Flu evolving in the human body ecosystem

The field of disease ecology is a fast-evolving one as ecologists realize more and more that the insides of animals and plants are really like small-scale ecosystems, encompassing the same rules as large-scale ecosystems, like species interactions, environmental variability and evolutionary change.

Katia Koelle of Duke University gave a talk yesterday about evolution in the H3N2 virus — not to be confused with the H1N1 swine flu, even though it infects both pigs and humans. Since its emergence in 1968, this H3N2 virus has evolved at lightning speed: about one-third of the amino acids that make up its hemagglutinin protein (where the “H” comes from) have changed.

“That’s a huge amount of evolution,” Koelle said in a Duke statement. “If there’s a new escape mutant that can actually so change the protein’s configuration that our antibodies can’t recognize the virus anymore, that means it’s going to have a huge advantage and infect more of us. “

Koelle and her colleagues developed a model to simulate the evolution of the virus in different hosts.  Just like animals evolve differently in different environments where there are different selective pressures, the virus evolves differently in different hosts. Farmed pigs live only about two years, whereas the average lifespan for humans is about 80 years.  The virus thus evolves quickly in humans, where there’s ample time for the virus to be wiped out by the hosts’ immunity. In pigs, though, new susceptible hosts appear much more often, so the virus has less pressure to evolve.

“How much of an advantage a new virus strain has will depend on how many people have gotten infected in the past,” Koelle said in the statement.