Pediculosis capitis

 

Salient features

 

·       This worldwide infestation is caused by bloodsucking, wingless, six-legged insects that live only on the hairs of the scalp

 

·       Diagnosis is usually confirmed by the presence of 0.8 mm egg case (“nits”) firmly attached to scalp hairs

 

·       Head lice are spread by head-to-head contact as well as by fomite transmission

 

·       Resistance to traditional over-the-counter preparations (pyrethrins, permethrin) is growing

 

·       Newer treatments, including topical ivermectin, are now available

 

 

Introduction

Lice are bloodsucking, wingless, six-legged insects that live only on the hairs of the scalp.

 

Epidemiology

 

Head lice infestations occur worldwide and are most common in children between the ages of 3 and 12. Infestation with head lice is more frequently observed in girls than boys, probably due to their tendency to have longer hair as well as the common exchange of brushes, barrettes and other hair accessories.

 

Head lice are quite common in the Indian subcontinent, where hair oils and creams are frequently used.

 

 

Pathogenesis

 

The head louse, Pediculus capitis, is a highly host-specific insect. The adult female is a greyish white insect 3–4 mm long and the male is slightly smaller. The claws on the legs are adapted for grasping hair.  These obligate human parasites feed exclusively on the blood of the host approximately every 4–6 hours. More than 95% of infested individuals have fewer than 100 adult lice in their scalps. During her lifespan of approximately 30 days, the female louse lays an average of about seven eggs daily on hair shafts. The eggs measure 0.8 mm in length and are usually laid close to the scalp for warmth; in general, eggs located within 1 cm of the scalp are unhatched. In warm climates, however, eggs may be found 15 cm or more from the scalp, especially in the area above the nape of the neck. The eggs are cemented to individual hairs by means of a chitinous material secreted by the female's accessory glands. They are oval, flesh colored and have a lid (operculum) capping the free end of the egg. The operculum is pushed off by the emerging louse nymph. Once the louse nymph has emerged, the empty egg case or ‘nit’ appears white, and is easier to see than the intact eggs close to the scalp surface. After 10 days, the eggs hatch producing larvae, which are referred to as nymphs or “instars.” Instars look like miniature adult louse and go through three stages of development that take 14 days for full maturation.

Head lice typically survive less than 2 days away from the scalp without a blood meal; however, given an appropriate temperature (28–32°C) and humidity (70–90%), survival has been reported at 4 days. Eggs can survive and hatch after 10 days away from the host. Nits can survive for 10 days away from the scalp.

Transmission is by means of direct head-to-head contact, optimal conditions for transfer being when hairs are parallel and slow moving or by indirect (fomite) transmission through combs, brushes, blow-dryers, hair accessories, upholstery, pillows, bedding, helmets, or other headgear. Lice can be dislodged by air movement, blow-dryers, combs, and towels, and passively transferred to fabric, facilitating new infestations. Spread of lice is encouraged by poverty, poor hygiene and overcrowding.

 

Clinical features

 

Skin findings of head lice infestations are limited to the scalp, behind the ears and the nape of the neck. Scalp pruritus is the characteristic manifestation of head louse infection and varies among patients. In the first infestation, it may take 2–6 weeks before pruritus is evident, reflecting the development of a delayed immunologic response to the irritant components of the lice saliva or excreta. In repeat infestations, pruritus develops within the first 24–48 hours. However, a number of individuals are asymptomatic and can be considered “carriers”.  Secondary bacterial infection may occur as a result of scratching, and concomitant head louse infection must always be considered in cases of scalp impetigo. Head lice can carry S. aureus and Str. pyogenes on their surfaces and are a common cause of pyoderma of the scalp^..

Bites of the mites may produce erythematous macules or papules, but usually an examiner only finds excoriations, erythema, and scaling of the scalp and posterior neck. Other findings may include a low-grade fever, regional lymphadenopathy, and irritability.

The empty egg cases (nits) occur in greatest density on the ­parietal and occipital regions. However, on naked‐eye inspection, they may be confused with peripilar keratin casts (‘pseudonits’; hair muffs) or dried globules of cheap hair lacquer.

 

Investigations

 

Detection of adult lice and nymphs provides evidence of an ‘active’ infestation, whereas the presence of eggs and empty egg capsules (nits) alone merely indicates that infection has occurred at some time. The most reliable method of diagnosing current active infestation is by detection combing, which has been shown to be superior to direct visual examination of the hair and scalp. This is an important criterion for several reasons:

1.   Individuals who do not have evidence of active infestation should not receive chemical treatment.

2.   Individuals should have live lice present on the head before using pediculicides, not just eggs alone.

3.   Children who do not have evidence of active infestation may be inappropriately excluded from school.

 

Management

 

Principles of management

The choice of treatment is based on the efficacy and potential toxicity of different agents and ease of access to prescription remedies. Pediculicides remain the mainstay of therapy. With all topical preparations (regardless of package instructions), two applications, 1 week apart, are advisable in order to: (1) kill any nits that survived treatment; (2) better defend against the seemingly growing resistance to most pediculicides; and (3) reduce the risk of re infestation by means of fomites.

