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Chapter: Clinical Dermatology: Infestations

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Lice infestations (pediculosis)

Lice are flattened wingless insects that suck blood. Their eggs, attached to hairs or clothing, are known as nits. The main feature of all lice infestations is severe itching, followed by scratching and secondary infection.

Lice infestations (pediculosis)

Lice are flattened wingless insects that suck blood. Their eggs, attached to hairs or clothing, are known as nits. The main feature of all lice infestations is severe itching, followed by scratching and secondary infection.

Two species are obligate parasites in humans: Pediculus humanus (with its two varieties P. humanus capitis, the head louse, and P. humanus corporis, thebody louse) and Phthirus pubis (the pubic louse).


Head lice

Cause

Head lice are still common, affecting up to 10% of children even in the smartest schools. The head louse itself measures some 3–4 mm in length and is greyish, and often rather hard to find. However, its egg cases (nits) can be seen easily enough, firmly stuck to the hair shafts. Spread from person to person is achieved by head-to-head contact, and perhaps by shared combs or hats.

Presentation and course

The main symptom is itching, at first around the sides and back of the scalp and then more generally over it. Scratching and secondary infection soon follow and, in heavy infestations, the hair becomes matted and smelly. Draining lymph nodes often enlarge.

Complications

Secondary bacterial infection may be severe enough to make the child listless and feverish.

Differential diagnosis

All patients with recurrent impetigo or crusted eczema on their scalps should be carefully examined for the presence of nits.

Investigations

None are usually required.

Treatment

Malathion, carbaryl and permethrin preparations (Formulary 1) are probably the treatments of choice now. They kill lice and eggs effectively; malathion has the extra value of sticking to the hair and so protecting against reinfection for 6 weeks. The policy whereby public health authorities rotate their use, with the aim of lessening the risk of resistant strains emerging, has fallen out of favour now.

Lotions should remain on the scalp for at least 12 h, and are more effective than shampoos. The applica-tion should be repeated after 1 week so that any lice that survive the first application and hatch out in that interval can be killed. Other members of the family and school mates should be checked. A toothcomb helps to remove nits but occasionally matting is so severe that the hair has to be clipped short. A systemic antibiotic may be needed to deal with severe secondary infection. Some recommend, as an alternative to the treatments mentioned above, that the hair should be combed repeatedly and meticulously with a special ‘detection comb’abut the efficacy of this method has still to be established. However, a head louse repel-lent, containing 2% piperonal, is available over the counter and may be worth a trial for those who are repeatedly reinfested. Systemic ivermectin therapy is reserved for infestations resisting the treatments listed above.


Body lice

Cause

Body louse infestations are now uncommon except in the unhygienic and socially deprived. Morpholo-gically the body louse looks just like the head louse, but lays its eggs in the seams of clothing in contact with the skin. Transmission is via infested bedding or clothing.

Presentation and course

Self-neglect is usually obvious; against this back-ground there is severe and widespread itching, espe-cially on the trunk. The bites themselves are soon obscured by excoriations and crusts of dried blood or serum. In chronic untreated cases (‘vagabond’s disease’) the skin becomes generally thickened,eczematized and pigmented; lymphadenopathy is common.

Differential diagnosis

In scabies, characteristic burrows are seen. Other causes of chronic itchy erythroderma include eczema and lymphomas, but these are ruled out by the finding of lice and nits.

Investigations

Clothing should be examined for the presence of eggs in the inner seams.

Treatment

First and foremost treat the infested clothing and bedding. Lice and their eggs can be killed by high tem-perature laundering, by dry cleaning and by tumble-drying. Less competent patients will need help here. Once this has been achieved, 5% permethrin cream rinse or 1% lindane lotion (USA only) (Formulary 1) may be used on the patient’s skin.


Pubic lice

Cause

Pubic lice (crabs) are broader than scalp and body lice, and their second and third pairs of legs are well adapted to cling on to hair. They are usually spread by sexual contact, and most commonly infest young adults.

Presentation

Severe itching in the pubic area is followed by eczema-tization and secondary infection. Among the excoria-tions will be seen small blue-grey macules of altered blood at the site of bites. The shiny translucent nits are less obvious than those of head lice (Fig. 15.2). Pubic lice spread most extensively in hairy males and may even affect the eyelashes.


Differential diagnosis

Eczema of the pubic area gives similar symptoms but lice and nits are not seen.

Investigations

The possibility of coexisting sexually transmitted dis-eases should be kept in mind.

Treatment

Carbaryl, permethrin and malathion are all effective treatments. Aqueous solutions are less irritant than alcoholic ones. They should be applied for 12 h or overnight to all parts of the trunk, including the peri-anal area and to the limbs, and not just to the pubic area. Treatment should be repeated after 1 week, and infected sexual partners should also be treated. Shaving the area is not necessary.

Infestation of the eyelashes is particularly hard to treat, as this area is so sensitive that the mechanical removal of lice and eggs can be painful. Applying a thick layer of petrolatum twice a day for 2 weeks has been recommended. Aqueous malathion is effective for eyelash infestations but does not have a product licence for this purpose.

 

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