Tuesday, April 28, 2009

About Influenza

Human influenza is caused by infection with influenza type A and type B viruses. These viruses consist of 8 "genes" or segments of single-stranded RNA. Two major surface antigens can be found in influenza viruses - HA (hemagglutinin) and NA (neuraminadase). The hemagglutinin protein is used by the virus to recognize, attach to, and enter a cell. This results in influenza infection. Unfortunately, both the HA and NA genes are very good at slightly changing their structure over time. The figure below shows a model of influenza virus structure taken from Hampson, AW, Med. J. Aust. 2006 Nov 20;185(10 Suppl):S39-43 which is available free online and is recommended reading.

Influenza changes its antigenic makeup via antigenic drift (small changes) and antigenic shift (large changes). Pandemic influenza (a widespread infectious disease) occurs at irregular intervals of about twice a century and usually results from antigenic shift which occurs in Influenza A. The figure below (taken from the Hampson review article) shows how genetic reassortment through antigenic shift is thought to occur.

What is key is that pigs are susceptible to both avian and human influenza A viruses. This "dual-infection" provides an opportunity for the avian and human influenza virus to share and switch (reassort) their genes. This often results in the appearance of a much more infectious and pathogenic influenza virus. But, this is not always the case.

Below is a link to the current swine flu epidemic showing where cases are occuring in worldwide.

Current swine flu cases map

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