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).
Antiparasitic Agents
Class Summary
Treatment options include pyrethrins with piperonyl butoxide (A-200, Pronto, R&C, Rid, Triple X), permethrin 1% lotion (NIX), permethrin 5% (approved to treat scabies but occasionally used to treat lice; Actin, Elimite), malathion 0.5% (Ovide), lindane 1% shampoo and lotion, ivermectin 0.5% lotion (Sklice), spinosad 0.9% (Natroba), benzyl alcohol 5% lotion (Ulesfia), isopropyl myristate (Resultz), dimethicone (LiceMD Pesticide Free, NYDA) and a mineral oil–based shampoo (Nix Ultra shampoo).
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
- Inhibition of metalloproteinases
Resistance to pediculicides has increased over recent years. Therapeutic agents can be rotated to slow the emergence of resistance.
Pyrethrins/piperonyl butoxide (A-200 Maximum Strength, LiceMD Complete, Pronto Plus Complete Lice Removal System, RID Lice Killing)
Pyrethrins are first-line treatment in head, pubic, and severe body louse infestation. They are available as a mousse or shampoo. 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. Piperonyl butoxide has been added to pyrethrin products to enhance efficacy and to minimize the potential for resistance.
Pyrethrin/piperonyl butoxide products should be applied to dry hair and left on for 10 minutes, then rinsed off. If live lice are noted after 9 days, a second application is recommended.
Permethrin (Nix, 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. The product should be applied to recently shampooed hair that is damp and left on for 10 minutes then rinsed off. A second application in 7 days is recommended.
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) may be more effective in some cases. Strict adherence to the treatment regimen is essential.
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, Soolantra)
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.
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)
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.
Isopropyl myristate (Resultz)
Dissolves the wax covering on the exoskeleton of head lice. This in turn causes dehydration and subsequent death. Owing to the physical mode of action, lice are unlikely to develop resistance to this product.
Mineral oil topical (Nix Ultra Shampoo)
Mineral oil can be used as a safe and effective alternative to insecticide-based pediculicides. There is less risk for development of resistance because of the physical mode of action of mineral oil.
Dimethicone topical
A highly effective head lice treatment, with a low risk of adverse effects. May serve as a potentially less toxic and an alternative to resistance-prone pesticide-containing products.
Abametapir (Xeglyze)
Topical 0.74% lotion indicated as a single 10-minute treatment for head lice in patients aged 6 months and older. It acts by inhibiting metalloproteinase, which is critical to egg development and survival of lice.
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Nit on a hair. Note the thin, translucent cement surrounding the hair shaft. Photo courtesy of David Shum, MD Western University, London Ontario.
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Two empty nits from Pediculus humanus capitis. Note the open shells still attached to the hairs and the porous operculi through which the lice have hatched. Photo courtesy of David G. Schaus.
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Three specimens of Pediculus humanus capitis.
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Pediculus humanus corporis.
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Phthirus pubis. Note the crab-like appearance.
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The head louse, Pediculus humanus capitis, has an elongated body and narrow anterior mouthparts. Body lice look similar but lay their eggs (nits) on clothing fibers instead of hair fibers.
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The pubic louse, Pthirus pubis, is identified by its wide crablike body.
