In the course of reading this post, you will most likely make 4 assumptions, all of which will be wrong. The first assumption you probably already made when you read this title of this post, and assumed that Awesome Wife and I spent the weekend giving each other a hard time of sorts. That was not the case. The title is literal: we spent the weekend picking nits- specifically the eggs, or more correctly egg cases- of lice out of Twin B’s hair.
At this point you have already likely made the 2nd incorrect assumption, to which we shall return- and of which I will disabuse you- momentarily.
Parenthood brings many joys, some of which I’ve occasionally mentioned in this blog, but it also brings frustrations and headaches. And while in the big picture it isn’t anywhere near as bad as hearing that your child has been arrested or impregnated, it’s still kind of a drag to learn that your kid has picked up head-lice.
To her credit, Twin B figured it out first, after a week+ of itching and mild complaining, which AW and I- to our chagrin- largely ignored*. But a visit to the doctor quickly confirmed the infestation in her and Bird Whisperer’s heads. It turns out that a number of schoolmates have also been afflicted; if you’re a Salt Lake City reader with children at Beacon Heights elementary school, you might want to do a quick nit check.
*In our defense, Twin B has a history of overstatement and excessive melodrama when it comes to various perceived ailments, so perhaps there was a bit of a Peter & the Wolf effect at play.
All About Lice
Lice of course are human parasites. No, really I mean it. Not like politicians or insurance companies or FOX News commentators, but real human parasites, in that they live on a human body, and consume the products of that body, specifically blood. Lice are remarkably well-adapted to life on their hosts, with specialized claws that allow them to latch onto the hairs of their hosts and travel rapidly up or down them, as well as a saliva which when dried serves as a glue to attach egg-cases to individual hair strands. They reproduce quickly, from hatching to laying eggs in just 21 days.
Lice, insects of the order Phthiraptera, are a diverse and highly successful group, with over 3,000 species, most of which are specifically adapted to, and have co-evolved with, a specific host. They infest every order of birds and the vast majority of mammals*. Most bird species host between 2 and 6 different species of lice; most mammals between 1 and 3. Humans host 3. Or 2, depending on who’s counting.
*Whales, dolphins, bats, monotremes and a few other mammals- including one we will visit later on in the post- are lice-free.
The second assumption you may have made is that the Watcher Family lives in abject filth, and this I assure you is not the case. The Watcher Family enjoys a thoroughly first-world lifestyle, with daily showers, clean bedding and clothing, and daily room-straightenings*. But one of the interesting things about head-lice is that they very often infest the heads of children with good hygiene.
In fact, according to the UK’s National Health Service, lice may actually have an easier time infesting a clean head of hair, as it may be easier to attach egg-cases to clean strands. The typical first-world head-lice host, BTW, is a child between 5 and 11 years of age**. And somewhere between 60% and 80% of child head-lice hosts are girls.
*Although to be honest, it is difficult to keep the clutter in Bird Whisperer’s room under control. No matter how often it is “straightened”, it quickly reverts to its naturally chaotic state. He’s somewhat of a night owl, and late at night AW and I can hear various pacing, jumping, banging and crashing from his room downstairs. We’re not exactly sure what he’s up to, but suspect he is developing either a small aircraft or a suitcase-warhead.
**This is probably the only really compelling argument I’ve come across for home-schooling.
Side Note: Here’s a fascinating little tidbit about head-lice: they have a way tougher time infesting the heads of African-Americans. In fact a 1985 study showed African-American children to have less than 3% the infestation rate as Euromerican kids! But interestingly, African kids in Africa do routinely get head-lice. What gives?
American head-lice are largely descended from those brought over by European colonists and have claws well-adapted for clinging to European head-hairs, which are generally round-ish in cross-section. African head-hairs are more oval in cross-section, and our Euro-lice have a tougher time gaining a hold on them.
All head hairs are slightly oval in cross-section. In general the rounder a strand of hair is, the straighter that hair lies on the head. Most head-hairs average about 0.1mm in thickness, but they can range from 0.04mm to 0.25mm. Hair thickness also varies with race, with Africans usually having thicker head-hairs than Europeans.
Most head-lice infestations involve fewer than 20 live, adult lice at any one time. An adult female lays 3-5 egg-cases per day, so you generally see far more nits than there are actual lice on a given head. It takes roughly a month for a full-blown infestation to develop from the time the first egg-laying female makes her way into a head of hair. Contrary to popular belief, lice don’t jump, but are transmitted through close contact and shared brushes, combs, hats, clothing and bedding. Twin B’s good friend next door- who was also afflicted- and she frequently engage in dress-up games involving exchanged hats, scarves, dresses, etc. Here at home the Trifecta is fond of weekend sibling “sleep-overs” in which they all play and then sleep in 1 bedroom. In several of these sleepovers, we’ve noticed that Twin B and Bird Whisperer- who are particularly close- wind up sharing a pillow (pic right).
