Therapy Analysis - Influenza
Strains of influenza
There are currently three known genera of influenza; these are termed influenza A, B and C, and are classified based on variations in the nucleoprotein antigen. Subtypes A and B cause the annual epidemics of 'flu, while the rarer pandemics are due to influenza A, which is the most virulent subtype, and also causes the most serious symptoms in humans. Influenza viruses are further classified according to two types of glycoproteins, haemaglutinin (H number) and neuraminidase (N number), present on the viral surface. There are 16 different H antigens (H1-16) and 9 different N antigens (N1-9).
Changes to the haemaglutinin and/or neuraminidase surface antigens greatly affect the viral pathogenic profile. Major changes create an antigenic shift, which produces a strain significantly different to existing prevalent strains. This can cause a pandemic due to a widespread lack of immunity against this 'new' virus. Conversely, minor accumulative changes cause the annual influenza outbreaks, where the antigens are altered, but still recognisable to the immune system, so a more efficient and rapid response to infection can be mounted.
The infamous Spanish 'flu pandemic was caused by an H1N1 strain, while the 1968 Hong Kong 'flu pandemic, which killed around 700,000 people (primarily school children), was caused by an H3N2 strain. Furthermore, the H3N2 strain is a predominant strain of seasonal influenza. In addition to antigen classification, influenza viruses are commonly also classified according to the species in which they are endemic. This has given rise to many of the species-specific 'flu strains, with human, bird, swine, dog and cat variants having been identified.
