Pandemic influenza

Pandemic influenza is defined as virulent human influenza that causes a global outbreak, or pandemic, of serious illness. There are three requirements for a pandemic: (1) novel HA or NA types (thus no pre-existing immunity in population); (2) a highly virulent strain of influenza virus; and (3) easy human-to-human spread. A worldwide influenza pandemic occurred in 1918. At least 21 million people (possibly 50-100 million) worldwide died from the ,Spanish flu' - the most devastating plague in human history. Many of the deaths were in healthy young adults and two-thirds occurred during a 4-month period [7, 8]. The propensity for high mortality in previously healthy young adults was a very unusual feature of the 1918 influenza virus. Routine seasonal influenza viruses cause the highest mortality in the very young (< 6 months of age) and the elderly (> 65 years old). The increased virulence of the 1918 virus in healthy young persons has been hypothesized to be due to a "cytokine storm", but the biological mechanisms are not known. This has disturbing parallels to the mortality pattern exhibited by the recent H5N1 avian influenza viruses (see below).

Two pandemics of influenza have swept the world since the "Spanish flu" of 1918 (H1N1): the "Asian" flu pandemic of 1957 (H2N2) and the "Hong Kong" flu pandemic of 1968 (H3N2). These pandemics were milder, with an estimated 2 million deaths in 1957 and 1 million deaths in 1968. These data suggest that flu pandemics occur when the virus acquires a new HA and/or NA. The pandemic of 1957 probably infected more people than 1918. However, the availability of antibiotics to treat the secondary infections that are the usual cause of death resulted in a much lower death rate. In addition, the 1918 influenza virus was likely more virulent than the viruses from the 1957 and 1968 pandemics.

In 1997, Taubenberger et al. [9] reported partial sequences of five influenza genes recovered from the preserved lung tissue of a U.S. soldier who died from influenza in 1918. Continued work by this group led to the sequencing of the entire genome of the 1918 virus [10-16]. Phylogenetic analysis of the genomic sequence data suggests that the 1918 virus was derived from an avian-like influenza virus a short time (perhaps a few years) before the start of the pandemic, but the origin is still not known. This virus has been recreated in a highly secure biocontainment facility at the CDC using the technique of reverse engineering [15]. Studies of this recreated 1918 virus in mice suggested that the HA and NA from the 1918 strain are major determinants of virulence [15]. The contribution of other genes to virulence has not been completely determined [12, 17-20]. However, this work suggests that a reassortant human influenza virus containing only an HA and/or NA gene from a highly virulent strain could cause severe disease and high mortality similar to that caused by the 1918 virus. This has important implications for the possible outcome of a pandemic that could occur due to avian influenza viruses.

Swine Influenza

Swine Influenza

SWINE INFLUENZA frightening you? CONCERNED about the health implications? Coughs and Sneezes Spread Diseases! Stop The Swine Flu from Spreading. Follow the advice to keep your family and friends safe from this virus and not become another victim. These simple cost free guidelines will help you to protect yourself from the swine flu.

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