Tangent: Rationally, I get why head lice still exist in the first world, but intuitively it still stumps me. Given that they can’t survive longer than a day or so off-head, it amazes me that we haven’t gotten rid of head lice. I have 2 ideas for eradicating them:
First, Planet-Wide Quarantine. Everyone would go to their own room with food and water and lice-killing shampoo and stay there for a month. Of course everyone in the world would need their own private room with running water and a refrigerator and little infants would have to be tended by robot nurses… OK so, it’s kind of a dumb idea now, but maybe in the future we could do it, and who knows what else we’d eliminate? The flu? The common cold? Multi-level-marketing? (Actually this idea is weirdly appealing to me in that I’d finally get to catch up on my reading…)
Second, Planet-Wide Head-Shaving. Everyone shaves their head on the same day. Then we all do it again 10 days later so that no newly-hatched lice (from shaved-off nits) would have a place to feed or lay eggs. How about that?*
*Actually, it wouldn’t eradicate body lice, which would probably cross back over into being head lice soon enough… OK, so both these ideas suck. But I’m going to make up for it later on in the post with an Awesome Half-Baked Theory.
Different Lice For Different Parts
Humans host 3 types of lice, which are sometimes considered 3 distinct species and other times 2 species, one with 2 subspecies. In any case, 2 of the 3 are very similar and closely-related, and the other is way different. The 2 similar types are Head Lice, Pediculus humanus capitus, and Body Lice, P. humanus humanus. The 2 types are practically indistinguishable and can interbreed, but in the “wild” (i.e. on you) they occupy different habitats. Body Lice lay eggs in clothing, are transmitted by shared clothing or bedding and bite parts of the body that come into contact with these items. Interestingly, while Head Lice are not known to transmit any diseases, Body Lice are a transmission vector for Typhus.
DNA studies of Head and Body Lice reveal 2 interesting things. The first is that they seem to have diverged roughly 100,000 years ago, and the second is that they appeared to have “crossed-over” from hair to clothing multiple times. These 2 observations suggest that the evolution of Body Lice may have been coincident with the “invention” of clothing, which presumably came about sometime in the last 100,000 years or so and which was probably not a single “invention”, but likely came about a number of times and locations independently.
But it’s the 3rd human-specific louse species that’s really interesting. This is Crab Lice, Pthirus pubis, which infests primarily pubic hair, but also armpit hair and even eyelashes*, and which is radically different in form from Head and Body Lice. Head Lice never infest pubic hair, and Crab Lice never infest head hair. Why not?
*Which is how and why Crab Lice can sometimes afflict prepubescent children.
Pubic hair is coarser, thicker and has a much flatter cross-section (photo right, not mine*) than head hair. Each species has claws adapted to its own type of hair, and neither can gain a foothold (or clawhold) in the other type of hair.
*Of course it’s not mine. What, you think I bought an electron microscope? I told you already, I blew the blog-budget on STICKERS.
OK, so now we know what lice are, how our kids “catch” them and why which types favor which locations. But where do they come from?
The Bizarre Evolution Of Lice
Lice have been co-evolving with primates for at least 25 million years. Our own Head and Body Lice are fairly closely-related to Pediculus schaeffi, the louse that infests Chimpanzees.
Side Note: With Chimpanzees, Gorillas and other non-human primates, there’s no distinction between Head, Body and Pubic Lice because there’s no such distinction between the hairs these primates have in different regions. Alone among primates, only humans have distinct head and pubic hair. Or facial hair for that matter, as I mentioned in this post.
DNA research on Chimpanzee and Human Head/Body Lice show that they diverged from a common ancestor a little over 6 million years ago. And in light of what we know about human evolution, this makes perfect sense. Chimpanzees are our closest living relatives, and DNA evidence indicates that we last shared a common ancestor somewhere between 5 and 7 million years ago. So far, so good; the co-evolution story looks nice and tidy.
After Chimpanzees, Gorillas are the living things most closely-related to us. It’s thought that we and they last shared a common ancestor a little over 7 million years ago*. So a reasonable reader- or at least one still paying attention- might reasonably assume that Human Crab Lice and Gorilla Lice last shared a common ancestor somewhere around 7 million years ago. This would be the 3rd wrong assumption. DNA evidence indicates that these 2 lice species shared a common ancestor only a little over 3 million years ago. How can this be?
*To be clear, Chimpanzees are more closely-related to us than they are to Gorillas. The 7 million-year-ago split referred to here was between the ancestor of Gorillas and the common ancestor of both Chimpanzees and Humans.
