Therapy of head lice: Update and recent trends

September 2001

With a new school year upon us, it is timely to review a common pediatric condition affecting school children – head lice infestation. While no new drugs or new randomized trials have been reported recently, the literature does include recent data applicable to the appropriate management of head lice infestation.

Resistance

The current literature on head lice infestation frequently includes reference to resistance of head lice to the commonly used medications. Numerous anecdotal reports appear attesting to what drug to use if resistance to the first-line therapies is suspected. Scant scientific evidence exists documenting the effectiveness of these alternatives. As well, the term “resistance” is commonly used to describe continued infections that may be due to improperly treated infections, or erroneous diagnoses (and thus not true head lice infections).

It is also important to consider that head lice resistance patterns and incidence rates of resistance are not well known and have not been well studied. Studies scientifically describing resistance patterns have only just begun to appear in the literature. Certainly, the term “resistance” as applied to head lice may not carry the same connotation (ie, qualitative measurement) as does bacterial resistance to antibiotics.

It is often assumed that the diagnosis of head lice infestation is quite difficult, for head lice are small, they closely match the color of their surroundings (ie, the patient’s hair), and they quickly move when hair is parted to locate them. Other objects found in hair can easily be confused with active head lice infestation, such as dandruff.

Empty eggshells or eggshells containing dead embryos from older, treated infestations can be misinterpreted as representing active infections. Empty eggshells can remain attached to hair shafts for long periods. Thus, it is often stated that a diagnosis should be based primarily on the visualization of mobile lice, or perhaps on the presence of many eggs close (within 0.25 inch) to the scalp.

Eggs are laid close to the scalp and are attached to hair shafts by a durable glue-like substance. The eggs can remain on the hair for a considerable time, long after the juvenile louse (nymph) has hatched. Thus, as hair grows, the empty egg shells can remain on the hair for a considerable time, and eggs located greater than 0.25 inch from the scalp are not likely to indicate active infection.

Diagnostic accuracy

A study recently evaluated the accuracy of head lice infestation diagnoses (Pollack, 2000). The study’s purpose was to determine the appropriateness of pediculicidal therapy and school exclusion because of suspected head lice infestation. Specimens of suspected head lice were sent by health care workers and the lay public to a Boston laboratory where they were analyzed.

Samples were received from 42 states and from several other countries. Over 600 submissions were received from over 500 subjects and it was determined that lice or their eggs were present in less than 60% of the samples. Of these, only 53.3% included a louse or a viable nit. The other submitted samples contained either empty nits, nits containing dead embryos, or non-louse material (eg, dandruff). The researchers also assessed the diagnostic accuracy of those submitting samples, and found that few of the samples submitted by physicians contained louse-derived material.

While most samples submitted by parents, school nurses, or teachers included louse-derived material, nits that were submitted frequently were nonviable. The authors also retrieved information of recent pediculicidal therapy and related its appropriateness to the submitted samples. It was found that over one-half of those without active infestation had used at least one course of an over-the-counter (OTC) pyrethroid product in the previous 4 weeks. While this may not completely demonstrate the appropriateness of product use (ie, used appropriately several weeks prior), it does offer some sense of the potential for overuse of pediculicides.

Although numerous anecdotal reports of treatment failure of head lice infestation with OTC products are appearing, the actual incidence of resistance of head lice to these products has not been well studied in the United States. As well, relatively few studies have been published which have scientifically evaluated the susceptibility of head lice to permethrin (Nix, Warner-Lambert), the active agent in many commonly used products.

The methodology for determining nonsusceptibility is also not well defined, as it is with determining bacterial nonsusceptibility to antibiotics. Head lice nonsusceptibility has been reported from several other countries and has been evaluated by determination of louse mortality after exposure to a specific concentration of permethrin for a specific time. These studies have generally shown longer periods necessary to kill head lice populations as compared with studies of lice several years prior, indicating development of nonsusceptibility. How these studies may apply to head lice in the United States is not known.

Pollack also evaluated head lice susceptibility to permethrin taken from children in the United States and Borneo (where permethrin is uncommon). Unlike other studies, this report used graded concentrations of permethrin. It was found that few of the head lice from U.S. children were killed by permethrin (in concentrations up to 1%, the concentration in OTC products), as compared to a much higher mortality in head lice taken from children from Borneo.

Nearly all the U.S. children had been treated with pyrethroid products, thus indicating a biased sample of lice potentially more resistant to common treatments. Thus, while it is quite possible that head lice are developing resistance to the commonly used pediculicidal products, the incidence of this resistance, especially in the United States, is still not well defined. Until further studies defining nonsusceptibility of head lice in the United States are reported, it is premature to conclude that head lice resistance is rampant in many children.

Treatment recommendations

Despite anecdotal reports and few qualitative studies of head lice nonsusceptibility, the current standard therapies should still be used. The pyrethroid products, permethrin and pyrethrin (Rid, Bayer), which are available OTC, should still be used first-line. It is imperative that these products be used properly, for they differ in their application times and procedures (Table). Patient reports of treatment failure, and thus implications of resistance, may actually be due to incorrect application.

Second-line therapies include malathion (Ovide, Medicis) or lindane. Resistance to lindane has been reported, but the incidence of this resistance in the United States is not well defined. Although lindane use may produce significant toxicity, adverse effects are uncommon if it is used appropriately. It should not be used in premature infants and children with underlying seizure disorders. Caution should be exercised when lindane is used in children with nonintact skin, or in children younger than 2 years of age.

Other products have been advocated for use with difficult cases. However, these products are not approved for such use by the FDA, and their use is based mainly upon anecdotal reports, and not controlled trials. Elimite (Allergan), approved for use for the treatment of scabies, contains permethrin 5%, the same active ingredient in Nix, although at a higher concentration. However, some evidence from resistance studies indicates that head lice nonsusceptibility to permethrin may be concentration independent. Other products that have been used for difficult to treat cases include ivermectin (Stromectol, Merck) or trimethoprim-sulfamethoxazole.

Treatment failures

Before treatment failures are attributed to resistance, it is important for clinicians to evaluate other factors that may also lead to treatment failures. Appropriate use of the product should be verified, with emphasis on application time, application to wet or dry hair, or prior use of hair conditioning products, as these factors may decrease product efficacy. Treatment of close contacts should also be discussed, as reinfection may be misinterpreted as a treatment failure. It is also important to verify a correct diagnosis, as other artifacts in the hair may be interpreted as continued active infection.

Empty eggshells, or eggshells containing killed embryos alone do not indicate active infection. A study recently published (Williams, 2001) attempted to determine the probability that children with nits alone (eggshells with a developing embryo, empty eggshells, or eggshells containing a killed embryo) will become infected with lice. It was found that most children with nits alone did not become infested, thus implying that treatment for all children with nits is excessive.

Conclusions

While limited evidence suggests that head lice have developed resistance to commonly used pediculicides, the incidence of this resistance, and risk factors for contracting resistant head lice, are poorly understood. The standard pediculicidal products should continue to be used. Although unapproved alternative products are available, their efficacy and safety have not been clearly established. Prior to resorting to one of these products, clinicians should verify that an active infection truly exists, and that recent treatments were appropriately applied.

For more information:

• Williams LK, et al. Lice, nits and school policy. Pediatrics. 2001;107:1011-1015.
• Pollack RJ, et al. Overdiagnosis and consequent mismanagement of head louse infestations in North America. Pediatr Infect Dis J. 2000;19:689-693.
• Pollack RJ, et al. Differential permethrin susceptibility of head lice sampled in the United States and Borneo. Arch Pediatr Adolesc Med. 1999;153:969-973.