A Planet of Viruses - Carl Zimmer [7]
Unfortunately, a poorly adapted flu virus can evolve into a well-adapted one. Flu viruses are particularly sloppy at replicating their genes, so many new viruses acquire mutations. These mutations are like random changes to the letters in the flu’s recipe. Some of the mutations have no effect on viruses. Some leave them unable to reproduce. But a few mutations give flu viruses a reproductive advantage. Natural selection favors these beneficial mutations, and flu strains can become better at infecting humans as mutation after mutation accumulates. Some mutations help the virus by altering the shape of the proteins that stud the virus shell, allowing them to grab human cells more effectively. Other mutations help the flu virus cope with human body temperature, which is a few degrees cooler than that of birds.
Human influenza viruses have also adapted to a new route from host to host. In birds, the viruses travel from guts to water to guts. In people, the virus moves from airways to droplets to airways. This new route also causes the flu rise and fall with the seasons. In places like the United States, most flu cases occur during the winter. According to one hypothesis, this is because the air is dry enough in those months to allow virus-laden droplets to float in the air for hours, increasing their chances of encountering a new host. In other times of the year, the humidity causes the droplets to swell and fall to the ground.
When a flu virus hitches a ride aboard a droplet and infects a new host, it sometimes invades a cell that’s already harboring another flu virus. And when two different flu viruses reproduce inside the same cell, things can get messy. The genes of a flu virus are stored on eight separate segments, and when a host cell starts manufacturing the segments from two different viruses at once, they sometimes get mixed together. The new offspring end up carrying genetic material from both viruses. This mixing, known as reassortment, is a viral version of sex. When humans have children, the parents’ genes are mixed together, creating new combinations of the same two sets of DNA. Reassortment allows flu viruses to mix genes together into new combinations, as well.
As scientists get a closer look at the genes of flu viruses, they’re discovering that reassortment has played a major role in the natural history of the flu. A quarter of all birds with the flu have two or more virus strains inside them at once. The viruses swap genes through reassortment, and as a result they can move easily between bird species. And sometimes, on very rare occasions, an avian influenza virus can pick up human influenza virus genes through reassortment. That can be a recipe for disaster, because the new strain that results can easily spread from person to person. And because it has never circulated among humans before, no one has any defenses that could slow the new strain’s spread.
Reassortment is important for other reasons than viruses jumping the species barrier. Once bird flu viruses evolve into human pathogens, they continue to swap genes among themselves every flu season. This ongoing reassortment allows the viruses to escape destruction. The longer a flu strain circulates, the more familiar it becomes to people’s immune systems, and the faster they can squelch its spread. But with some viral sex, an old flu strain can pick up less familiar genes and become harder to fight.
Humans are not the only hosts who have picked up flu viruses from birds. Horses, dogs, and several other mammals have also picked it up. And in April 2009, the world became painfully aware that flu viruses also infect pigs. An outbreak of so-called swine flu jumped from pigs to humans. It first surfaced in Mexico and soon spread over the entire planet.
The history of this particular flu strain, called Human/Swine 2009 H1N1, is a tangled tale of genetic mixing and industrialized agriculture. Pigs seem to have just the right biology for reassortment; some of their receptors can easily accept human flu viruses, while other receptors welcome