Gorillas Gave Us the Itch

Among primates, humans have the dubious distinction of having not one, but two types of sucking lice--one that nests on the head and the other that calls pubic hair home. Where did these buggers come from? A new study suggests that while we've been sharing head lice with chimps since before we were even separate species, pubic lice came from gorillas--and not very long ago.

Each year, 3 million people in the United States catch a case of pubic lice, otherwise known as "crabs." It's an important public health problem, but evolutionary biologist David Reed of the Florida Museum of Natural History in Gainesville prefers to look at it as an evolutionary conundrum. Typically, lice share the same evolutionary history as their hosts. For example, if a population of hosts splits into two and each of the isolated populations begins to evolve into separate species, then the parasites evolve too. Human head lice, for example, share a common ancestor with a chimp-based louse, and the two lice are closely related--just as chimps and humans are. But the closest relative to human pubic lice is not found in chimps, but rather gorillas, which are more distant kin from humans. Reed wanted to know why.

He and his colleagues compared the DNA sequences from two genes in various lice and calculated when the different species appeared. As expected, human and chimp Pediculus (head lice) began to diverge about the same time as hominids and chimps started down independent evolutionary paths, about 6 million years ago. The gorilla and pubic lice (both Pthirus species) split just 3.5 million years ago, however, about 4 million years after gorillas and the line that led to chimps and hominids diverged. Thus rather than co-evolving with our ancestors from the very beginning, these lice were acquired at a later point in time, Reed and his colleagues report today in BMC Biology.

Because lice rarely survive more than a few hours off their hosts, this jump required close contact between the two species. "They must have been in the same place at the same time," Reed points out. Vincent Smith, a cybertaxonomist at the Natural History Museum in London, says the study "raises some interesting questions about what gorillas and our hominid ancestors were doing in such close proximity to share each other's lice.

Head Lice Are Lurking!

While many insects have become dormant as the cold weather settles in, head lice are now thriving—keeping nice and toasty warm on humans. Head lice are human parasites, spending their entire lives on human heads. Head lice are spread between people from head-to-head contact, or by sharing clothing or grooming items.

From kids piling their hats and coats together at school to those amusing moments when you try on all of the funny holiday hats at the store, head lice are getting many opportunities to travel and spread.

You are probably wondering now what these sneaky little critters look like. The human head louse adult is about the size of a sesame seed and grayish-white to tan in color. Head lice eggs, called nits, look like tiny yellow dots and are often mistaken for dandruff, though you cannot brush them off. Adult lice lay nits on hair shafts close to the scalp.

Once the nits hatch, the lice will feed on very small amounts of blood drawn from the scalp. Their bites can cause intense itching and small patches of inflammation. Head lice do not transmit disease, but heavy infestations can cause severe scalp irritation. Also, persistent scratching by an infested person can lead to secondary infections.

Nits need to be removed mechanically, using a special comb. Lice cannot survive off their human host for longer than 48 hours, and therefore, do NOT live in the environment! This means that there is no reason to apply pesticides to a home or business in response to a head lice outbreak. Also, head lice are host specific, meaning they prefer to stay on humans, so there is no need to worry about treating the family pet or that the pet can spread head lice.

What’s new in the head lice arsenal?

In regard to school management, the new guidance reiterates its assertion that children should not be excluded from school or school events because of lice, and that screening for nits is not a good indicator of infestation. In fact, such screenings have been shown to have little effect on the incidence of head lice and are not cost effective. For example, the AAP highlights a study in which, of 1729 children screened for head lice, only 31% of the 91 children with nits had an active live lice infestation. Another 18% with nits developed an infestation within 2 weeks of observation.

“Because of the lack of evidence of efficacy, routine classroom or schoolwide screening should be discouraged,” the AAP says. “Although children with at least 5 nits within 1 cm of the scalp were significantly more likely to develop an infestation than were those with fewer nits (32% vs 7%), only one- third of the children at higher risk converted to having an active infestation. School exclusion of children with nits alone would have resulted in many of these children missing school unnecessarily.”

Additionally, says the AAP, lice infestations have low contagion in classrooms. Between that and the fact that children who are diagnosed have likely been infested a month or more by the time of diagnosis, students diagnosed with a lice infestation should remain in class but close head contact with others should be discouraged. Alerting an entire classroom of parents also should be questioned, the AAP says, citing sentiments from experts that “because of the relatively high prevalence of head lice in young school-aged children, it may make more sense to alert parents only if a high percentage of children in a classroom are infested.”

“No-nit” policies that exclude children from school activities until all nits are removed also should be abandoned according to many health professionals, the report says. “International guidelines established in 2007 for the effective control of head lice infestations stated that no-nit policies are unjust and should be discontinued because they are based on misinformation rather than objective science,” the report states. “The American Academy of Pediatrics and the National Association of School Nurses discourage no-nit policies that exclude children from school. However, nit removal may decrease diagnostic confusion, decrease the possibility of unnecessary retreatment, and help to decrease the small risk of self-reinfestation and social stigmatization.”

Do head lice change color?

According to the research of R. Hoeppli, ancient Chinese medicine used lice to cure “high fever and severe headaches as if the skull is cracking.” A paste made from 300 – 500 black lice, spread on the head, was said to do the trick. I must admit, I’d rather have 500 pureed lice on my head than 500 live ones, but the question this raised for me was where one would find black lice. Is there such a thing?

Head lice that I’ve seen have ranged from pale ivory through a golden – sometimes reddish – brown; magnified, they are transparent. Lice that have fed have a black mass inside – presumably blood in the process of being digested – but are not, themselves, black. There is a colour range, to be sure. But black? A literature search turned up some interesting things:

  • Head lice may have a natural way of blending in without actually changing color. Ibarra and Hall wrote: “Eggs and lice are well camouflaged, reflecting the colour of their surroundings.”

  • Newly hatched lice that have not fed are transparent (Meinking) and do not have color until after they’ve fed.

  • Color that matches the background has been shown to have evolved in other species of lice. This, however, refers to colour change over generations, not within the life span of a single organism.

The ‘wisdom’ that human head lice change color depending on the hair color of the host is oft repeated on websites and in non-academic publications. Published scientific information to back it up, however, appears nonexistent. Similarly, parasitology texts and laboratory identification references do not mention it

I remain highly skeptical that our head lice can change color within one generation, or that black head lice actually exist. I conclude that the Chinese remedy called for human head lice that had fed and had blood in their guts..