The A BC of influenza viruses

Vexing

All influenza viruses are divided into three types , A, B and C depending on the virus structure. Type A is responsible for lethal influenza pandemics whereas type B causes smaller localized outbreaks. Less common and more stable than other strains, type C has milder symptoms. Influenza B and C are usually found only in humans. But type A influenza infects both people and animals, including birds, pigs, horses, whales and seals.

Influenza A viruses are divided into subtypes based on two surface proteins

(1)              Hemaggleetinin  HA

(2)              Neuraminidase   NA

Influenza Virus Structure

        Fifteen distinct HA Subtypes and nine NA subtypes exist, But they can combine to form a number of other subtypes, some of which normally are specific to a single species. For example, subtypes H1N1,H1N2 and H1N3 usually cause influenza in humans, whereas H7N7 and H3N8 viruses cause disease in horses. At least 15 flu subtypes offect binds, the most virulent of which is H5N1. Until recently, avian subtypes have rarely been found in humans or in animals other than pigs.

Type A influenza viruses are further divided into strains, which are constantly evolving. And it is exactly this-the ability of influenza viruses to changes their genetic make up and to swap genes indiscriminately-that makes them so unpredictable and potentially deadly.

Bird Flu Virus

Faster than you can say Antigenic:

All living things change, but influenza A viruses change quickly, constantly and sometimes cataclysmic-ally. This takes place in two ways:

Antigenic drift: These are small permanent, ongoing attentions in the genetic material of a virus. Because viruses are not able to repair genetic errors that take place as they reproduce, new strains are continually replacing old ones. Once you have a particular strain of flu, you develop antibodies to it, but those antibodies won’t protect you from new strains. In the same way, the flu vaccine you received last season won’t ward off this years bug.

Antigenic shift: This occurs when influenza A subtypes from different species a bird and a human, for example- trade and merge genes. The result is an entirely new strain, different from either of the parent viruses. Because no natural immunity to the new strain exists, it can spread quickly causing widespread illness and death. And when one of the original subtypes is a human influenza virus, the new virus has the ability to spread easily from person to person and the potential to become a global epidemic.