Abstract and Introduction
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Head lice (Pediculus humanus capitis
) are ectoparasites, meaning that they live on or within the skin rather than inside the body. Annually, approximately 6 to 12 million U.S. children aged 3 to 11 years become infested with head lice.
Lice infestations are a substantial economic burden on the health care system. Although difficult to assess, the annual direct and indirect costs of infestations, including treatment, lost wages, and school-system expenses, are estimated to be $1 billion.[2,3]
Preschool-age and school-age children are most commonly infested, with household members also at increased risk.
Not only is lice infestation a nuisance, but if it is left untreated, it may result in a secondary bacterial infection. Infestation has nothing to do with hygiene practices or hair length, nor do lice transmit disease.
Despite these facts, the social stigma, embarrassment, and anxiety resulting from infestation can be considerable for both the child and the parent(s).[2,4]
Louse saliva, which contains vasodilators and anticoagulants, is injected into the host while the louse is feeding on the host’s blood. The host usually develops a sensitivity to the saliva, resulting in pruritus; however, it may take 4 to 6 weeks for the sensitivity and itching to manifest.
The adult head louse is tan to grayish-white and 2 to 3 mm long (about the size of a sesame seed), with six legs. The female louse can live 3 to 4 weeks, and upon maturation she can lay up to 10 eggs per day. The eggs, which are cemented to the base of the hair shaft approximately 4 mm from the scalp, are pigmented to match the hair color of the host, rendering them difficult to see. Eggs farther than 1 cm from the scalp likely are not viable. Once hatched, the eggs are easier to see, as the empty casings appear white. The term nit
denotes the empty casing; however, some experts also use the term to describe the viable egg containing the nymph.
The life cycle of head lice consists of three phases: 1) egg without an eyespot (the eyespot signifies a developed nervous system), 2) egg from time laid to time hatched, and 3) nymph (newly hatched louse) to mature egg-laying louse. From the time of laying, an egg takes an average of 4 days to develop an eyespot and 8.5 days to hatch. It takes a nymph an average of 9.7 days to develop into a mature egg-laying adult.
Once mature, the female louse can mate and lay eggs in approximately 1.5 days. Eggs cannot hatch at room temperature; they need body heat to incubate.
Even under optimal conditions, 10% to 30% of eggs fail to hatch.
Lice usually cannot survive away from the host for 24 hours. Lice do not hop or fly; they can only crawl. However, there have been some reports of lice being ejected from the scalp as a result of static buildup from brushing dry hair. Transmission usually occurs from direct head-to-head contact, and rarely from contact with the infested individual’s brush, comb, or hat.
Since lice avoid light and crawl quickly, it is difficult to detect a live louse on the scalp. A quicker, more efficient way to diagnose head lice infestation is to use a nit comb.
Compared with direct visualization, using a nit comb reduces the number of false negatives.
Some experts believe that the use of a lubricant, such as oil, conditioner, or water, slows the lice down and facilitates diagnosis.
Usually it is easier to see eggs at the nape of the neck or behind the ears.
A diagnosis of lice infestation should not be based on detection of nits alone. When diagnosis is made by the detection of nits alone, up to 66% of children are treated unnecessarily. Treatment should not be administered if living lice cannot be found.
Preventing head lice is difficult, especially in young children. Avoiding head-to-head contact is the surest way to prevent transmission. Children should be taught not to share personal items such as combs, brushes, and hats. Prompt treatment of infested individuals can aid in prevention.
The optimal treatment would be readily available, easy to use, effective and safe, affordable, and non–resistance-promoting.
ophylactic treatment is not recommended.
OTC and prescription-only medications are approved for use in the U.S.
These products, which are pediculicidal (kills live lice only) or ovicidal (kills lice and eggs), act either on the louse’s nervous system or as occlusive agents.
Many reports exist of resistance to the OTC agents; however, in the case of treatment failure, improper use by the patient should be suspected first. Some experts recommend removing nits with a nit comb after treatment.
