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Sidebar Feature   |  Spring 2006

New bird flu strains develop from genetic mutations

All influenzas start as variants of bird flu, or avian influenza, but biochemical processes can change the disease into one that affects other animals and humans. Bird flu viruses are classified according to the combination of two types of proteins, hemagglutinin (H) and neuraminidase (N), found on the virus cell surface. The 16 H proteins and nine N proteins form a large number of viruses for which birds are the natural hosts.

The bird flu that has been making news amid speculations of a pandemic has the H5 protein and the N1 protein; hence, it's called “H5N1.”

H5N1 viruses

H5N1 viruses budding from infected cells. (Photo courtesy Rick Bright, Centers for Disease Control and Prevention)

New flu strains develop when H and N combinations change. Shifts occur when human or swine influenza genes mix with an avian variety, says Suresh Mittal, a Purdue University virologist in the School of Veterinary Medicine. This usually happens in pigs, often called the “virus crock pot” because, unlike most animals, their cells are receptive to viruses from pigs, humans and birds.

Once new flu strains mutate to highly pathogenic human-to-human transmission, they're no longer considered bird flu, Mittal says.

Flu genetic alterations work a bit like a jigsaw puzzle. As two viruses grow in the same animal, they replicate pieces of their genomes and then reassemble them. A new virus is born during the rebuilding process when one virus takes a piece of genome from the other virus to fill an empty spot.

Pandemic potential exists when one of these new viruses is introduced into the human population that has no previous exposure to it. This lack of immunity allows easy virus transmission from person to person.



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