Head Lice: A Review of Topical Therapies and Rising Pediculicidal Resistance

Jason Sneath, MD¹ and John W. Toole, MD, FRCPC²

¹ Department of Dermatology and Skin Science, University of British Columbia, Vancouver, BC, Canada
² Section of Dermatology, University of Manitoba, Winnipeg, MB, Canada

Introduction

Head lice infestations (Pediculosis capitis) are a worldwide problem with prevalence estimates typically ranging between 1-3% in elementary school aged children.^1,2 Although this obligate parasite is a nuisance, infestation does not pose a health risk. Infestations tend to occur more frequently in females,³ and less frequently in black children,⁴ as it may be more difficult for lice to grasp their oval-shaped hair shafts. It is not associated with poor hygiene. Infestation occurs across all levels of society, but occurs more frequently under conditions of overcrowding. Recent evidence suggests increasing frequency of topical treatment failure may be related to a growing resistance to the neurotoxic pediculicides that have been the first-line treatment for the last 40 years.⁵ Herein, we will review the current topical treatment options, including newer non-pediculicidal options.

Overview of Facts on Lice

• Pediculus humanus capitis (the head louse) is a 2-4 mm blood sucking, wingless insect.
• A louse cannot jump, but rather has 6 legs adapted for crawling along hairs at 23 cm per minute.⁶
• A louse will feed every 3-6 hours.
• Prior to feeding, the louse injects saliva into the skin.
• The life span is approximately 4 weeks and the female lays 6-8 eggs per day.
• Eggs hatch in 8 days, leaving their shell (“nit”) cemented to the base of the hair.
• Head lice spread by head contact, shared fabrics, shared combs, and other fomites that are commonly in contact with the scalp and hair.⁷
• A louse can survive 2-3 days away from a human host.
• Pets are not vectors.

Top

Diagnosis and Symptoms

Many affected individuals report no symptoms, but the most commonly reported symptom is scalp pruritus.⁸ The pruritus is thought to be caused by hypersensitivity to the louse saliva that is injected into the scalp during feeding, but the itching often does not begin until 1-4 weeks after infestation. Although any part of the scalp may be colonized, there seems to be a predilection for the nape of the neck and post-auricular areas.

Skin Findings

• Often there are no significant findings on the skin.
• Pruritic, papular lesions may be found at the nape of the neck.
• There may be excoriations on the scalp.
• Secondary staphylococcal infection is possible.
• Possible enlargement of cervical / nuchal lymph nodes.

Hair Findings

• True infestation is confirmed by the presence of live adult lice or nymphs (hatched immature lice) present on the scalp with nits.
• The presence of nits alone does not confirm infestation, as an empty nit can remain cemented to a hair even after the infestation has cleared.
• The distance of the nit from the scalp can be a clue to the duration of the infestation, as it moves with the hair away from the scalp when hair grows.
• A nit within 0.6 mm of the scalp is usually a viable egg.

Diagnosis is best made by wet or dry combing the scalp with a fine-toothed nit comb with teeth spaced 0.2 mm apart. One study comparing wet combing with visual inspection found that wet combing accurately diagnosed infestation 90.5% of the time, as compared to 28.6% with visual inspection.²

Directions for Detection by Wet Combing⁹

• Saturate hair with a conditioner.
• Remove tangles with a regular comb.
• With the nit comb against the scalp, comb to the end of the hair.
• Check the comb for lice after each pull by visual inspection and by cleaning the comb with a tissue and inspecting the contents.
• Dispose of the tissue in a plastic bag.
• Comb the entire scalp at least 5 times.
• Seal the plastic bag and dispose of it.
• If infestation is confirmed, rinse off all conditioner prior to treatment.

Treatment Options

Management

Traditionally, topical pediculicides have been the mainstay in the initial treatment of pediculosis. They are widely available without a prescription, which has contributed to the difficulty in gathering data on the true prevalence of infestation. Easy access and improper use has likely contributed to the significant resistance that has developed against topical pediculicides. Knockdown resistance (kdr) is a heritable insensitivity to dichlorodiphenyltrichloroethane (DDT), the pyrethrins, and the pyrethroids. A recent study examining lice collected in Quebec, Ontario, and British Columbia found the allele for resistance present in 97.1% of the 274 lice sampled.⁵ These findings suggest that a significant resistance to the traditional first-line treatment options exists within Canada.

