thumb|Pediculus humanus capitis by [[Des Helmore]]
The head louse (Pediculus humanus capitis) is an obligate ectoparasite of humans. Head lice are wingless insects that spend their entire lives on the human scalp and feed exclusively on human blood. Head lice cannot fly, and their short, stumpy legs render them incapable of jumping, or even walking efficiently on flat surfaces. However, the degree of separation is contentious as they can produce fertile offspring in a laboratory.
A much more distantly related species of hair-clinging louse, the pubic or crab louse (Pthirus pubis), also infests humans. It is morphologically different from the other two species and is much closer in appearance to the lice which infest other primates. Louse infestation of the body is known as pediculosis, pediculosis capitis for head lice, pediculosis corporis for body lice, and phthiriasis for pubic lice.
Adult morphology
thumb|upright=1.4|Head louse crawling on a hairbrush
Like other insects of the suborder Anoplura, adult head lice are small (2.5–3 mm long), dorsoventrally flattened (see anatomical terms of location), and wingless. The thoracic segments are fused, but otherwise distinct from the head and abdomen, the latter being composed of seven visible segments. Head lice are grey in general, but their precise color varies according to the environment in which they were raised.
Thorax
thumb|upright|Head louse gripping a human hair
Six legs project from the fused segments of the thorax. Eggs are usually laid on the base of the hair, 3–5 mm off the scalp surface.
To attach an egg, the adult female secretes a glue from her reproductive organ. This glue quickly hardens into a "nit sheath" that covers the hair shaft and large parts of the egg except for the operculum, a cap through which the embryo breathes.
After hatching, the louse nymph leaves behind its egg shell, still attached to the hair shaft. The empty eggshell remains in place until it is physically removed by abrasion or the host, or until it slowly disintegrates, which may take six months or more.
- Viable eggs that will eventually hatch
- Remnants of already-hatched eggs (nits)
- Nonviable eggs (dead embryo) that will never hatch
Of these three, only eggs containing viable embryos have the potential to infest or reinfest a host. However, a no nit policy is a common public health measure to prevent transmission of lice. Some authors have therefore restricted the definition of nit to describe only a hatched or nonviable egg:
thumb|Louse hatching
Others have retained the broad definition, while simultaneously attempting to clarify its relevance to infestation:
In British and Irish slang, the term "nit" is often used across different age groups to refer to head lice themselves.
Development and nymphs
thumb|Development of Pediculus humanus humanus ([[body lice), which is similar to that of head lice (Pediculus humanus capitis)]]
Head lice, like other insects of the order Phthiraptera, are hemimetabolous. The time required for head lice to complete their nymph development to the imago lasts for 12–15 days. Mating attachment frequently lasts more than an hour.
Factors affecting infestation
The number of children per family, the sharing of beds and closets, hair washing habits, local customs and social contacts, healthcare in a particular area (e.g., school), and socioeconomic status were found to be significant factors in head louse infestation. Girls are two to four times more frequently infested than boys. Children between 4 and 14 years of age are the most frequently infested group.
Behaviour
Feeding
All stages except eggs are blood-feeders and bite the skin four to five times daily to feed. They inject saliva which contains an anticoagulant and suck blood. The digested blood is excreted as dark red frass.
Position on host
Although any part of the scalp may be colonized, lice favor the nape of the neck and the area behind the ears, where the eggs are usually laid. Head lice are repelled by light and move towards shadows or dark-coloured objects in their vicinity.
Transmission
Lice have no wings or powerful legs for jumping, so they use the claws on their legs to move from hair to hair.
Distribution
Approximately 6–12 million people, mainly children, are treated annually for head lice in the United States alone. In the UK, it is estimated that two-thirds of children will experience at least one case of head lice before leaving primary school. High levels of louse infestations have also been reported from all over the world, including Australia, Denmark, France, Ireland, Israel, and Sweden.
Archaeogenetics
Analysis of the DNA of lice found on Peruvian mummies may indicate that some diseases (such as typhus) may have passed from the New World to the Old World, instead of the other way around.
Genome
The sequencing of the genome of the body louse was first proposed in the mid-2000s and the annotated genome was published in 2010. An analysis of the body and head louse transcriptomes revealed these two organisms are extremely similar genetically. Indeed, transcriptome analyses raises the possible that they differ due to phenotypic flexibility and this is "probably a result of regulatory changes, perhaps epigenetic in origin, triggered by environmental signals."
Unlike other bilateral animals, the 37 mitochondrial genes of human lice are not on a single circular chromosome but are extensively fragmented. For the head louse, and the body louse, they are on 20 minichromosomes, for the pubic louse 14 minichromosomes and the chimpanzee louse,18 minichromosomes.
Mitochondrial clades
Human lice are divided into three deeply divergent mitochondrial clades known as A, B, and C. Three subclades have been identified, D (a sister clade of A), E (a sister clade of C), and F (a sister clade of B).
Clade A
- head and body: worldwide
- found in ancient Roman Judea