A likely explanation appears to be a “host-switch” of Gorilla Lice from Gorillas to humans roughly 3 million years ago. Lice species can occasionally switch hosts*, and such a switch from proto-Gorillas to proto-humans would explain the genetic history of Crab Lice.
*As can bacteria or viruses. Recent examples of host-switching viruses include “Swine Flu”, “Bird Flu”, and of course HIV.
Gorilla hair BTW is thicker and coarser than human head hair, and is more similar to human pubic hair, which would explain why the host-switch was only to our lower half. But it raises an awkward question: How did we get it? Among humans Crab Lice is overwhelmingly transmitted via sexual contact. Did ancient hominids and Gorillas get it on?
Not necessarily. Remember, Crab Lice are pubic lice for us, but they’re body lice for Gorillas. Lice could have been transmitted between the 2 species via shared bedding/leaves or even via a predator-prey relationship*.
*Which is apparently how humans acquired HIV from Chimpanzees sometime in the last half-century, not just once, but at least 3 times. 1 of those host-switches led to the current worldwide AIDS epidemic.
OK, so assuming such a host-switch occurred from Gorillas to hominids, the astute, still-paying-attention reader might well make a 4th very reasonable assumption: that Pthirus- the Gorilla & Human Crab Lice genus- and Pediculus- the Chimpanzee & Human Head/Body Lice genus- last shared a common ancestor around the time when Gorillas and Chimps/Humans diverged, somewhere around 7 million years ago. Sadly, that reader would be wrong. DNA evidence indicates that Pthirus and Pediculus last shared a common ancestor some 13 million years ago.
Scientists make mistakes, and the “molecular clocks” revealed by mitochondrial DNA are constantly being revised and updated*, but a difference between 7 and 13 million years is extremely unlikely to be a mistake. 13 million years coincidentally is about the time of divergence of the common ancestor of Humans, Chimps and Gorillas from that of our next closest-living relative, the Orangutan. But Orangutans (both subspecies- there are 2) have no lice.
*For example, the divergence estimate for Head and Body Lice was initially estimated to be ~70,000 years ago, but more recently revised to 107,000 years ago.
So it’s unclear where Pthirus came from. One explanation is that Pthirus and Pediculus speciated for unknown reasons of geography, isolation or what-not and then both infested the common ancestor of Gorillas Chimpanzees and humans, but that Pediculus later became extinct in Gorillas while Pthirus later became extinct in Chimps/Humans, before re-infesting humans some 3 million years ago.
Conjectural Tangent: I’ve got another “theory*”, almost certainly completely wrong, but it’s my blog, so here goes:
*As frequent readers of this blog well know, all of my “theories” are completely half-baked.
The 13 MYA date is way fishy. Orangutans are the sole survivors of a once diverse and broad-ranging sub-family of primates, Pongidae, which included the now extinct Gigantopithecus, who at 10 feet tall was the largest primate ever to have lived. Perhaps Pthirus does date from the Pongidae-Homininae split and evolved as lice species adapted to some (now-extinct) Pongid species. If this species came into contact (common habitat, predator-prey, etc.) with the ancestors of Gorillas after the Gorilla-Chimp/Human split, an earlier host-switch could explain the presence of Pthirus on Gorillas*.
*My “theory” falls short in that it doesn’t explain the absence of Pediculus on Gorillas. 2 possibilities are that a) Gorillas lost Pediculus subsequent to the Gorilla/Chimp split or b) that Pediculus was the result of yet another host switch from some other primate to the common Chimp/Human ancestor, sometime after the Gorilla/Chimp split, but before the Chimp/Human split. OK now I’m getting really wacky**.
**But in my defense, when you spend hours and hours picking nits, you think about all kinds of weird stuff.
As far as Orangutans go, it’s worth noting that they practice a far more solitary lifestyle than Humans, Chimpanzees or Gorillas, which presumably makes them poorer hosts for lice. But we don’t know whether their Pongid ancestors from ~13 million years ago were as solitary, and perhaps the ancestors of Orangutan had lice but lost them as they evolved their present-day solitary lifestyle.
Anyway, regardless of the exact origins, the evolution of human lice is absolutely fascinating. But, as I so often do in these long, run-on, tangent-strewn posts, I have saved the best for last.
Let’s get back to our own Head Lice, P. humanus. DNA evidence shows that modern-day human head-lice populations break down into 2 distinct clades. A clade is a monophyletic* group consisting of a single common ancestor and all its descendants. One of these clades occurs all over the world, and the other occurs only in the New World. Now here’s the weird thing: these 2 clades diverged roughly 1.2 million years ago.
*For an easy explanation of monophyly vs. para- or polyphyly, see this post.
On the face of it, this makes no sense. Modern humans almost certainly evolved from a single small ancestral population sometime in the last 200,000 years, and left Africa only 100,000 years ago. How could our head lice have diverged over a million years ago?
