Pediculus Humanus
Human head, body and pubic lice
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Genus: Pediculus humanus (capitis or corporis)
&
Genus: Phthirus Pubis (pubis)
Head lice are parasites (living off a human host) with all peoples of the world potentially susceptible to infestation with either.
Pediculus Humanus exists as:
• Capitis – (head louse) normally inhabits the scalp regions and occasionally the body.
• Corporis – (body louse) may differ anatomically from the head louse from which it may have evolved. It rarely inhabits the scalp.
Pediculus Humanus
Wingless six legged creatures measuring 1.0 to 4.0 mm.
Adults are greyish-white/brown in colour, nymphs (hatched larvae) become blood red from feeding. They may inhabit and infest the entire scalp, but are initially found near the hairline at the lower occipital and auricular margins.
They require a source of human blood to survive acquired by puncturing the skin. This invasion injects insect’s saliva, causing pruritis (variable levels) and excoriations, which in sensitive patients may lead to secondary infection.
The female louse may produce a total of 300 ova in a lifetime (9 -12 per day).
The louse possesses a flattened jointed body with jointed limbs ( Arthropoda ); each leg is equipped with a claw by which it clings to hair-shafts. It has two antennae.
Infestations occur without regard to social status or levels of personal cleanliness. Long-haired Caucasoid female children from crowded environments are at greatest risk. Fastidiously clean hair provides no immunity to infestation. Adults are significantly less vulnerable to infestation.
Transmission is mainly by direct contact – head to head, sharing headwear, combs and brushes. Head lice survive on combs brushes, chair backs, hats, and scarves for up to 48 hours. As they do not survive off-host beyond this period, fumigation or spraying premises is probably unnecessary.
The Pediculus ‘welds’ its Ova (egg or ‘nit’) onto the hair shaft approx. 2-4 mm above scalp level. Ova hatch at temperatures of above 21 ° C. within 5-8 days. The vacated ‘shell’ remains attached to the hair shaft and becomes increasingly distanced from scalp level due to hair growth at the nominal 1-2 cm per month.
Ova are note removed by shampooing, but may sometimes be combed off the hairshaft at its distal end.
Symptoms: irritation scratching and possible secondary bacterial infection.
Pubic Lice – Genus: Phthirus Pubis, or crab louse –
Its name is synonymous with its distinctive shape. It measures approx. 2.cm in width and length (Herms & James 1961).
The insect is slow moving and may remain in situ for lengthy periods.
Of its three pairs of legs, the two hind pairs possess claws.
Ova incubation is approx. seven days. Infestations primarily involve the pubic region in adults, but these lice may be located anywhere on the body.
In post natal infants the eyebrows and lashes may be inculcated.
Treatments currently available for lice (not always satisfactory)
There are different classes of parasiticides which act on the central nervous system of the louse. Infestation may respond to chemical treatments formulated either as creams, shampoos, or lotions.
Over the counter medicines may not provide long-term satisfaction (e.g. the Permethrin based products).
A lice repellent containing Piperonal 2% (Rappell) is available.
An electric combing device which delivers small shock waves has been heralded as a satisfactory means for killing the insects in some persons. However the ova are usually unaffected.
A recently introduced silicone based treatment marketed as Hedrin ( a cutaneous solution containing 4% Dimeticone w/w.) suffocates the louse and she ‘explodes’
Take advice regarding such local treatment from a registered pharmacy/trichologist/health centre/.
All chemicals must be used with care, as they can be inherently toxic to humans.
Consult a doctor before using these medicines on infants or during pregnancy.
Medication -free treatment for headlice infestations: Daily applications of heavy creamy conditioners and thorough (30 minute) combings with a fine tooth-comb to dislodge the creatures preferably over a basin.
As re-infestation requires just one hidden egg to hatch, continue this procedure with diminishing frequency for several weeks after the last creature has been thus removed.
Treatment of an infested person should involve the removal of all clothing, and treating the person with the parasiticide.
All family members may be treated on the same day and subsequently as recommended to control any lice hatching.
Clothes and bedclothes should be isolated in plastic bags for 14 days, then dry cleaned.
Alternative : Place items in a freezer. Freezing for a minimum period of six hours is lethal to ova and lice.
Very short hair reduces the probability of infestation/re-infestation.
Prof. Barry Stevens FTTS.
Pediculosis capitis. by Dr Sajjad Ahir MD LTTS (Dermatologist)
History: Lice have been man’s frequent companions for millennia and no race is known to have been spared their attentions. Two species of louse infest man. Pediculus humanus and phthirus pubis. Pediculus humanus occurs in two distinct populations. P. humanus capitis the head louse, and P. humanus corporis, the body louse. The head louse is occasionally found on the body, but the body louse is rarely seen in the scalp.
