Pigs are a “mixing vessel” for new strains of flu

The influen­za virus­es that cir­cu­late dur­ing an ordi­nary flu sea­son are bad enough. But every so often, a new and more dan­ger­ous strain of influen­za virus starts cir­cu­lat­ing. The worst strains of flu are hybrids of ordi­nary flu virus­es with strains that don’t nor­mal­ly infect human beings. Some researchers believe that the dan­ger­ous new strains of flu arise when unre­lat­ed strains of flu virus meet and swap genes in domes­tic pigs.

There are two basic ways in which a new strain of the flu can get start­ed. One is through ordi­nary muta­tion. Influen­za virus­es mutate incred­i­bly fast because their genes are in a sin­gle strand of RNA. With­out the “back­up copy” that you find in dou­ble-strand­ed DNA, they can’t use their host cell’s proof­read­ing equip­ment to find and cor­rect copy­ing mis­takes in their genes. Here’s a tech­ni­cal expla­na­tion, for peo­ple who like math. It explains why influen­za virus­es mutate about 300 times faster than poliovirus and even about 10 times as fast as HIV.

Nor­mal­ly, an unsta­ble genome would be a bad thing. That’s because most muta­tions are harm­ful to the virus itself. A virus with an unsta­ble genome would tend to die out unless it repro­duced itself in large num­bers very quick­ly and had the abil­i­ty to recom­bine with relat­ed virus­es. But under those cir­cum­stances, the genet­ic insta­bil­i­ty turns into an advan­tage! It enables the virus to stay one step ahead of the host’s immune sys­tem. We can get the flu again and again because the flu that’s going around this year is a lit­tle bit dif­fer­ent from the strain that went around last year. The anti­bod­ies we made last year aren’t a per­fect match for this year’s flu.

The very worst flu strains are the ones that are rad­i­cal­ly dif­fer­ent from the ones that had been going around in ear­li­er years. They can infect more peo­ple and cause a more seri­ous ill­ness. These pan­dem­ic flu strains seem to rep­re­sent a com­bi­na­tion of genes from dif­fer­ent strains of flu: some ordi­nary human flu strains plus some unfa­mil­iar type of flu that nor­mal­ly infects wild birds. That’s why there’s been so much hys­te­ria late­ly about “avian flu.” Pigs seem to be a per­fect “mix­ing ves­sel” for new and dead­ly strains of flu, such as the “swine flu” that caused mil­lions of deaths at the end of World War I.

Pre­vent­ing future flu pan­demics would be sim­ple. All we have to do is stop rais­ing domes­tic poul­try and domes­tic swine. At the very least, the use of antivi­ral drugs in agri­cul­ture should be banned. Oth­er­wise, they will no longer be use­ful against flu.