Pediculosis

Abstract

Pediculosis is the result of infestation by one of two families of sucking lice. The Pediculidae include the agents of head and body lice. Public lice belong to the family Pthiridae. Pediculus humanus capitis (the head louse) and Pediculus humanus humanus (the body louse) look identical. The two differ by the slightly larger size of the average body louse, as well as where they reside and deposit their eggs. Head lice live in the scalp hair and deposit their ova on hair shafts. Body lice live in the seams of clothing and deposit their ova on fabric fibers, usually in the seams of the clothing. Although body lice are common vectors of disease, head lice have not been proven to act as disease vectors. This having been said, the two are so closely related that it is naive to believe that head lice will never be shown to spread disease.

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Pediculosis Capitis

It is estimated that pediculosis capitis affects 6 to 12 million people per year in the United States. (1) Louse infestation crosses all regional and socioeconomic boundaries, affecting rich and poor alike. African-American children are less likely to acquire head lice, but may occasionally present with public lice in the scalp. Transmission of head lice occurs through close physical contact as well as through fomites such as hats. Transfer between individuals is optimal when hairs are parallel and slow-moving. (2) This suggests that periods of rest together are more likely to result in louse transmission than periods of vigorous play.

Diagnosis

Head lice move rapidly when the hair is disturbed, and may not be easy to find. Nits (egg cases) are firmly cemented to the hair shaft and are much easier to find. The heaviest infestation is generally in the post-auricular scalp. Shampooing followed by combing with a fine toothed comb can aid in the identification of lice. Each head louse is the size of a sesame seed, and is red-brown in color. Young nymphs are tan and much smaller. The presence of a variety of stages of development suggests established infestation.

Lice are wingless, elongated, and dorsoventrally flattened. They are insects with 3 well-defined body segments and 3 pairs of clawed legs. Pediculus nits are attached to the hair shaft at a wider angle than Pthirus nits, which lie close to the hair shaft. (1) Lice typically lay nits within 1 to 2 mm of the scalp surface, and only nits close to the scalp are indicative of active infestation. For practical purposes, nits that are located within a quarter inch of the scalp should be counted as a sign of active infestation. This may not be true in very humid climates, where lice may lay nits anywhere along the length of the hair shaft. It takes about 2 weeks for nits to hatch and for the nymphal forms to mature to adulthood.

Although pruritus is common, patients with lice can be asymptomatic, and those with few symptoms may represent a reservoir for reinfestation in the classroom. During a louse epidemic, all children should be inspected for lice, not only those children manifesting pruritus. On examination, the presence of lice or nits, excrement, erythematous papules, excoriations, or cervical lymphadenopathy suggest a diagnosis of pediculosis. A morbilliform rash, or pediculid, may accompany the infestation and may be the presenting sign.

During louse inspections, the use of louse combs increases the diagnostic yield. (3,4) A variety of combs are available. In general, metal combs are easier to use than plastic combs, and any comb is easier to use if the hair is washed and a conditioner is applied. Acid rinses make the hair easier to comb. Battery powered combs that produce a static electric current are readily available on the Internet. There is ample testimonial evidence that these combs are beneficial, but large well designed studies are lacking.

Lice should be examined microscopically or with a hand lens to differentiate them for psocids (primitive louse-like arthropods that can live in decaying books in the school library and cause scalp infestation). (5) As compared with head lice, psocids have larger, rounder heads, large pincer-like mouthparts, immense hind legs and long antennae. Nits must be differentiated from hair casts (remnants of the inner root sheath that slide easily along the hair shaft). Hair casts may be present by the hundreds and lead to misdiagnosis, unnecessary treatment, and exclusion from school.

Pediculosis Corporis

Body louse infestation is common among the homeless in urban areas, and is seen worldwide in refugee camps and wherever crowding and poor living conditions occur. Body lice are vectors for Bartonella quintana, an important cause of fever and endocarditis, as well as for relapsing fever, trench fever, and epidemic typhus.

Pediculosis Pubis

Crab louse infestation manifests not only in the public area, but on the hairy areas of the arms, legs, buttocks, chest, and back. Eyelashes may also be affected. Pubic louse infestation most commonly represents a sexually transmitted disease, and some data suggest that almost a third of the patients have another concurrent sexually transmitted disease. Risk factors should be assessed, and testing for syphilis, HIV, gonorrhea, and chlamydia should be considered. In one retrospective study, adolescents with pubic lice were twice as likely as uninfested adolescents to have chlamydia or gonorrhea. (6)

Treatment Options

Targeted therapy of only those individuals in a classroom who have evidence of louse infestation can be effective if routine louse inspection is carried out. Those with no sign of infestation do not require treatment. (7)

The ideal agent for the treatment of lice would be free of harmful chemicals, readily available without a prescription, easy to use, and inexpensive. Although promising agents are being evaluated, chemical pediculicides currently remain the mainstay of treatment of head lice. At one time, malathion, permethrin, and pyrethrins were similar in efficacy. (8,9) Unfortunately, resistance is now common in populations where pediculicides have been used heavily. (10-12) Resistance to permethrin often correlates with resistance to natural pyrethrins and other pyrethroids. (13) The pattern of resistance in an area generally follows the pattern of pediculicide use. Malathion, which was not marketed in the United States for a number of years, has performed well in populations with permethrin resistance. (14) In Britain and Australia, malathion resistance has been reported, and lice resistant to both pyrethrin and malathion have been documented in Britain. (15)

