The head louse ( Pediculus humanus capitis) is an obligate parasitism of .
Lice differ from other hematophagic ectoparasites such as in spending their entire lifecycle on a host.
The non-disease-carrying head louse differs from the related disease-carrying body louse ( Pediculus humanus humanus) in preferring to attach eggs to scalp hair rather than to clothing. The two subspecies are morphologically almost identical, but do not normally interbreed. From genetic studies, they are thought to have diverged as subspecies approximately 30,000–110,000 years ago, when many humans began to wear significant amounts of clothing.
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.
Adult morphology
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.
After feeding, consumed blood causes the louse body to take on a reddish color.
Head
One pair of antennae, each with five segments, protrudes from the insect's head. Head lice also have one pair of
compound eye. Eyes are present in all species within the Pediculidae family, but are reduced or absent in most other members of the Anoplura suborder.
Like other members of the Anoplura, head louse mouthparts are highly adapted for piercing the skin and sucking blood.
These mouth parts are retracted into the insect's head except during feeding.
Thorax
Six
insect leg project from the fused segments of the thorax.
As is typical in the Anoplura, these legs are short and terminate with a single
claw and opposing "
thumb".
Between its claw and thumb, the louse grasps the hair of its host.
With their short legs and large claws, lice are well adapted to clinging to the hair of their host. These adaptations leave them incapable of jumping, or even walking efficiently on flat surfaces. Lice can climb up strands of hair very quickly, allowing them to move swiftly and reach another host.
Abdomen
Seven segments of the louse
abdomen are visible.
The first six segments each have a pair of spiracles through which the insect breathes.
The last segment contains the
anus and (separately) the
genitalia.
Sex differences
In male lice, the front two legs are slightly larger than the other four. This specialized pair of legs is used for holding the female during copulation. Males are somewhat smaller than females and are characterized by a pointed end of the abdomen and a well-developed genital apparatus visible inside the abdomen. Females are characterized by two
Gonopod in the shape of a W at the end of their abdomens.
Eggs and nits
Like most insects, head lice are
oviparous. Females lay about three or four eggs per day. Louse eggs (also known as
nits) are attached near the base of a host hair shaft.
Eggs are usually laid on the base of the hair, 3–5 mm off the scalp surface.
In warm climates, and especially the tropics, eggs may be laid or more down the hair shaft.
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.
Each egg is oval-shaped and approximately 0.8 mm in length.
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.
Empty shells are matte, collapsed, and white. The term nit may include any of the following:
-
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.
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
Head lice, like other insects of the order
Phthiraptera, are
hemimetabolous.
Newly hatched nymphs will
Moulting three times before reaching the sexually mature adult stage.
Thus, mobile head lice populations may contain eggs, nits, three nymphal
instars, and the adults (male and female) (
imago).
during head louse development is subtle. The only visible differences between different instars and the adult, other than size, is the relative length of the abdomen, which increases with each molt,
as well as the existence of reproductive organs in the adults. Aside from reproduction, the behavior of nymphs is similar to that of adults. Like adults, nymphs feed also only on human blood (hematophagia), and cannot survive long away from a host.
Outside their hosts lice cannot survive more than 24 hours.
The time required for head lice to complete their nymph development to the imago lasts for 12–15 days.
Nymph mortality in captivity is about 38%, especially within the first two days of life.
Reproduction and lifespan
Head lice reproduce sexually, and copulation is necessary for the female to produce fertile eggs.
Parthenogenesis, the production of viable offspring by virgin females, does not occur in
Pediculus humanus.
Pairing can begin within the first 10 hours of adult life.
After 24 hours, adult lice copulate frequently, with
mating occurring during any period of the night or day.
Mating attachment frequently lasts more than an hour.
Young males can successfully pair with older females, and vice versa.
Experiments with P. h. humanus (body lice) emphasize the attendant hazards of lice copulation. A single young female confined with six or more males will die in a few days, having laid very few eggs.
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.
Normally, head lice infest a new host only by close contact between individuals, making social contacts among children and parent-child interactions more likely routes of infestation than shared combs, hats, brushes, towels, clothing, beds, or closets.
Touching heads is by far the most common route of lice transmission.
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
Mummy 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 Bilateria, 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
Clade D (sister of clade A)
-
head and body: Central Africa, Ethiopia, United States
Clade B
-
head only: worldwide
-
found in ancient Roman Judea and a 4,000-year-old Chilean mummy
Clade F (sister of clade B)
-
head and body: South America
Clade C
-
head only: Ethiopia, Nepal, Thailand
Clade E (sister of clade C)
See also
External links
