Trying To Keep Ahead of Lice: A Therapeutic Challenge

C. E. Malcolm, MD, CCFP¹ and J. N. Bergman, MD, FRCPC²

1. University of British Columbia Student Health Services, Vancouver, Canada
2. Department of Dermatology and Skin Science, University of British Columbia, and The Pediatric Allergy Dermatology Centre (PADC), Vancouver, Canada

Background

The social, economic, and educational impact of head lice infestations is considerable. It is most commonly seen in school-aged children, and girls are more commonly affected than boys. New therapeutic options are now available that may help clinicians to keep ahead of lice.

Infestation

• Caused by the obligate ectoparasite Pediculus humanus capitis; can survive for 1–2 days away from the scalp.
• Transmission most commonly occurs through:
• Close physical contact, especially head-to-head contact
• Fomites, such as hats.
• Transfer is optimal when hairs are relatively stationary and parallel, i.e., while children are at rest.
• Eggs are glued to the hair close to the scalp in egg castings, or nits. Nits within 1cm of the scalp should be counted as a sign of active infestation.

Clinical Presentation

• Most common symptom is pruritus.
• Occurs due to sensitization to either louse salivary or fecal antigens.
• May be so intense that excoriations and secondary bacterial infection may occur.
• Diagnostic gold standard is finding a live louse or nymph on the scalp, or a viable egg attached to the hair. Microscopic examination of the nit may aid in this determination.
• Nits alone are not proof of active infection.
• Not finding a louse does not completely rule out infestation.
• Louse combs increase the diagnostic yield.

Treatment Options

Pediculicides: Neurotoxic Agents

• Includes permethrin, permethrin-based products, malathion, and lindane.
• Not recommended for children < 2 years of age. Off-label use based on clinical judgement.
• Avoid hair conditioner before applying; it may coat the hair and protect the lice and nits.

Permethrin-based Products

• OTC extracts of natural pyrethrins from chrysanthemums combined with piperonyl butoxide to increase stability and effect.
• Neurotoxic to lice, but not ovicidal; even after two treatments viable lice and eggs may remain.
• Contraindicated in patients allergic to ragweed, chrysanthemums, or other permethrin products.

Permethrin 1%

• Historically considered standard treatment; however issues of resistance have made it necessary to explore new alternatives.
• It is a poorly absorbed synthetic pyrethrin with pediculicidal and ovicidal activity.
• Leaves a residue on the hair and remains active for 2 weeks following application.
• Wash hair, rinse with water, towel dry, then apply to entire scalp and hair for 10 minutes and rinse out. Treat again 7-10 days later.

Malathion 0.5%

• Can be applied for 10 minutes or overnight and repeated in 1 week.
• Can cause stinging of the skin and eyes.
• Should be used with caution:
  • base is flammable.
  • may lead to respiratory depression if ingested (there are no reported cases).
• No significant resistance has been reported in the US.

Lindane 1% Lotion

• Second-line treatment to be used as an alternative when other treatments have failed.
• Has limited ovicidal activity; kills lice by causing CNS stimulation and respiratory paralysis.
• Higher side-effect potential including neurotoxicity and bone marrow suppression.
• Contraindicated in children < 2 years, pregnant women, and nursing mothers.

Oral Agent – Ivermectin

• An antihelminthic drug and effective pediculicide
• Suggested for off-label use in the treatment of head lice at a dosage of 200ìg/kg, to be repeated in 7-10 days.
• Possible neurotoxicity is a concern; safety and efficacy remain to be established.
• No resistance has been reported to date and it may be used after failure with topical pediculicides.
• May be useful for extensive infestations or infestations with multiple types of ectoparasites.
• Should not be used in children weighing < 15kg.

Non-neurotoxic Pediculicides

• Exoskeleton Integrity Dehydration Pediculicides
• This is new nonpesticide, nonprescription, behind-thecounter product containing isopropyl myristate 50% and ST-cyclomethicone 50% (Resultz™).
• Recently approved by Health Canada for the treatment of lice in persons aged 4 years and older.
• Works by dissolving the waxy exoskeleton that covers the lice.
• Apply first to dry hair, scalp, and the nape of the neck, leave in place for 10 minutes, then rinse. Repeat in 1 week.
• Phase II clinical trials document a higher success rate (no live lice) compared with traditional pediculicides (57% isopropyl myristate 50% and ST-cyclomethicone 50% vs. 22% with .33% pyrethrin + 4% piperonyl butoxide; 77.1% isopropyl myristate 50% and ST-cyclomethicone 50% vs. 20% with permethrin 1%). [Data on file – Altana Pharma.]
• Other phase II studies showed a 97% (28 of 29 patients) success rate.[Kaul N, et al. In vivo efficacy and safety of an experimental pediculicide rinse. Presented at: the 63rd Annual Meeting of the American Academy of Dermatology, New Orleans, Feb 2005.]
• Well tolerated with mild local erythema or pruritus being the main side-effect.

Dry-on Suffocation Based Pediculicide

• Originally marketed as Nuvo® Lotion; it was later discovered to be Cetaphil® Gentle Skin Cleanser.
• Reported 96% success rate when applied to the scalp, dried with a hair dryer (for ~30 minutes), and removed during the next day’s bath.[Pearlman DL. Pediatrics 114(3):e275-9 (2004 Sep).]
• Reviews found that the study did not use proper methods of diagnosing lice, was anecdotal, and was not a welldesigned, randomized control study.[Roberts RJ, et al. Lancet 365(9453):8-10 (2005 Jan); Burkhart CG, et al. J Am Acad Dermatol 54(4):721-2 (2006 Apr).]
• Given encouraging preliminary results, further study is warranted.

Mechanical

• Nit combing is labor intensive and somewhat painful; should not be used alone.
• Application of a 8% formic acid rinse or a 1:1 mixture of white vinegar and water followed by combing with a nit comb can aid in nit removal.
• The only treatment recommended for children < 2 years of age.

Environmental Interventions

• Decontaminate clothing, linen and towels by washing in hot water (60°C) or dry-cleaning.
• Treat combs and brushes with boiling water, alcohol, bleach, or soak in a disinfectant solution (e.g., 2% Lysol^®).
• Examine all household members and close contacts and treat concurrently if infested.
• Notify the school.
• Treat bedmates prophylactically.

Treatment Categories

Treatment Failures and Resistance

Resistance to permethrin and lindane is common in populations where these pediculicides have been heavily used. Treatment failures can also be a result of reinfestation from:

• an untreated classmate
• an inadequate quantity of pediculicide applied
• the improper duration of product application.

A second treatment of the prescribed pediculicide should be administered 7-10 days after the start of treatment to kill all active stages of the louse. Resistance should be suspected if live lice are still present 2-3 days after the second application of a product has been used correctly and no other cause for failure can be identified.

• If lice are present after 2 correctly applied treatments, resistance is certain.
• Resistant infestations should be treated with an agent from a different class of pediculicides

Conclusion

Lice have developed resistance to some pediculicides and it is expected that with ongoing use, these pediculicides will probably become less effective. These products can still be used effectively to treat nonresistant lice. New products are now available in Canada that may prove to be equal to or more effective/safe than the standard neurotoxic pediculicides, while
at the same time minimize the problem of treatment-resistant lice.