In recognition of the developing resistance, there has been an increased interest within Canada to explore effective non-pediculicidal options. A recent study found the efficacy of isopropyl myristate 50% to be significantly higher (57%) than the standard treatment with pyrethrin 0.33% + piperonyl butoxide 4%.¹⁰

While non-pediculicidal therapy may be efficacious against treatment resistant infestations, re-infestation from close contacts and fomite transmission is a common problem. Along with treatment, it is important to decontaminate the environment.

Environmental Decontamination¹⁴

• Family members and close contacts should be examined and be treated for any infestation.
• Any clothing, linens, combs, toys, and fabrics used by the individuals in the 3 days preceding treatment should be decontaminated.
• Fabrics can be washed in high heat and put in a hot dryer for 20 minutes.
• Items that cannot be washed can be sealed in a plastic bag for 14 days or placed in the freezer for 24 hours.
• Brushes can be soaked in rubbing alcohol for 1 hour.
• Floors and furniture can be cleaned by vacuuming.
• Spraying the home with a pediculicide is not recommended.
• No nit policies at schools are unnecessary.

Management Tree

(scalp pruritus, papules at the nape of the neck, or known close contact with another infested individual)

Examine scalp using fine-toothed nit comb (0.2 mm spacing) Wet hair with lubricant, such as hair conditioner (optional), and comb front to back with the comb against the scalp

By wet or dry combing (presence of live adult louse or nymph)

Choose treatment (based on patient preference, age, and local resistance to topical pediculicides)

Take measures to decrease risk of reinfestation and spread (treat all family members, notify school, and decontaminate environment)

Manual Removal

Some patients may prefer to attempt mechanical treatments prior to topical therapy. Wet combing, as described earlier, can be both diagnostic and therapeutic. To attempt this method the patient should wet comb the entire scalp until no more lice are found every 3-4 days for 3 weeks, or at least 2 weeks after the last adult louse was found.¹⁴

Treatment Failure

Treatment failure is commonly a result of inadequate or improper treatment, resistance, or reinfestation. If environmental decontamination was performed and the treatment was properly administered, then immediate retreatment with a different agent is advised.

Conclusion

Head lice infestation is a common problem for children in Canada. The first-line treatment of using topical pediculicides is unfortunately not as effective as it once was because of a heritable resistance that seems to be rising in prevalence. Topical non-pediculicides may be an effective option in the case of failed treatment due to louse resistance to standard treatment.

References

1. Harris J, et al. Commun Dis Public Health 6(3):246-9 (2003 Sep).
2. Jahnke C, et al. Arch Dermatol 145(3):309-13 (2009 Mar).
3. Counahan M, et al. J Paediatr Child Health 40(11):616-9 (2004 Nov).
4. Centers for Disease Control and Prevention. Fact sheet: head lice. Available at: http://www.cdc.gov/lice/head/factsheet.html. Accessed June 28, 2010.
5. Marcoux D, et al. J Cutan Med Surg 14(3):115-8 (2010 May-Jun).
6. Ko CJ, et al. J Am Acad Dermatol 50(1):1-12 (2004 Jan).
7. Burkhart CN, et al. J Am Acad Dermatol 56(6):1044-7 (2007 Jun).
8. Mumcuoglu KY, et al. J Med Entomol 41(4):803-6 (2004 Jul).
9. District Health Authority Public Health Services of Nova Scotia. Guidelines for treatment of pediculosis capitis (head lice), August 2008.
10. Kaul N, et al. J Cutan Med Surg 11(5):161-7 (2007 Sep-Oct).
11. Mumcuoglu KY, et al. Isr Med Assoc J 4(10):790-3 (2002 Oct).
12. US Food and Drug Administration public health advisory on lindane.
13. Position Statement from Infectious Diseases and Immunization Committee, Canadian Paediatric Society. Paediatr Child Health 13(8):692-704 (2008 Oct).
14. Goldstein AO, et al. (2010 Jan).