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Lice... the mere mention of them is enough to make most people all itchy. Well, get ready to get scratching, because we've compiled seven squirm-inducing lice facts on this week's List Show!

Hosted by: Hank Green
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Sources:
https://projects.ncsu.edu/cals/course/ent425/library/compendium/phthiraptera.html
http://www.cdc.gov/parasites/lice/head/gen_info/faqs.html
http://www.cdc.gov/parasites/lice/head/biology.html
http://www.livescience.com/41028-lice-reveal-clues-to-human-evolution.html
http://www.livescience.com/9225-humans-lice-clothed-naked-hairless-bodies.html
https://blogs.scientificamerican.com/guest-blog/of-lice-and-men-an-itchy-history/
https://www.sciencedaily.com/releases/2008/02/080207120006.htm
http://www.cfp.ca/content/58/8/839.long
http://www.zoologia.hu/list/Why_infest.pdf
http://pediatrics.aappublications.org/content/114/3/e275.full
http://www.livescience.com/55695-lice-resistant-to-treatment.html
http://jme.oxfordjournals.org/content/early/2016/03/30/jme.tjw023
http://news.bbc.co.uk/2/hi/health/4534540.stm
http://phthiraptera.info/Publications/38690.pdf
http://mbe.oxfordjournals.org/content/28/1/29.full.pdf
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2083419/
http://news.nationalgeographic.com/news/2007/03/070316-gorilla-lice.html
http://www.smithsonianmag.com/science-nature/nitpicking-the-lice-genome-to-track-humanitys-past-footsteps-29121653/
http://www.livescience.com/27523-lice-genes-reveal-human-migration.html
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4978820/
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0057619#pone.0057619-Burgess2

 Intro (0:00)


Hank: Lice. Little creatures can strike fear into the hearts of parents and school administrators around the world and hundreds of millions of people deal with lice infestations every year. But as annoying as they are humans and lice have shared a long, itchy history.

They have evolved along with us and they've come along for the ride as we migrated around the world. They've even affected the outcomes of our wars. So here are seven facts about these pests that you might or might not want to know depending on how itchy you are right now. Just like- just talking about it just talking about it!

[SciShow intro plays]

 Over 5,000 Species (0:43)


There are more than 5,000 species of lice, they are tiny and wingless and they are experts at feeding on birds and mammals. And they have to be experts: they are obligate parasites being that their life cycle depends on finding a host to feed on.

In general lice can be divided into two groups: sucking lice which have special suction mouthparts and chewing lice which have jaw like mouthparts. Both types sound pretty gross.

Depending on the feeding mechanism they use different kinds of lice can infest lots of different warm-blooded animals with some lucky exceptions like bats. Adult lice only live about 30 days. In that time they mate and then the females lay eggs called nits right at the base of the hair shaft.

These nits hatch in about a week and become small immature nymphs which stay attached to the hair shaft and start feeding on blood from the skin. In about two weeks the nymphs mature into adult lice and start the cycle all over again. They feed on blood several times a day. Nymphs can't live without blood for more than a few hours and adults die within a couple of days.

So for lice finding and sticking to a host is key. Luckily, out of all of those thousands of species there are only three types of lice that are known specifically to infect humans: head, body, and pubic lice.

Head lice or Pediculus Humanus Capitis are probably the most well-known. They are common pests but head lice aren't typically very dangerous they're just very itchy. Humans can also be infested with Pthirus Pubis the pubic louse AKA crabs.

The pubic louse is smaller and rounder with large claws adapted to attaching to coarser pubic hairs. It can also infest other parts of the body with coarse hair like eyelashes and eyebrows. Like head lice, pubic lice don't really transmitted disease they just cause lots of itching.

  Body Lice cause Disease(2:27)


Then there's body lice. They're so closely related to head lice that they're considered different types of the same species, but the body lice can be dangerous. They've been associated with diseases like Typhus, Trench Fever, and Louse-born Relapsing Fever. That's because body lice can carry the bacteria that cause these diseases and the poop from an infected louse is contaminated. If human host scratches a bite or even just has a small cut or scrape, the bacteria in the feces can get into the bloodstream.

Typhus and Louse-Born Relapsing Fever are often fatal. Trench Fever isn't deadly but it is severe; it causes very high fevers for days at a time.

Body lice were common in trench warfar,e where soldiers were crammed together in unsanitary conditions, and those lice would spread disease. Part of the reason Napoleon failed in his invasion of Russia in the early 1800s eighteen hundreds was that tens of thousands of his soldiers died from Typhus spread by lice. And the trench fever was a huge problem during both world wars. So body lice are the ones that you probably want to avoid.

 Head Lice might be beneficial (3:27)


The best way to prevent body lice is to avoid contact with other people who have it and to wash your clothes and body on a regular basis. But it turns out that another way to avoid getting body lice might be getting head lice.

A group of researchers from Hungary proposed this idea in 2012, based on the fact that humans often touch head to express affection, like when you lean your head against your friend's head while you're taking a photo.

