Free-Floating Hostility

Sunday, January 15, 2006

Epi Shack: Drug Resistant Flu Strains

I have an update to my previous briefing on Flu Epidemics. As always, you should feel free to ignore this information, but better you hear it from me than some putz on TV. I'm planning to study infectious disease this semester, so the next time I post about this I may have a more sophisticated understanding of Influenza. But for now...

The Event: Last night the CDC called a press conference, notwithstanding the fact that it was Saturday night of a Holiday weekend, to announce that this year's dominant flu strain is almost completely resistant to two of the four antiretroviral drugs used to treat it. Here is the AP article on the press conference.

Skipping to the trolls: Antiretrovirals are used on people who have already been infected with the Influenza virus. The flu vaccine on the other hand prevents infection by giving your immune system good intel on the virus so that when you are exposed you don't have to get sick first to destroy it. This year, as long as doctors get the message in time and can prescribe the two newer drugs (Tamiflu and Relenza), most patients who have not yet died of seasonal flu will have a good shot at recovery. But there are two obvious implications.

1. Drug resistance does not arise spontaneously (see below). Antiretrovirals are probably being obtained without prescriptions as a response to the high publicity this year about Influenza and then misused. If the new antiretrovirals are abused the way the old ones have been it is only a matter of time before influenza strains become resistant to Tamiflu and Relenza as well. It's not that these new drugs are so much more powerful as much as that they're still new that is allowing them to work.

2. In the case of a flu epidemic, we can't count on antiretroviral drugs. Right now the particular strain of flu known as Avian flu (which gets the most hype because it has such a high mortality rate) is not showing high resistance, but that could change in a heartbeat. The finding that prompted the CDC press conference is that 90% of the virus samples of this year's dominant strain of flu were found to be drug resistant. Last year only 11% of the same strain was resistant, and the year before that it was 2%. So just because Avian flu is currently treatable if diagnosed in time doesn't mean that it will stay that way. Near-universal vaccination is the only meaningful preparation, and we still don't have the capacity in this country or anywhere to produce that much vaccine quickly in an emergency. There is an existing vaccine for this year's strain of flu and for the Avian strain, but to make enough for a national or international population requires resources and infrastructure that are not yet in place.

A quick rundown on drug resistance (skip this part if this is old hat to you): Bacteria and viruses both have the capacity to mutate rapidly; that is, when they reproduce, a relatively high proportion of copies will contain a random mis-copied gene. If that new version of the gene inhibits that bacterium's or virus' function, it will die or never live. But if, by chance alone, it conveys resistance against a drug that is designed to kill or inhibit it, then this new mutant will thrive while all of the "correctly" copied originals will be killed off by the drug. A situation is created in which we confer selective advantage on the disease agent that we're powerless to treat (at least with that drug).

The context in which this usually arises is someone failing to take their medication for the full course. Let's say you see your doctor because you're feeling crummy and your doctor diagnoses you with flu. She prescribes an antiretroviral (amantadine, say) and you follow her instructions. The meds work, and your flu symptoms recede. Because you're feeling so much better, you either forget or decline to take the rest of the pills in your bottle. But the bad news is that just because your symptoms have receded doesn't mean your immune system is done; it basically just means the inflammatory response phase is over. So a virus living in your system that would be destroyed if the full dose was taken instead survives in conditions that cultivate the drug-resistant form of the virus. The same, by the way, is true for bacteria and antibiotics, which is why most doctors now warn you to take antibiotics for the full course of the prescription no matter how good you feel.

Take home message: get your flu shot, and do your best to encourage any children (who are the most effective at transmitting the disease), elderly or immunocompromised people in your lives get one too. Do not self-medicate on antiretroviral drugs or antibiotics, do follow prescription labels, do agitate for disease preparedness.

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