Biology Reference
In-Depth Information
1
Evolution's Fast Lane
In essence, it's a destructive form of molecular burglary; flu gets into the building,
cracks the safe, takes what it wants; and wrecks the place on its way out. 6
Pete Davies
The most ferocious of man-eaters is an innocuous companion of wild ducks and other waterfowl. At the
end of every summer, as millions of ducks and geese mass in Canadian and Siberian lakes for their annual
migration, influenza blooms. As researchers first discovered in 1974, the virus replicates harmlessly but
vigorously in the intestinal tracts of juvenile birds and is copiously excreted into the water. 7 Other birds
ingest this viral soup until as many as one-third of the young ducks and geese are producing influenza.
In northern lakes, moreover, diverse strains of influenza coexist in the same population, even within an
individual duck; one study in Alberta found twenty-seven different subtypes in a community of mallards,
pintails, and bluewinged teals. 8
During their migrations to the Gulf Coast and southern China, the birds continue to shed virus in their
feces for as long as one month, increasing the likelihood of the infection spreading to other species of wild
and domestic birds. By late fall, however, duck influenza fades to invisibility. Some virologists believe that
enough smoldering infection survives in the birds to be rekindled the following August. Others surmise that
influenza is tough enough to survive winter under lake ice. In any event, ducks and influenza both return to
the same lakes year after year. The cycle, in fact, may be hundreds of thousands, perhaps millions, of years
old. In the opinion of one textbook, it is “a classical example of an optimally adapted system.” 9 Influenza
prospers while ducks remain otherwise unharmed.
Influenza in humans, pigs, and other mammals, on the other hand, is far from such a happy equilibrium;
indeed, it is a radically different system of host-parasite interaction due to a variety of factors. In the first
place, the virus usually infects the respiratory tract rather than the gut and spreads by an aerosol rather
fecal-oral route. Second, it is highly pathogenic, causing an acute respiratory infection that sometimes kills
the host. Third, in contrast to genetically stable wild-duck influenzas, the species-jumping versions are ex-
traordinary shape-shifters that constantly alter their genomes to foil the powerful immune systems of hu-
man and mammalian hosts. The pandemic threat stems especially from this capacity for ultrafast evolution-
ary adaptation.
Influenzas are classified into three major genera: A, B, and C. Influenzas B and C have been domestic-
ated by long circulation in human populations. “Genetic studies,” a leading expert explains, “suggest that
[they] . . . diverged from the avian influenza A viruses many centuries ago.” 10 Influenza C is a cause of
the so-called common cold, while B produces a classic winter flu, especially among children. Neither is a
pandemic threat, although B is responsible for some of the annual influenza mortality in susceptible pop-
ulations. Influenza A, on the other hand, is still wild and very dangerous. Although its primary reservoir
remains among ducks and waterfowl, it is in the early stages of crossing over to humans and other bird and
 
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