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Head lice therapies revisited

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Head lice therapies revisited
Craig G Burkhart MD MPH1, Craig N Burkhart MD MSBS2
Dermatology Online Journal 12 (6):3

1. Medical University of Ohio at Toledo and Ohio University of Osteopathic Medicine, Sylvania, Ohio.
2. University of North Carolina at Chapel Hill, Department of Dermatology, Chapel Hill, North Carolina

Pediculosis capitis, or head lice infestation, is attributed to the highly host-specific insect, Pediculus humanus capitis, which has infested humans since antiquity. This infestation has proved difficult to eradicate. Indeed, earlier societies people often shaved their heads and wore wigs in an attempt to rid themselves of the discomfort and risk of disease associated with this insect.

Even today, treatment for head lice is not always effective, and options are limited. Present therapies can be divided into non-pesticidal formulations, topical over-the-counter agents, and prescription topical remedies.

Nonpesticidal therapies include fine-tooth combing, Vaseline petroleum jelly, mayonnaise, olive oil, and the like. Inasmuch as the overall composition of the nit-sheath protein is chemically quite similar to that of keratin [1], there is no simple remedy to loosen the nits without damaging the hair shaft. Therefore, removing the nits with a comb is difficult, tedious, time consuming, somewhat painful, and not reasonable in most situations. There is a definite misconception that petroleum jelly or mayonnaise actually kill or smother lice. However in vitro testing shows that placing an adult louse for several hours into petroleum jelly or mayonnaise does not affect viability [2]. After rinsing the solution off the louse, it quickly regains total motor capabilities. An overlooked factor in insecticide resistance testing is the ability of lice to assume an apparent moribund state and later to resurrect from seeming death. Insects are less dependent than mammals on continuous nervous control of respiration and circulation. Thus, with efficacy testing in head lice, the end point must reflect irreversible morbidity or death. Agents claimed to provide therapeutic success must be more critically assessed.

Over-the-counter insecticides consist of two related products, permethrin and pyrethrin (with a synergist, piperonyl butoxide). Although these products were highly therapeutic when first introduced, resistance has become common [3]. Treatment failure in most instances can be attributed to drug resistance rather than poor compliance or re-infestation.

Prescription insecticidal agents for head lice are also associated with drug resistance. However, the malathion-containing insecticide in America, unlike in Europe, is combined with an additional insecticide and has not met with resistance yet [4]. Specifically, Ovide® (malathion Taro Pharmaceuticals, Hawthorne, New York) in addition to malathion, contains 78 percent isopropyl alcohol, terpineol, dipentene, and pine-needle oil. The inclusion of plant insecticides (terpineol, dipentene, and pine-needle oil) in this vehicle offers measurable pediculicidal and ovicidal effects [5].

Resistance to insecticides occurs by modified target sites, enhanced detoxification, decreased sensitivity of nerve membranes, and decreased availability of the insecticide at the primary target site [6]. The latter factor may be mediated by decreased cuticular penetration, faster degradation by metabolism, storage in insensitive tissues, or increased elimination. Insect resistance has developed by means of genetic mutations. For example, the housefly has developed three distinct locations on its DNA genome for evolving adaptations to pyrethroid insecticides [7]. Given that insects have the genetic capabilities of overcoming environmental stresses, one has to assume that resistance to present formulations will be forthcoming. Thus, development of other treatment options is continually warranted.

Another alternative is ivermectin. This drug binds selectively to specific receptors of neurotransmitters that function in the peripheral motor system of invertebrates. Ivermectin is FDA approved for treatment of strongloidiasis and onchocerciasis. There are numerous articles on its use for scabies, but a dearth of information in its use for head lice [8]. Oral ivermectin exerts its insecticidal effects only on lice that are feeding from their hosts. Because the plasma half life of oral ivermectin is 16 hours, a second dose on day 8 is recommended in order to kill nymphs that hatch after the initial dose, before they become fertile. Ivermectin at 200 micrograms per kg given on days 1 and 8 appears to be very effective treatment for pediculosis capitis. There is concern about using this drug in patients who weigh less than 15 kg, or those who are pregnant or breast-feeding.

Topical ivermectin (1%) is reported to be highly successful in the management of infestations in animals. Our experience with this concentration has also proved totally effective in human head lice. Ivermectin can be compounded in propylene glycol or in an oil-base formulation [9]. We also have used ivermectin in a polymer base, inasmuch as this base would limit systemic absorption of the insecticide. Our in vitro studies in which the adult lice are inundated with 1 percent ivermectin solution for 10 minutes followed by rinsing with water show that the lice lose muscle control within 20 seconds and, more importantly, never regain any motor control over the ensuing 24-hour vigil.

The present choice of treatment for head lice can be based on efficacy, potential toxicity, and louse resistance patterns to various insecticides in one's particular geographic area. In practice, prescription Ovide® is the preferred choice on the basis of superior efficacy to all other available insecticides. Over-the-counter pyrethrins and permethrin would be considered first-line therapy only if cost issues are paramount. Of note, ivermectin appears to be the next new topical insecticide on the horizon for head lice.


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