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This is ALL THINGS CONSIDERED from NPR News. I'm Michele Norris.

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And I'm Robert Siegel.

A body louse is rarely a welcome visitor. And if one louse is bad, a load of lice is literally lousy. Not only do they cause intense itching, but they can also spread disease.

To combat an enemy, you have to understand it, and that is why scientists decided to sequence the genome of the human body louse. NPR's Joe Palca has that story.

JOE PALCA: Before we get into the genetics, let's go over a little basic louse biology. First of all, without you and me, there would be no human body louse.

Dr. BARRY PITTENDRIGH (Geneticist, University of Illinois, Urbana-Champaign): It absolutely has to have the human host in order to survive.

PALCA: That's Barry Pittendrigh, a louse geneticist at the University of Illinois. I reached him in Turkey, where he had just finished attending the Fourth International Congress on Phthiraptera. Phthiraptera is the technical name for lice, and if you can spell that word, you've probably won a spelling bee at some point in your life.

Anyway, as Pittendrigh says, body lice can only survive on a human being. They get most of their nutrients from human blood, most but not all. They don't get vitamin B5. For that they have to rely on a bacterial endosymbiont. And before you can say can I have a definition, please, an endosymbiont is an organism that lives inside another organism and helps it out.

Dr. PITTENDRIGH: So in the absence of this endosymbiont, the body louse does not get the vitamins it needs to survive.

PALCA: So a louse with a healthy endosymbiont and access to your blood is a happy louse.

Human lice come in two varieties: head lice and body lice. Head lice are an itchy pain but otherwise largely harmless. Body lice can spread diseases.

Dr. JOHN M. CLARK (Entomologist and Pesticide Toxicologist, University of Massachusetts): Things like relapsing fever and epidemic typhus and trench fever that have killed, you know, millions and millions of people.

PALCA: That's John M. Clark of the University of Massachusetts. A few years ago, he and Pittendrigh decided it would be a good idea to learn more about the louse genome. So they sent some louse DNA to a colleague in Texas.

Dr. CLARK: And he called us back all excited because the genome turned out to be 108 megabases, which is enormously small.

PALCA: About a third the size of a bumblebee genome - and yes, lice are insects. Now, as they report in the journal PNAS, they have the complete sequence of all the letters of DNA that make up the louse genome.

May Berenbaum is also a louse geneticist. She says the louse genome has a mere 11,000 genes, and there's a reason lice can make do with so few genes.

Dr. MAY BERENBAUM (Geneticist, Cornell University): Over their evolutionary association with humans, they had sort of a cushy existence.

PALCA: Since they live on us, they don't have to hunt for food. It's right under their feet. So the louse genome is missing a lot of the genes other insects use to sense their environments. They're also missing a lot of genes to break down toxins.

Dr. BERENBAUM: There's only a dozen of these genes that deal with toxins, which is half of what honeybees have and a third of what the fruit fly has.

PALCA: And Berenbaum says there's a good reason for that, too. Human blood doesn't have many toxins.

Dr. BERENBAUM: They don't need an elaborate system for breaking down chemical poisons because over their evolutionary association with humans, they haven't had to deal with them until relatively recently.

PALCA: But now the poor, dear lice have to deal with the chemical poisons we use to try to kill them off. It could get even worse for lice in the future. Berenbaum says the genome sequence may suggest new ways to kill them off.

Joe Palca, NPR News, Washington.

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