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 chrysanthemums has been alleged but inadequately documented.
Spinosad (Natroba) suspension is approved for the topical treatment of head lice in patients 4 years of age and older. Spinosad is derived from the fermentation of a soil Actinomyces bacterium and is toxic to P humanus with no appreciable absorption from topical application. It is recommended that the 0.9% suspension be applied to the hair and scalp for 10 minutes and then rinsed out. A repeat treatment may be applied 1 week later if live lice are present.
Ivermectin (Sklice) 0.5% lotion is approved for head lice treatment in patients 6 months of age and older. Ivermectin is toxic to P humanus, resulting in paralysis and death of the parasite. The pharmacology of ivermectin is discussed in Chapter 53. The lotion should be applied to the hair and scalp and rinsed out after 10 minutes. Ivermectin is for single use only and should not be repeated without health care provider recommendation.
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 is available as a 1% 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 recommendation calls for a single 60 mL 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. 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 crotamiton 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.