Current Treatments for Scabies and Pediculosis

M.L. Elgart, MD


Clinical Professor of Dermatology, Medicine and Pediatrics The George Washington School of Medicine, Washington, DC, USA

ABSTRACT

Scabies and lice have afflicted man since ancient times. Permethrin is generally the treatment of choice for head lice and scabies, because of its residual effect. Toxicity and absorption are minimal. Ivermectin should be reserved for cases where permethrin fails.

Key Words:
scabies, lice, permethrin, ivermectin, malathion, lindane

How do we get rid of scabies and lice? Treatment options that were formerly available, included sulfur, crotamiton lotion (Eurax), and 25% benzyl benzoate¹. Sulfur in 5–10% petrolatum is relatively cheap, but must be applied on three successive nights to be effective. It is considered the safest treatment for pregnant women and very young children, although there are no studies to confirm the lack of toxicity.

For many years lindane was the preferred therapy until concern was voiced about its efficacy and safety. Permethrin, malathion and, most recently, ivermectin have become treatments of choice.

Lindane

For scabies

For the past 50 years, lindane has been the preferred therapy for scabies. This agent needs to be used on successive nights to ensure that the eggs and live mites are adequately exposed to treatment.

For lice

For lice, lindane has a 90% success rate, but there are concerns about side effects involving the central nervous system if improperly used.

Seizures secondary to this medication have been reported, particularly when this medication was applied to wet skin or to skin altered by inflammatory changes that cause easy absorption. The absorption rate may be ≥ 40 times that of permethrin, leading to a significant potential for toxicity². Bathing prior to application should be avoided. Hair should be washed with plain shampoo and dried thoroughly before applying lindane. Children should not apply lindane without adult supervision.

While the topical lotion and shampoo are still available, the topical cream has been withdrawn from the US market. However, it is still available in Canada.

Permethrin

Permethrin is a synthetic compound based on the insecticidal components of naturally occurring permethrins¹². It kills both organisms and eggs, and because of its lack of percutaneous absorption, toxicity is not a consideration. Weekly applications have been very successful in preventing reinfection.

For scabies

The 5% permethrin preparation kills the organisms and eggs, and has an extremely low rate of absorption, making the toxicity potential nonexistent. Weekly applications have been extremely successful in preventing reinfection. It is probably the most reliable topical scabicide.

For lice

The 1% Permethrin crème rinse is effective, although the 5% lotion may be used if the 1% fails. Washing removes excess medication, but the protective residual remains for about a week, thereby reducing the possibility of reinfestation.

For each of these topical applications, the entire skin, head to toe must be treated, including the fingernails, toenails, soles of the feet, the umbilicus, and the perianal area. When anatomic areas are missed, it is impossible to distinguish reinfection from resistant organisms. All people with whom the patient has come into contact must also be treated to avoid reinfection.

Ivermectin

This drug was released for human use in the US in 1996, for onchocerciasis.

It is taken orally, at a dose of 0.2mg/kg, or two-6mg tablets for a 60kg person. It does not protect against reinfestation, though, so may require a follow-up course of treatment. Merck, who developed ivermectin, has donated over a million doses for the treatment of onchocerciasis in Africa. This eradication program occurred without significant side effects.

Table 1: Common treatments for scabies and head lice. Prices are US average wholesale prices. Retail prices are about 40% higher.
*There is some information indicating that the Kwell brand has been discontinued in the US by the manufacturer, Reed and Carnick.

For scabies

A number of clinical studies have shown ivermectin to be an excellent scabicide. A recent study¹⁰ compared the efficacy and safety of ivermectin and lindane when treating human scabies in 53 patients in Buenos Aires, Argentina. Ivermectin was as effective as lindane, and because of its ease of administration, it was felt to be a worthwhile tool for improving compliance and controlling infestations. Several reports^6-9,17 have stressed the advantage that ivermectin provides in managing the eradication of scabies, including the control of outbreaks of infestation in institutional settings.

For lice

Ivermectin has been reported to be effective in removing head lice. However, a second treatment after 10 days may be needed, because it does not affect the viable eggs on the hair shafts⁵.

The US FDA has not approved Ivermectin for treatment of lice or scabies, and the appropriate doses and dosing schedules for these conditions have not yet been established¹⁶.

Malathion

For lice

Malathion lotion should be applied to dry hair, and then allowed to dry on the scalp. After several hours, the hair can be combed to remove nits and lice. Success rates of ≥ 90% have been reported³.

Ovide Lotion (Medicis), composed of 0.5% malathion in 78% isopropanol, was recently approved by the US FDA for treatment of head lice. The same formulation has been available in the US twice before, as Prioderm (Purdue Frederick) and Ovide (GenDerm)¹².

Malathion is an irreversible cholinesterase inhibitor that is hydrolyzed by plasma carboxylesterases much faster in mammals than in insects and is considered safe. Resistance to malathion has been reported⁴.

Nit Removal

After using any of the above treatments, the nits may be dead, but will remain on the hair. Most school authorities will not allow students to return unless the nits are gone. The National Pediculosis Association advocates vigilant daily nit removal for at least 7 to 10 days following the initial treatment. Alternatively, malathion lotion loosens the attachment of the nits to the hair.

Reports of Resistance

Resistance to permethrin in developed countries has been reported in 1999^11,13,18. In one study, researchers compared the response of head lice to permethrin in US children, where pediculicides are readily available, to children from Borneo, where such products are unknown. They concluded that head lice in the US are less susceptible to permethrin than those found in Borneo¹¹.

A study from England evaluated resistance to permethrin and malathion in lice samples and concluded that head lice are resistant to current over-the-counter treatments in England18. Treatment failure can be predicted after repeated applications, even with more concentrated formulations¹¹. There have been reports of resistance for lindane¹⁴ and malathion^4,15, but not for ivermectin.

Other Treatment Options

Other treatment options that are sometimes used include heavy greases such as petrolatum, which is rubbed into the scalp, and the hair placed under a towel or in a net overnight. The petrolatum probably closes breathing holes and suffocates the lice. Physostigmine ointment, used for eyelash infestation of pubic lice, may have a similar mechanism of action. These therapeutic approaches have not undergone rigorous testing.

Alternative Approaches

Alternative approaches are used and enthusiastically accepted in our present climate. However, there are no well conducted studies that support any alternative medical claims at present.

Conclusion

Permethrin, because it leaves a residual on the skin and hair for several days after use, is the preferred treatment for lice and scabies. Its residual effect on the skin discourages re-infestation and lasts up to a week. The 5% lotion is used for scabies, and the 1% for head lice–although the 5% lotion may be used if the 1% fails. Toxicity and absorption are minimal. Ivermectin should be considered for cases in which there is an increased number of organisms (e.g., “Norwegian” scabies) or a failure following the use of permethrin. Malathion is a third treatment option.

References

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