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Bird flu. Swine flu. They sound like diseases only farmers and veterinarians should have to worry about, yet world health authorities fear them even more than so-called "human flu." Influenza is a zoonosis, meaning it is transmitted between humans and animals, which makes influenza epidemiology unique. How does the influenza virus jump the species barrier?
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How Viruses Evolve
Influenza mutates very fast. Every time the virus replicates, there is an average of about 1.4 mutations in the daughter viruses — permanent changes in the genome. Many of these mutations will be deleterious, causing the mutant virus strain to die out and be replaced by better-adapted versions.
Other changes will have no effect on the daughter viruses' physical characteristics, called its phenotype. This is because more than one combination of bases (the information units of the genome) can code for the same result.
Still other changes will have an effect on the phenotype that is neutral to the virus's abilities. A minor change in the virus's protein coat often has no direct effect on its function. Despite not affecting function, these mutations can be beneficial if they give the virus a new "identity," preventing the host's immune system from recognizing it.
Most rarely of all, a mutation will give a daughter virus a new ability. By chance, once in a great while, a mutation will coincidentally give the virus a protein that enables it to invade a new kind of cell — the cells of a different host species. A disease that can be passed from animals to humans is called a zoonosis, and Influenza A is a zoonotic virus.
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The Opportunity to Jump Species
Imagine that such a mutation appears during an outbreak of H5N1 avian flu in a flock of domestic chickens. Humans have close contact with their food animals, and the virus that by chance acquired the ability to infect humans will have an opportunity to cause illness in the chicken farm's staff.
The mutated virus may be able to reproduce successfully enough to make its host sick, but not enough to spawn daughter viruses that can infect more members of that species. But one species of influenza has another strategy for driving its evolution.
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Antigenic Shift: Influenza A's Special Trick
Influenza A is capable of phenomenon called antigenic shift, which could be described as "virus sex." In antigenic shift, very different strains of Influenza A trade large sections of their genome. This process requires these different strains to be present in the same individual at the same time. That individual could hypothetically be a member of the chicken farm's human staff, but there are other possibilities as well.
Pigs are thought to be a sort of laboratory where different subtypes of Influenza A can trade genes, creating new strains. In addition to hosting their own strains of Influenza A, the so-called swine flu, pigs can be infected by both bird flu and human flu. They may be a critical link in the evolution of a strain of H5N1 bird flu that scientists fear may one day devastate human populations.