Pediculosis and Pthiriasis (Lice Infestation) Medication

Updated: Mar 01, 2018
  • Author: Lyn C C Guenther, MD, FRCPC, FAAD; Chief Editor: Michael Stuart Bronze, MD  more...
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Medication

Medication Summary

The goal of therapy is to eliminate lice and eggs. Chemical pediculicides are the mainstay of pharmacotherapy. With most medications, treatment should be repeated in 7-10 days (the time needed for the eggs to hatch) because nits are less effectively killed than adults. Eyelash infestation can be treated effectively with petrolatum ointment (eg, Vaseline).

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Antiparasitic Agents

Class Summary

Pubic lice can be treated with over-the-counter permethrin 1% lotion and a mousse containing pyrethrins and piperonyl butoxide. [14] Malathion lotion 0.5% is currently not FDA approved to treat pubic lice.

One study reported efficacy of 4% dimethicone lotion (a silicone-based lotion believed to disrupt the louse's ability to manage water) in the treatment of head lice. [37] Another study found that 4% dimethicone lotion was a significantly more effective than malathion in most patients. [38]

Dimethicone (NYDA) is not approved for use in the United States but is available in Canada and Europe. Dimethicone replaces the air in the breathing system of lice, nymphs and nits (eggs), thickens quickly, then causes suffocation and death. Dimethicone can be used in individuals aged 2 years or older. This product should eliminate head lice infestation after one application, Apply to dry hair and leave on for 8 hours. After it has been on for 30 minutes, using the supplied NYDA lice comb, comb the dead lice and eggs. Repeat after 8-10 days to ensure complete removal of head lice.

Isopropyl myristate is not available in the United States (it is currently in phase III clinical trials), but it is available in Canada and Europe. It is a non–insecticide-based drug containing isopropyl myristate, an ingredient commonly used in cosmetics. Its mode of action is a mechanical process that weakens the waxy shell of lice, resulting in internal fluid loss and dehydration.

Treatment options include pyrethrin shampoos, permethrin 1% cream rinse, permethrin 5% cream, 0.5% and 1% malathion (Ovide), topical ivermectin (Sklice), spinosad, and benzyl alcohol lotion 5% (Ulesfia).

Parasite biochemical pathways are different from the human host; thus, toxicity is directed to the parasite, egg, or larva. Mechanism of action varies within the drug class. Antiparasitic actions may include the following:

- Inhibition of microtubules causing irreversible block of glucose uptake

- Tubulin polymerization inhibition

- Depolarizing neuromuscular blockade

- Cholinesterase inhibition

- Increased cell membrane permeability, resulting in intracellular calcium loss

- Vacuolization of the schistosome tegument

- Increased cell membrane permeability to chloride ions via chloride channels alteration

Resistance to pediculicides has increased over recent years. Therapeutic agents can be rotated to slow the emergence of resistance. Benzyl alcohol lotion needs to be applied twice, but it might be an easier and safer alternative to lindane and malathion.

Permethrin (Acticin, Elimite)

Permethrin is the drug of choice recommended by most authorities as the first line of treatment in head, pubic, and severe body louse infestation, especially for infants older than 2 months and small children. This agent is a neurotoxin that causes paralysis and death in ectoparasites. It is more effective than crotamiton in treating symptoms and reducing the chances of secondary bacterial infection. One benefit of permethrin is a residual effect in the hair for several hair wash cycles.

Resistance probably has developed in many areas, however. Physicians in some countries select different pediculicides on a rotating basis to discourage development of resistance.

Permethrin is very effective in killing adult lice and nymphs but is not as effective in killing nits (eggs). Patients should wash hair with a nonmedicated shampoo.

An over-the-counter (OTC) 1% concentration (permethrin lotion) may be insufficient for treatment of pubic lice and for some cases of head lice. The 5% prescription preparation marketed for scabies (Elimite, Acticin) may be more effective in some cases. Strict adherence to the treatment regimen is essential.

