Permethrin is toxic to Pediculus humanus, Pthirus pubis, and Sarcoptesscabiei. Less than 2% of an applied dose is absorbed percutaneously.Residual drug persists up to 10 days following application.It is recommended that permethrin 1% cream rinse (Nix) be applied undiluted to affected areas of pediculosis for 10 minutes and then rinsed off with warm water. For the treatment of scabies, a single application of 5% cream (Elimite, Acticin) is applied to the body from the neck down, left on for 8–14 hours, and then washed off. Adverse reactions to permethrin include transient burning, stinging, and pruritus. Cross-sensitization to pyrethrins or chrysan-themums has been alleged but inadequately documented.
The gamma isomer of hexachlorocyclohexane was commonly called gamma benzene hexachloride, a misnomer, since no benzene ring is present in this compound. Percutaneous absorption studies using a solution of lindane in acetone have shown that almost 10% of a dose applied to the forearm is absorbed, to be subsequently excreted in the urine over a 5-day period. After absorption, lindane is concentrated in fatty tissues, including the brain.
Lindane (Kwell, etc) is available as a shampoo or lotion. For pediculosis capitis or pubis, 30 mL of shampoo is applied to dry hair on the scalp or genital area for 4 minutes and then rinsed off. No additional application is indicated unless living lice are present 1 week after treatment. Then reapplication may be required.
Recent concerns about the toxicity of lindane have altered treatment guidelines for its use in scabies; the current recommen-dation calls for a single application to the entire body from the neck down, left on for 8–12 hours, and then washed off. Patients should be retreated only if active mites can be demonstrated, and never within 1 week of initial treatment.
Concerns about neurotoxicity and hematotoxicity have resulted in warnings that lindane should be used with caution in infants, children, and pregnant women. The current USA package insert recommends that it not be used as a scabicide in premature infants and in patients with known seizure disorders. California has prohib-ited the medical use of lindane following evaluation of its toxico-logic profile. The risk of adverse systemic reactions to lindane appears to be minimal when it is used properly and according to directions in adult patients. However, local irritation may occur, and contact with the eyes and mucous membranes should be avoided.
Crotamiton, N-ethyl-o-crotonotoluidide, is a scabicide with some antipruritic properties. Its mechanism of action is not known. Studies on percutaneous absorption have revealed detectable levels of crotami-ton in the urine following a single application on the forearm.
Crotamiton (Eurax) is available as a 10% cream or lotion. Suggested guidelines for scabies treatment call for two applications to the entire body from the chin down at 24-hour intervals, with a cleansing bath 48 hours after the last application. Crotamiton is an effective agent that can be used as an alternative to lindane. Allergic contact dermatitis and primary irritation may occur, necessitating discontinuance of therapy. Application to acutely inflamed skin or to the eyes or mucous membranes should be avoided.
Sulfur has a long history of use as a scabicide. Although it is non-irritating, it has an unpleasant odor, is staining, and is thus dis-agreeable to use. It has been replaced by more aesthetic and effective scabicides in recent years, but it remains a possible alter-native drug for use in infants and pregnant women. The usual formulation is 5% precipitated sulfur in petrolatum.
Malathion is an organophosphate cholinesterase inhibitor that is hydrolyzed and inactivated by plasma carboxylesterases much faster in humans than in insects, thereby providing a therapeutic advan-tage in treating pediculosis . Malathion is available as a 0.5% lotion (Ovide) that should be applied to the hair when dry; 4–6 hours later, the hair is combed to remove nits and lice.
Benzyl alcohol (Ulesfia) is available as a 5% lotion for the treat-ment of head lice in patients older than 6 months. The lotion is applied to dry hair and left on for 10 minutes prior to rinsing off with water. Because the drug is not ovacidal, the treatment must be repeated after 7 days. Eye irritation and allergic contact derma-titis have been reported.