General guidelines for the use of chemical pediculicides have included that lotion and liquid formulations are preferable to shampoos as the latter expose the insects to relatively low concentrations of insecticide with subsequent poor efficacy and which, in the long term, might favor the development of resistance. Preparations with an aqueous basis are less likely to irritate an excoriated scalp than alcoholic solutions, do not irritate the bronchi of asthmatics and are not flammable.

Family members should be examined, and treated only if they show evidence of active infestation by the presence of live lice. Nits may be removed with a fine‐toothed comb. Treatment has most chance of success if it is applied or undertaken correctly and if all affected individuals in the household are treated simultaneously. People should be advised to check whether treatment was successful by detection combing on day 2 after completing a course of treatment, and again after an interval of 7 days.

 

Alternative treatment

 

Resistance of head lice to insecticides led researchers to look for alternatives.

Physical treatment is an alternative to the use of chemical agents; the ‘Bug Busting’ wet‐combing method has been promoted as a treatment for head lice. The technique involves ordinary shampooing of the hair, followed by the application of generous amounts of conditioner, and combing using a fine‐toothed comb to remove lice. This procedure is repeated every 4 days for 2 weeks. Shaving the head is usually not acceptable because of psychosocial impact.

 

Permethrin

 

Permethrin is the only synthetic pyrethroid that is used worldwide for head lice. The OTC 1% permethrin cream rinses and lotions are applied for 10 minutes; however, an 8–12-hour application of the 5% cream for scabies is an alternative therapy. Unfortunately, resistance to even the higher-concentration products has developed in head lice and other insects.

 

Ivermectin

 

A 0.5% topical ivermectin preparation was FDA-approved in 2012 for the treatment of head lice in patients ≥6 months of age. Topical ivermectin has been shown to kill permethrin-resistant head lice, and the viability of lice hatched from treated eggs is severely compromised. The lotion is convenient (i.e. applied to dry hair, left for 10 min, then rinsed with water), which should increase compliance.

Oral ivermectin represents another therapeutic option for resistant head lice infestations. Oral Ivermectin 200 mcg/kg of on days 1 and 8 is an option for patients whose disease is resistant to topical therapies. As in scabies, children who weigh less than 33 pounds (15 kg) and pregnant or breastfeeding women should probably not receive oral ivermectin since safety data on these populations are not available.

 

Spinosad

In 2011, 0.9% spinosad topical suspension was approved by the FDA as a new therapy for head lice in children ≥4 years of age. It is a fermentation product of the bacterium Saccharopolyspora spinosa that induces muscle spasms and paralysis in lice when applied topically. A clinical trial showed that 0.9% spinosad cream rinse left on for 10 minutes without nit combing had superior efficacy to 1% permethrin cream with nit combing, and spinosad had no significant clinical or laboratory side effects.

TREATMENTS FOR HEAD LICE
Treatment	Group	Administration on days 1 and 8	Concerns	Efficacy & resistance
Permethrin cream rinse or lotion (1%)	Synthetic pyrethroid	Topical application for 10 minutes to clean, dry hair	None	Poor–fair; resistance common
Permethrin cream (5%)†	Synthetic pyrethroid	Topical overnight application to clean, dry hair	Allergic contact dermatitis in individuals with sensitivity to formaldehyde	Poor–fair; resistance common
Spinosad cream rinse (0.9%)	Bacterial fermentation product	Topical application for 10 minutes to dry hair	None	Good; no resistance noted to date
Ivermectin solution (0.5%)++     Ivermectin (available as 3 mg tablets)	Avermectin       Avermectin	Topical application for 10 minutes to dry hair     Oral dose of 200 mcg/kg	Potential skin and eye irritation       Potential CNS toxicity; not recommended for children weighing <33 pounds (15 kg), breastfeeding mothers or pregnant women (category C)	Excellent; no resistance noted to date

⁺Approved for individuals >/= 2 months of age

⁺⁺Approved for individuals >/= 6 months of age

 

 

 

Causes of therapeutic failure for head lice

·        Misunderstanding of instructions

·        Non‐compliance

·        Resistance

·        Inappropriate instructions on head lice products or from health professionals

·        High cost of products

·        Misdiagnosis

·        Psychogenic itch

·        Incomplete ovicidal activity

·        Inappropriate preparation (e.g. shampoo)

·        Insufficient dose–time, frequency and/or quantity of product applied

·        Failure to retreat

·        Reinfestation

·        Live eggs not removed

 

Prevention

Patients should be counseled in at least some effective measures to prevent re infestation by fomite transmission. After treatment, treated individuals should wear clean clothing, and all clothing, hats, pillow cases, towels and bedding used during the previous week should be washed in hot water (50°C at least) and dried at high heat. Non washables should be dry-cleaned, ironed, put in the clothes dryer without washing, or stored in a sealed plastic bag in a warm area for 2 weeks. Combs and brushes may be washed in very hot water (65°C) or may be coated with the pediculicide for 15 minutes. Floors, carpets, upholstery (in both home and car) play areas, and furniture should be carefully vacuumed to remove any hairs with viable eggs attached. Fumigation of living spaces is not recommended and pets do not need to be treated because they do not harbor the human head louse. There is no reliable evidence that strict “no nit” policies at schools are necessary to prevent spread of the infestation, and they should be abandoned in favor of family education regarding effective treatment.