Flu viruses: A factsheet

Flu viruses: A factsheet

Here is a factsheet on strains of the human influenza virus and the bird flu virus.

Human Influenza Viruses

* TYPE A: The most dangerous and most widespread type, found in wild birds as well as poultry, pigs, whales, horses and seals. Subtypes of type A viruses are designated according to two proteins on the surface of the virus: H, for haemaggluttinin, and N, for neuraminidase.

Thus bird flu's official codename is type A(H5N1), while "Spanish flu," which killed tens of millions of people after World War I, is type A(H1N1).

There are 15 different H subtypes and nine N subtypes.

They can undergo minor mutations every two or three years, a process called antigenic drift.

But they can also swap genes with other viruses, a process called antigenic shift. This typically happens through small changes in amino acids (the building blocks of proteins) in the haemaggluttinin. The classic vessel for this is the pig, which can harbor both avian and human viruses.

Antigenic shift is very rare but can create a lethal new subtype against which no-one has any immunity. So far, only three subtypes are known to have done this (H1N1, H2N2 and H3N2, responsible for global pandemics in 1918, 1957, 1968 and 1977).

* TYPE B: These viruses can also cause epidemics but only circulate widely among humans.

* TYPE C: The most innocuous, causing a mild respiratory illness but not epidemics.

Bird Flu

Avian influenza (bird flu) is an infectious disease of birds that was first identified in Italy 100 years ago. All birds are thought to be susceptible to it but some species are more resistant to it than others.

There are 15 subtypes of bird flu, two of which (subtypes H5 and H7) have been the most lethal for poultry flocks. Those two subtypes have also been transmitted to humans.

H5N1, at the center of the health scare that is unfolding in Asia today, first leapt the species barrier to humans in Hong Kong in 1997, killing six out of 18 cases.

H7N7 bird flu broke out in the Netherlands in 2003, causing the death of a veterinarian and conjunctivitis and other mild illness in 83 others.

A third subtype, H9N2, was detected among three Hong Kong children in 1999 and 2003, but all three were mild cases.

Sources: World Health Organization (www.who.int), U.S. Centers for Disease Control and Prevention (www.cdc.gov). -- AFP