How Flu’s Mutations Threaten Birds, Pigs and Humans

New strains of influenza are constantly emerging. Although the virus is associated with winter flu epidemics in people, wild migratory birds are its main target -- and are responsible for much of its global distribution. From them it may jump into mammals, especially pigs, where new strains can incubate. In general, novel flu viruses are a threat for birds, sometimes a problem for pigs, but are seldom a major issue for humans. Occasionally, though, new strains capable of infecting people do emerge, raising concern because of the potential for them to mutate and become better suited to person-to-person transmission, like the H1N1 swine flu that sparked a pandemic in 2009.

Horses, ferrets, dogs and even sea otters are susceptible to flu, but birds and pigs are the main worry for humans. The possibility of a pandemic arises when flu is passed from a wild bird to a human, usually via a domesticated bird like a chicken, or a pig. Sometimes the animal is also infected by a human flu strain, and they combine to produce a variant like the H1N1 swine flu that killed an estimated 284,000 people. People have little or no immunity to new strains, and existing vaccines don’t protect against them, so they spread easily. Flu pandemics have occurred four times in the last 100 years. In 1918, the most devastating of them killed as many as 50 million people. Among humans, flu is transmitted mainly via tiny droplets that they emit when they cough, sneeze or talk, although airborne transmission is thought to be possible.