Pyrethrins and piperonyl butoxide (A200 Maximum Strength, A-200 Lice Treatment, LiceMD, Licide, Pronto Complete Lice Removal System, Pronto Plus Lice Killing Mousse Plus Vitamin E, Pronto Plus Lice Killing Mousse Shampoo Plus Natural Extracts and Oils, Pronto Plus Warm Oil Treatment and Conditioner, RID Maximum Strength)

Pyrethrins are first-line treatment in head, pubic, and severe body louse infestation. These agents stimulate the parasitic nervous system, causing seizures and death of parasites. This is an older OTC agent that still appears to be effective. It lacks the residual action of permethrin and is more likely to require repeated applications. Pyrethrin products are contraindicated for patients with contact allergy to ragweed, turpentine, or chrysanthemums.

Malathion (Ovide)

Malathion is approved by the FDA for the treatment of head lice. It is an irreversible cholinesterase inhibitor that is hydrolyzed and therefore detoxified rapidly by mammals but not by insects; it is both ovicidal and pediculicidal. It binds to hair and provides some residual protection after therapy. Malathion is available as a 0.5% lotion for use in individuals aged 6 years or older. Malathion 0.5% lotion (Ovide) is flammable and should not be used in the presence of hot hair care products or near individuals who are smoking.

Ivermectin topical (Sklice)

Topical ivermectin causes parasite death by selective, high-affinity binding to glutamate-gated chloride channels located in invertebrate nerve and muscle cells. It is a topical pediculicide that treats head lice with a single 10-minute application without nit combing in adults and children aged 6 months or older.

Ivermectin (Stromectol)

Ivermectin binds selectively with glutamate-gated chloride ion channels in invertebrate nerve and muscle cells, causing cell death. Its half-life is 16 hours; it is metabolized in the liver. Ivermectin is the drug of choice for onchocerciasis and strongyloidiasis; it has been shown to be effective against pediculosis and has been used in mass epidemics. It is not effective against nits.

Few serious adverse effects have been reported when ivermectin is used to treat lice or scabies. This agent is not associated with evidence of selective fetotoxicity in pregnant women inadvertently exposed (based on limited data). Limited animal data also fail to show evidence of selective fetotoxicity.

Ivermectin is available in the United States as an oral 6-mg pill marketed for treatment of Strongyloides. Physicians have used this drug for lice and scabies (off-label use) in cases where such therapy was in the best interest of patients and conventional therapy failed. Health care providers in the United States are encouraged to read the FDA statement concerning off-label use of approved drugs, which appears in the Physicians' Drug Reference. In February 2012, topical 0.5% ivermectin (Sklice) was approved in the United States as a single-dose, 10-minute application without the need for nit combing in individuals aged 6 months or older.

Lindane

Lindane stimulates the nervous system of parasites, causing seizures and death. It is a chlorinated insecticide available as 1% lotion, cream, and shampoo. It is a second-line treatment if other agents fail or are not tolerated.

In March 2003, the FDA issued a public health advisory warning of increased risk of adverse effects of lindane treatment in persons who are young, small, or elderly. Heightened caution should be exercised if lindane is used in such patients.

Benzyl alcohol (Ulesfia, Zilactin)

Benzyl alcohol inhibits lice from closing their respiratory spiracles, allowing the lotion to obstruct the spiracles, which ultimately results in asphyxiation. It does not elicit ovicidal activity. This product contains 5% benzyl alcohol and is approved by the FDA for treatment of head lice in patients aged 6 months or older. The lotion is applied twice (initially and 1 week later) for 10 minutes.

Spinosad (Natroba)

Spinosad causes neuronal excitation in insects, followed by hyperexcitation, paralysis, and death. It is a pediculicide indicated for topical treatment of head lice infestations in individuals aged 4 years or older. This agent is available as a 0.9% suspension. The product is applied to dry hair as a cream rinse, left in for 10 minutes, and then shampooed out.

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