Humans do this all the time, but non-human primates don't. And there's another thing that's unique to humans: two separate kinds of lice for our heads and bodies. The team realized that there might be a connection between head touching and lice.

There's some evidence that exposure to body lice can provide some protection against them. When a separate group of researchers injected rabbits with an extract from body lice feces, the lice had a much harder time infesting them. They ate much smaller blood meals, laid fewer eggs, and fewer of the eggs that they did lay managed to hatch. And remember head lice are very closely related to body lice.

So it's possible that having a mostly harmless head lice infestation could provoke an immune response in humans protecting them from a much more dangerous body lice infestation later on. If that's true, the Hungarian researchers argue that affectionate head-touching might have evolved as a way to intentionally transmit head lice to your friends and loved ones helping protect them from body lice and the diseases they carry.

 Body Lice and Clothing (4:48)


But even though body lice are the bad kind, they can be super useful for researchers studying human history. It's easy for head lice to attach to thicker heads of hair, but we don't have much body hair as our early ancestors, so the lice needed a way to get up close and personal with our bodies. And genetic analysis of head lice and body lice show they diverged on the evolutionary tree around 170,000 years ago, which means that's probably when humans started experimenting with clothing. That makes sense since the kind of mini ice age began around a 130,000 to 180,000 years ago, so clothing may have become more of a necessity. But early humans lost the majority their body hair around a million years ago, so we spent a lot of time surviving in the wild without body hair or clothing.

Pubic lice can also tell us something about human evolution as specifically our ancestors sex habits, since pubic lice are mainly transmitted through sex. Head and body lice are super closely related to each other, but pubic lice are much more distantly related. It's not even the same genus. In fact, it shares a closer ancestry with a different species: Pthirus Gorrilae which infests gorillas.

Now these two species diverged around 3.3 million years ago when our Australopithecus afarensis ancestors were around. Which means that at some point Australopithecus and gorillas were in close enough contact for gorilla pubic lice to infest our ancestors, possibly sexual contact. But we know that pubic lice can also be transmitted by sharing bedding or towels, so it's possible that Australopithecus and gorillas were just sharing habitats and that our ancestors reused nests where gorillas had slept. Which would be... I would... I mean... I understand it's nature things happen nature but that would be less disturbing.

 Lice and human migration (6:30)


The genetics of lice can tell us about the history of human migration and civilization too, because when our ancestors left Africa and spread around the world they brought lice with them. Nits have been found on ancient artifacts like pieces of clothing and personal combs, and they've also been found preserved in the bodies of mummies from as far back as 10,000 years ago.

And studying the genetics of lice from these mummies, as well as modern samples from around the world, can help track how humans migrated out of Africa to Asia and the Americas. In a 2013 study for example, researchers looked at lice genes from samples collected from Asia, North America, Central America, and Europe, and they found that lice from Central America had more of their genome in common with lice from Asia, while lice from North America where more closely related to samples from Europe. Which means that lice in Central America were probably brought from Asia thousands of years ago, while the most common strains of lice in North America were spread by European settlers much more recently.

 Treatment resistant lice (7:27)


So studying lice can tell us a lot about our evolutionary history, but we're also having a huge impact on their evolution. We keep trying to kill them, and they keep becoming resistant to the treatments we use.

A type of chemical called a pyrethroids, for example, used to be great at killing off insects including lice. But starting around the 1990s they became much less effective on lice.

The lice had acquired genetic mutations in a specific gene that help them survive by reducing the sensitivity of the nervous system in response to the insecticide treatment. And when these so-called super lice survive treatment, they breed with each other and they pass along their resistance their offspring so you end up with even more super lice. But there are other classes of topical insecticide that do still work effectively.

Benzyl alcohol, for example, essentially suffocates the lice by targeting their respiratory system, though it can have side effects like skin irritation. There's also spinosad and ivermectin, which both contain compounds that affect the insect nervous systems. But eventually lice might develop similar genetic resistance to those drugs too.

There are other effective methods though. One study coated hair in a skin cleaner, specifically Cetaphil, and then blow dried it and fill it hardened into a kind of shrink wrap coating and left it there for eight hours. The researchers found that the hardened coating suffocated life with a ninety-five percent success rate. And in clinical testing heated air from a specially-designed blow dryer was shown to kill ninety-eight percent of nits and eighty percent of live lice.

But if all else fails you can always do what humans have been doing for thousands of years: take a fine-tooth comb and spend hours picking out nits and adult lice. Or of course just give up and shave your head entirely.

Thanks for watching this episode of SciShow which is brought to you by our patrons on Paetron, who paid to learn about lice today. Thank you for doing that! And also they paid so that you could learn about lice, unless you're one of the people who paid in which case... yeah! Thank you! If you want to watch more SciShow you can do that youtube.com/SciShow there's a subscribe button, it's right down below the video. Or it's at youtube.com/SciShow.