Based on life cycle, pediculicidal agents that act on the nervous system ideally should be applied twice. The first application kills all hatched lice. Nine days later (rather than 7 days, as some manufacturers recommend), a second application kills all newly hatched eggs not killed with the first application. If the second application is done on day 7, some eggs laid just before the first application have not yet hatched, based on the average hatching time of 8.5 days.
See Table 1
for application instructions.
Available OTC, permethrin is the least toxic of the pediculicides. The American Academy of Pediatrics (AAP) considers permethrin the first-line treatment of choice. It causes hyperstimulation of the nervous system of lice, resulting in paralysis and death.
Widespread resistance to permethrin has been reported, but the prevalence is unknown.
Resistance tends to vary geographically.
Permethrin leaves a residue on the hair that can kill any nymphs remaining after the first application. The use of conditioners and hair products with silicone-based additives may hinder permethrin’s adherence to the hair shaft and impair the residual effect. Adverse effects include pruritus, erythema, and edema.
This product is available OTC. Pyrethrin is extracted from the chrysanthemum flower and then formulated with PBO. Individuals who are allergic to ragweed may react to pyrethrins, although this is rare.
PBO works synergistically by inhibiting the louse enzymes that catabolize pyrethrins, thus extending pyrethrins’ action.
Pyrethrins with PBO are not completely ovicidal, and 20% to 30% of eggs will remain viable and hatch. This warrants a second treatment, ideally on day 9. Unlike permethrin, pyrethrins with PBO have no residual activity.
Resistance to pyrethrins with PBO has been demonstrated; however, the prevalence is unknown. Resistance seems to differ from community to community and from country to country.
Available by prescription only, malathion was reintroduced in 1999 after being removed from the market two different times based on concerns about its long application time, flammability, and odor.
Malathion is an organophosphate that inhibits cholinesterase activity, resulting in increased acetylcholine concentrations.
The excess cholinergic activity causes hyperstimulation of the louse nervous system and prevents feeding.
Malathion is highly ovicidal, and a single application is sufficient for most patients.
Superior activity over 1% permethrin has been demonstrated.[8,9]
Because of the increased permeability of infant scalps, malathion is contraindicated in neonates and infants.
Malathion has not been studied in children aged less than 6 years, and it is contraindicated in children aged 24 months and younger.
The drug is Pregnancy Category B.
Resistance has been reported in the United Kingdom, but the UK formulation differs from that used in the U.S. The U.S. formulation contains terpineol, dipentene, and pine needle oil, all of which are pediculicidal, and the combination of these ingredients with malathion is thought to have delayed resistance in the U.S.
The U.S. formulation contains 78% isopropyl alcohol, which renders it highly flammable. Patients should be counseled to avoid high temperatures and open flames (e.g., hair dryers, cigarettes) while the hair is still wet.
Adverse effects include a slight stinging sensation; however, because inadvertent transdermal absorption has occurred in agricultural use, the patient should be monitored for signs of excessive cholinergic activity, including increased sweating, salivary and gastric secretions, gastrointestinal and uterine motility, and bradycardia.
Approved by the FDA in April 2009, benzyl alcohol is available only by prescription. Benzyl alcohol is not neurotoxic or ovicidal; it is an occlusive agent and kills lice by asphyxiation.[2,10]
Because lice can close their respiratory spiracles when they are submerged in water or other occlusive agents, it is difficult to suffocate them.
The benzyl alcohol in Ulesfia prevents the lice from closing the spiracles and allows the vehicle (containing mineral oil and inactive ingredients) to penetrate and obstruct them, resulting in asphyxiation.
Ulesfia is approved for children older than 6 months and is Pregnancy Category B. The most common adverse effects include ocular irritation, application-site irritation, application-site anesthesia, and hypoanesthesia.
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Spinosad, an ovicidal agent, was approved by the FDA in January 2011. Spinosad is indicated for use in patients aged 4 years and older.[12,13]
The drug is derived through the fermentation ofSaccharopolyspora spinosa
, a soil Actinomycete
The mechanism of action (MOA) is hyperexcitability of the louse nervous system that results in paralysis and death.
