the two surface glycoproteins and in the eight RNA segments of the viruses that keep accumulating their genetic variability. Another notable characteristic is the lack of proofreading among RNA polymerases, contributing to replication errors of the order of 1 in 104 bases (in contrast with the much higher replication fidelity found among DNA polymerases, with errors of the order of 1 in 109 bases). Antigenic and genetic evidence show that the 1957 H2N2 Asian and the 1968 H3N2 Hong Kong pandemic strains were generated by genetic reassortment between human and avian flu viruses. Pigs seem to play an important role in interspecies reassortment and subsequent transmission of influenza viruses.

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