Lice, Human Louse

(Pediculus humanus)


1. Epidemic typhus: This disease is caused by a PPLO, Ricketsia prowazeki, which multiplies in the gut of lice. Spores are released through faeces within 5 days of infection and can remain viable for 4 months in the dry conditions. They get into the human system through contact with blood, through wounds, conjunctive or by inhalation into lungs. The disease spreads in epidemic form and is capable of causing 100% fatality. Reservoir hosts are sheep and goats.

2. Trench fever: This is caused by Ricketsia quintana. The disease was common among soldiers during the world war II, when soldiers had to spend several days trapped in trenches and lice transmitted the disease in epidemic form. The mode of transmission is similar to the epidemic typhus.

3. Relapsing fever: This disease is transmitted by a spirochaet, Borrelia recurrentis which breeds in the haemolymph of the louse and escapes when the louse is crushed or dies due to the parasite. It gets into the human blood through wounds or scratches. This disease spread in great epidemic form during the World War I and II.

4. Local urticaria and itching: This is also called the Vagabond disease. Itching, rashes and discoloration of the skin takes place due to the allergic reaction to the bites and blood sucking by lice.

Pediculus humanus

This is a 2-3 mm long insect with dorsoventrally flattened body. Head is small with reduced eyes and small, 5-segmented, filiform antenna. Mouthparts are highly modified, with a chitinous, spine-like stabber for puncturing the host skin. There is no proboscis or maxillary palps or other structures of usual mouth parts. Legs form a clinging apparatus by having a single curved claw that opposes a tibial thumb. Tarsus is single segmented. Tip of abdomen of female is bifid while that of male rounded and armed with a chitinous pointed aedeagus, which is the male copulatory organ.

Blood is sucked by the pumping action of pharynx. Both adults and young of both sexes feed on blood. Their mouthparts are in the form of fine stylets which, except when feeding, are withdrawn within the head. When food is required the skin of the host is pierced by means of the stabber which is thrust out and then gradually pushed farther and farther into the flesh.

At the same time a salivary secretion which prevents coagulation of the host’s blood is injected into the wound and enables the insect to suck it up without clots forming in the delicate mouth-parts. The adults can take up to one milligram of blood at a time. It is probable, however, as adults cannot live long without feeding, that they prefer to take smaller quantities at frequent intervals.

At temperatures at which they remain capable of active movement they survive starvation, after a meal, for about three days at 30°C and five days at 24°C. At lower temperatures they survive for about a week after feeding, but it must be remembered that at these lower temperatures they are not capable of active movement. When a louse takes a very large meal, blood may sometimes be seen flowing from its anus. At other times overeating results in a rupture of the gut and allows the blood to mingle with the body fluids; this causes the death of the louse within a comparatively short time. Adults are capable of fairly rapid active movement. They have been recorded as moving at a rate of 9 inches per minute; but they never move in straight lines.

Fecundity of female is 150-300 eggs/female. The eggs (called nit) are oval in shape. 0.8 mm long, with an operculum on one end and are glued to the host hair by a gelatinous sleeve. Incubation period is 6-7 days. Nymphs that hatch from the eggs are similar to the adults in structure as well as in habits. They grow by sucking blood of the host and mould twice to become adults. Nymphal period is 9 days that includes 3 instars. Total life cycle is completed within 15-16 days. Adults survive for 4-6 weeks. There are two forms of the human louse—head louse and the body louse, which differ in the following details:

Head louse (Pediculus humanus capitis)

Body louse (Pediculus humanus corporis)

  1. Found on head, among hairs.
  2. Longevity is 4 weeks.
  3. There is no seasonal variation.
  4. The louse is found more on children.
  5. Spreads through head contact.
  6. Black in colour.
  1. Found on body and in clothing.
  2. Longevity is about 6 weeks.
  3. There is greater density of lice in winter months.
  4. Occurs uniformly on all age groups.
  5. Spreads through clothing, bed etc.
  6. Dull whitish in colour.

The Crab louse (Phthirus pubis)

This species is restricted to the pubic region of human beings. Thoracic region is very broad and abdomen narrower than the thorax, 5-segmented and bears small paired appendages. They are sluggish lice and are not known to transmit any diseases. Fecundity is 50 eggs/female. The eggs hatch in 7-8 days and nymphs take 27-33 days to develop into adults. Otherwise they are similar to the human louse.

Body is much broader in proportion to its length and the legs have the appearance of being attached to the edge of the somewhat flattened body rather than to the underside. Its movements are slow and deliberate and are made with a somewhat crablike motion. As its common name implies, it is usually found infesting hairs of the pubic and perianal regions of both sexes. Like

Pediculus human us it is world-wide in its distribution, but much rarer, so that infestations of Phthirus pubis are less frequently seen.


Cleanliness and hygiene eradicates lice. In case of severity of infestation, clean shaving of head brings relief. Application of kerosene mixed with olive oil in equal ratio on the head kills lice. Lindane ointment, malathion (Lycil) and Mediker shampoo (also contains malathion) or any insecticide mixed with oil kills all lice on head. The application has to be repeated every week to kill nymphs emerging from eggs. Body lice can be controlled by steaming or boiling of clothes and bedding or rinsing clothes in weak insecticide solution.