The likeliest explanation is another host-switch, from another, now-extinct, hominid species, to us. About a million years ago is roughly when our ancestors are thought to have diverged from Homo erectus.
Homo erectus was a bipedal hominid who lived from somewhere around 2 million years ago to only about 30,000 years ago. It used fire and tools, hunted, and had a brain 2/3 the size of ours*. H. erectus is generally considered to be the ancestor of several later hominid species, including Neanderthals and us. Somewhere between 1 and 2 million years ago some “version” of H. erectus migrated out of Africa and across much of Asia, probably more than a million years ahead of Homo sapiens.
*For comparison, a Chimpanzee’s brain is 1/3 the size of ours.
Tangent: It’s worth pausing for a moment on the time-frame here. We tend to think of ourselves as the pinnacle of evolution. But our species has only existed in its present form for maybe a 150,000 – 200,000 years, we’ve only had real culture- with art and decoration and sewing- for the last 40,000 – 80,000 years, and only lived in the same places for maybe 10,000 years. H. erectus lived and roamed much of the world for 2 million years- apparently without wrecking it. Who’s the more successful hominid?
Modern humans migrated out of Africa sometime in the last 100,000 years, and throughout the Old World. More recently, probably 12,000- 15,000 years ago, humans migrated into Siberia, across Beringia and into North America. It’s suspected that en route modern humans encountered still-living H. erectus populations in Asia*, acquired the “New World Clade” of P. humanus from them, and carried it with them to the New World.
*Much as modern humans encountered Neanderthals in Europe, of which paleoanthropologists are pretty certain.
Side Note: Supporting evidence for this hypothesis is that Body Lice appear to have evolved exclusively from the “Worldwide Clade” of head-lice, implying that modern humans encountered the New World Clade well after the “invention” of clothing.
The story of lice and human evolution just gets curiouser and curiouser. More recently a third clade of human Head Lice may have been identified in Ethiopia and Nepal (of all places!), and this clade appears to have diverged from the Worldwide and New World clades some 800,000 years before the Worldwide- New World split.
Lice-removal kits include a shampoo, a gel and a comb. The shampoo- with which the entire Watcher family has now been treated- contains 2 active ingredients. The first is Pyrethrin (C21H28O3 or C22H28O5, diagram left). Though highly toxic, it’s a natural insecticide obtained from the seedcases of a species of Chrysanthemum, and is biodegradable. The second is Piperonyl Butoxide (C19H30O5, diagram right), which is a pesticide synergist, meaning that while it’s not a standalone pesticide, it dramatically increases the potency of several other pesticides, including Pyrethrin. While not especially (directly) toxic to humans, it is suspected to possibly be carcinogenic, and may even be linked to birth defects*. In any case, you don’t want to shampoo regularly with this stuff. We’ll re-treat in a week, after any surviving eggs have had opportunity to hatch.
*Neither claim has been proven.
The gel supposedly loosens nits from hairs; we found it somewhat effective for our short-haired boys, but for Twin B, whose thick tresses consumed >95% of our nit-removal efforts, we soon resorted to scissors, cutting a strand at a time. A buzz-cut would’ve saved us hours of time and frustration, not to mention enduring cricks in our necks, but that’s not really a humane option for an 8 year-old girl. Over the years Twin B has been a delightful daughter: smart, kind, and considerate, quick with a smile or a laugh. She’s been, quite honestly, our lowest-maintenance child so far, well worth the tedium of a weekend of nit-picking.
Note About Sources: Over the last few days I’ve read dozens of lice-related sites and papers. Although plenty of information is available on the topic, much of it- including specifics re: survival period of off-head lice, daily egg-laying capacities of female lice, survivability of eggs, rates/demographics of infestation,and efficacy of various treatments- varies widely between sources. In this post I’ve tried to stick with stats that seem most commonly cited, and made a few subjective calls along the way.
Side Note: Normally, when blogging about a medical-related topic for which so much of the available information is conflicting, I’d advise a reader to consult with their physician. Unfortunately the after-hours “KidsCare” physician to whom I took the Trifecta was remarkably poorly-informed about many of the details of lice, their lifecycle, and even treatment specifics. (I’ve half a mind to send him this post. Ironically the admitting nurse, who’d dealt with an infestation this past summer on her own daughter, was exceedingly helpful.)
The 2 best sources on lice evolution I found were this paper by DL Reed et al and this paper by Robin A. Weiss (which heavily cites Reed’s work.) As always I am exceedingly grateful to researchers and institutions who make their work freely available to the public.
Chemistry info on the active ingredients of lice-killing shampoo was obtained from Wikipedia and the National Pesticide Information Center. Info on human head-hair comes in part from Chemical and physical behavior of human hair, by Clarence R. Robbins.