Click here to read more from Slate on the swine flu.
The Department of Homeland Security has declared the swine flu a public-health emergency. Avian influenza remains active in Africa, Asia, and Europe. So pigs and birds each have their own form of influenza. Can all species get the flu?
Many other animals do contract influenza from time to time, but in most species it tends to die off quickly. Pigs, birds, humans, horses, and a few marine mammals like whales and seals have developed and sustained their own unique strains of the flu. In general, scientists won't label a strain as host-specific until most cases result from intraspecies transmission and the virus spans more than one generation of the host animal.
Birds are known to carry every single one of the 144 varieties of influenza virus, as defined by the shape of their surface proteins (ranging from the H1N1 strain to H16N9). For this reason, most scientists believe that all forms of the virus originated in birds and every flu is on some level a kind of bird flu.
Human, swine, and other host-specific strains are merely mutated avian strains that managed to infect a new kind of animal and spread across several generations. For the source virus to jump hosts, it would have to evolve a slightly different shape to its surface proteins, making them capable of binding to novel cell receptors. (This change usually does not represent an entirely different protein structure—i.e., the strain won't change from H1 to H2—but rather involves a shift of just a few molecules.)
Pigs happen to serve as efficient way stations for the transfer of influenza from birds to humans. That's because they have some bird-shaped cell receptors and some human-shaped cell receptors. An avian influenza strain can infect a pig without too much rearranging and then evolve the ability to infect humans. In addition, the same pig can harbor both human and avian flu. If the two strains exchange the right genetic information, they might produce a more versatile and dangerous offspring.
Mutation and infection alone are not sufficient to launch a species-specific strain of influenza. The new viral strain must be exposed to the new host species, and then it must keep spreading to more individuals before either the virus or host dies. For most species, those are pretty long odds. Avian influenza has infected cats, ferrets, and possibly dogs, but none of these mutant strains has survived long enough to be classified as a new, species-specific disease.
So why did the virus persist in humans, pigs, and birds? No one knows for sure, but humans and pigs tend to live in very close quarters. The survival of influenza among birds has two likely causes. First, most strains of avian influenza do not make their host sick, so the virus has plenty of time to evolve and be transmitted. Second, influenza survives quite well in cold water. When migratory birds settle briefly into small ponds, they leave the virus for the next set of travelers. Under these conditions, the disease spreads very easily.
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Explainer thanks David Swayne of the U.S. Department of Agriculture and Gary Whittaker of the Cornell University College of Veterinary Medicine.