In two clinical studies evaluating the efficacy of spinosad and permethrin under actual-use conditions (in the home), spinosad without nit combing was superior to permethrin with nit combing (study 1, 84.6% vs. 44.9%; study 2, 86.7% vs. 42.9%) for complete lice eradication 14 days after the last treatment. Most spinosad patients required only one application (63.8% and 86.2% in studies 1 and 2, respectively); most permethrin patients required two treatments (60.3% and 64.5% in studies 1 and 2, respectively).
These data suggest that spinosad is more effective and convenient than permethrin.
This product contains benzyl alcohol and is classified as Pregnancy Category B. The most common adverse reactions (≥1%) are application-site erythema, ocular irritation, and application-site irritation.
This product is no longer indicated
for the treatment of head lice. Lindane causes neurotoxicity in humans, and seizures have been reported in children. The AAP does not recommend the use of lindane, and the drug has been banned in California.
Table 1. Medication Application Instructions
Shampoo hair with nonconditioning shampoo, then towel it dry.
Apply to damp hair and leave on for 10 minutes.
Rinse off with warm water.
Repeat if live lice are seen after 7–10 days.
|Pyrethrins With PBO
Apply to dry hair.
Rinse out after 10 minutes.
Repeat after 9 days.
Apply an amount sufficient to entirely cover dry hair and scalp (keep eyes tightly shut).
Allow hair to air dry.
Hair should remain uncovered after application.
Shampoo hair after 8–12 hours.
Use a nit comb after shampooing to remove lice and eggs.
Repeat if live lice are seen after 7–9 days.
Apply to dry hair, using enough to completely saturate hair and scalp.a
Rinse off with water after 10 minutes.
Repeat in 7 days (although waiting until day 9 may be more effective).
Apply first to dry scalp, then to dry hair in amount sufficient to cover entirely.
Apply up to 120 mL (depending upon hair length).
Leave on for 10 minutes.
Using warm water, thoroughly rinse hair and scalp.
Repeat if lice are seen after 7 days.
Apply to dry hair in amount sufficient to coat hair and scalp (up to 1 tube).
Leave on for 10 minutes.
Rinse off with water.
For hair ≤4″ long, 1 bottle will suffice; hair up to 16″ long may require up to 3 bottles; hair longer than 22″ may require up to 6 bottles.
PBO: piperonyl butoxide.
Source: References 2, 7, 10, 12, 17.
Ivermectin is an anthelmintic agent structurally similar to macrolide antibiotics, but with no antibacterial activity. A single dose of 200 mcg/kg given orally and repeated 10 days later has been shown to be effective against head lice.
A recent study found a dose of 400 mcg/kg given on days 1 and 8 to be more effective than malathion.
Gina Wesley, OD, MS, FAAO, discusses cost considerations.
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A neurotoxic drug, ivermectin causes central nervous system (CNS) hyperstimulation, resulting in paralysis and death.
Although ivermectin does not readily cross the blood–brain barrier, young children may be susceptible to CNS adverse effects.[2,16]
Children weighing less than 15 kg should not receive ivermectin.
On February 7, 2012, the FDA approved a topical ivermectin lotion for the treatment of lice in patients aged 6 months and older.
This agent is used off-label to treat head lice. Although the MOA is unknown, SMZ/TMP is thought to eradicate symbiotic bacteria in the louse gut.
In one study, SMZ/TMP (10 mg/kg/day in 2 divided doses) administered for 10 days with permethrin 1% was superior to both SMZ/TMP and permethrin alone at 2 weeks (95%, 83%, and 79.5%, respectively) and 4 weeks (92.5%, 78%, and 72%, respectively).
There is anecdotal evidence that substances such as mayonnaise, margarine, herbal oils, and olive oil act as occlusive agents and are effective.Dimethicone products have been clinically shown to have ovicidal properties. The product kills eggs by obstructing their aeropyle (similar to obstructing the louse’s respiratory spiracles). Available OTC, LiceMD is formulated with dimethicone.
Shaving the head is an effective means of eradicating lice; however, for cosmetic reasons, it is an undesirable option. Wet combing with nit combs is an attractive alternative to insecticides for some parents. Combing may be facilitated by using shampoo or conditioner as a lubricant; however, evidence is insufficient to conclude that this is an effective method for eradicating lice.