Pediculosis Humanus capitis. The female head louse is 3-4 mm long; the male is slightly smaller and banded across the back. Its colour ranges from greyish-white to brown according to the skin colour of the human population it habitually parasitizes. The eggs, which are oval-lidded white capsules are each firmly cemented to a hair shaft adjacent to the scalp. After about a week the eggs hatch to produce larvae which resemble small adults. Feeding begins soon after hatching, each nymph filling with blood until they look like, rubies on legs. They undergo three moults during their first 10 days of life to reach maturity, darken in colour and commence mating.
The factors which regulate the larvae population in the individual scalp are not fully understood. The temperature and humidity of the scalp provide a favourable environment for incubation; hatching is slowed or will not occur, at temperatures below 22 oC. In most established infestations there are fewer than 10 adult lice. Lice and their eggs are most numerous in the occipital and post auricular regions of the scalp.
Epidemiology and incidence: The head louse occurs throughout the world. It is transmitted by direct contact, or by shared hats, caps brushes or combs. The louse can travel directly from one head to another an a pillow or a chair or may be transferred as eggs on shed hairs. Long fine hair and poor hygiene favour the establishment of infestation. Girls of low intelligence showed a higher incidence than others living under the same conditions. Members of large families have greater opportunities of exposure as do members of schools or other communities. The importance of social conditions in determining the prevalence of head lice was emphasized by a study of School children in Kassel in Germany. Infestation was most frequent among children of foreign workers and in children who physically or mentally handicapped. The prevalence of head lice in Britain was high in 1940 and the years that followed, but by 1960 was relatively uncommon, specially in rural areas. In recent years the infestation has again become more common, virtually epidemic in many years. An interesting observation, which is not fully explained, is that the prevalence of head lice is more than 30 times greater in American whites than in black attending the same school classes. The incidence is high in some villages in India, reaching about 18%.
Pathology: Nymphs and adults of both sexes suck blood, and in doing so inject their saliva. The pruritus which leads to scratching and hence to the secondary bacterial infection which may dominate the clinical picture is a manifestation of hypersensitivity to antigenic constituents of the saliva, There have been few investigations in humans of the immounological response to lice. There is wide individual variation in response and some subjects showed little or no reaction after months of daily exposure. In the investigation of home contacts of our patients we have seen very heavily infested subjects with no symptoms.
Clinical features. The clinical features of pediculosis are surprisingly variable. Pruritus, which may be intense, depends on the immune response to the salivary antigens of the louse and on the host’s threshold of perception. It is seldom completely absent. Characteristically it is most severe in the occipital region where the infestation is usually most heavy. Scratching introduces secondary bacterial infection, leading to impetiginous crusts and cervical adenitis. In sever cases a child may be pale, listless and febrile. A generalized erythematous dermatitis has been reported. If the infestation is of long duration and secondary infection is sever and persistent the hair may become densely matted by malodorous pus and exudates to give rise to the state formerly known as plica polonica. In those individuals in whom pruritus is slight or absent, the infestation may not be recognized until it is deliberately sought. Except in those few cases in which the population of lice as nymphs or adults is high , there is no mechanized dandruff, to be seen. The diagnostic feature in the presence of oval egg-capsules, popularly known as nits, firmly cemented to the hair shafts. They are most easily confused with hair casts but the latter slide freely along the shaft and are annular. Other foreign bodies have been responsible for misdiagnoses, which may have embarrassing consequences for public health administrators. With an hand lens the distinctive shape of the egg is easy to identify. It is particularly easy to see under wood’s light, which is useful if a large school population has to bee screened. Impetigo of the scalp is never an acceptable diagnosis until pediculosis has been reliably excluded.
Treatment: If secondary infection is sever, and particularly in the presence of adenitis or toxaemia, and antibiotic should be administered systemically. If the hair is densely matted it may be necessary to cut it. In most cases, however, treatment with a parasiticide is sufficient provided it is carried our thoroughly. Preparations of gammexane of 5% emulsion of DDT have been widely used with success for some years. The chosen application is rubbed into the scalp once daily for 5 days, and thoroughly washed off a week later. The hair is then carefully examined and any remaining eggs are removed with a fine-toothed comb. Where lice are resistant to gammexance and the DDT malathion 0.5 % in spirit has been successfully used. The lotion is applied liberally and allowed to dry. After 12-24 hours the hair is washed and combed. In vitro testing has shown that 0.5 % malathion lotion killed lice within 5 minutes and was highly ovicidal with only 5% of eggs hatching. Other preparation which may be successful are 1% lindance shampoo and 10% crotamiton. In one investigation of school children 20% of treated children were re-infested with in 2 months. This experience emphasizes the need to examine and treat all schools and home contacts, including of course those who are allegedly symptom-free. Because of widespread resistance problems, many health districts operate a rotation policy, now mainly between carbaryl, malathion and permethrin.
Sajjad Ahir MD LTTS