Mechanisms of resistance include knock-down resistance (relative nerve insensitivity), glutathione-S-transferase-based resistance (a cause of DDT and pyrethroid resistance), and oxygenase-based resistance that may be overcome by synergistic agents such as piperonyl butoxide. (16)

It is important to note that many treatment failures are really reinfestation from an untreated classmate, rather than a manifestation of drug resistance. Other causes of treatment failure include improper dilution and improper duration of application. The relative lack of ovicidal effect must be taken into account, and nit removal is recommended whenever possible. Lice close their respiratory spiracles when exposed to water, (17) and new agents should focus on mechanisms to improve penetration of the spiracular plate.

Pyrethrins synergized with piperonyl butoxide have long been available over the counter in the United States. Pyrethrins are natural compounds, derived from chrysanthemums. The fact that they are a "natural flower extract" may appeal to some patients, however, individuals allergic to Compositae plants must avoid these products. Several different formulations are available, including RID Mousse, RID Shampoo, A-200, R and C, Pronto, Clear Lice System. A-200 shampoo contains benzyl alcohol, which is synergistic with the pyrethrin. Resistance to these agents is a concern.

Permethrin 1% (Nix[R]) was first marketed as a prescription product, but is now widely used as an over-the-counter treatment for head lice. Resistance has been increasing in many geographic areas. As with the pyrethrins, the mechanism of action for permethrin is interference with sodium transport in the arthropod with subsequent depolarization of neuromembranes and respiratory paralysis. Although resistance is a growing problem, these agents have a favorable safety profile and are still widely used.

Malathion 0.5% (Ovide[R] is a relatively weak organophosphate cholinesterase inhibitor that works by causing respiratory paralysis in the arthropod. The vehicle contributes significantly to the pediculicidal effect. Unfortunately, the vehicle is also flammable. Currently, resistance to this agent has not been reported in the United States.

Lindane 1% (formerly Kwell[R] is a second line agent because of potential for toxicity. Still, it remains on the market as an alternative when other treatments have failed. It is an organochloride that kills lice by causing respiratory paralysis. The actual neurotoxic potential is quite low when used according to the manufacturer's instructions. (1,2) It does accumulate in the body with repeated exposure, and repeated use should be discouraged. It may also not be a good choice for very young children, or those with an impaired cutaneous barrier or a history of seizures.

Oral trimethoprim-sulfamethoxazole (TMP/SMX) has been shown to be effective in small studies of off-label use, and may be suitable for use in combination with topical agents. (20,21)

Long available in Britain, carbaryl-containing pediculicides have fallen out of favor, based in part on evidence that they may be carcinogenic. (22) There is also evidence of resistance to carbaryl. (23)

Ivermectin is an antihelminthic drug, but has also been suggested as a potential off-label therapy for pediculosis. (24,25) Topical ivermectin holds some promise, and oral ivermectin has generally been given as a single 200-mcg per kg dose. The drug is available in 3- and 6-mg forms. A second dose is generally given in a week to 10 days to kill emerging nymphs. The possibility of neurotoxicity has been a concern, including a report of deaths after the use of ivermectin in a nursing home. Some of these patients had previously been treated with other potentially neurotoxic agents, and there was a simultaneous unexplained decrease in the death rate on other wards. These facts have caused some to question the association, but as the drug appears to have some potential for neurotoxicity, and the safety and efficacy of the product for the treatment of pediculosis remains to be established, it should be used only when the benefits clearly outweigh risks.

Although most patients present with only a single infesting organism, among the homeless and in refugee populations, individuals may be infested by lice, scabies mites, fleas, and intestinal parasites. (26) The presence of other parasites may favor the use of an oral agent with activity against multiple pathogens. Ivermectin may be an appropriate choice in some such settings.

Shaving of all infested hair is effective and is still practiced in many parts of the world. Shaving is not likely to be accepted by most patients in western countries, and chemical pediculicides remain the mainstay of therapy. (27) Wetcombing (bug busting) was popularized in the United Kingdom and has been shown to have some degree of efficacy where resistance is common. Unfortunately, wetcombing was only half as effective as malathion in eradicating head lice in the United Kingdom. (28) A Belgian study also demonstrated a low cure rate. (29) Combing remains an important adjunctive measure, and application of diluted vinegar, a commercial preparation of 8% formic acid, or an enzymatic cream rinse can aid in the removal of nits. (30)

"No nit" policies that prohibit children from returning to school are controversial. Generally, they do more harm than good, as many children with non-viable nits are excluded from school while asymptomatic children with active infestation remain in the classroom. The presence of nits alone should not be the basis for excluding children from school. (35,1) Rather, routine nit inspections should be performed and affected children treated appropriately.

Alternative and "natural" topical treatments abound and are easily found on the Internet. Your patients will find them and ask about them. Although I do not advocate that you use them, it is best to have some working knowledge about them. Although various lipids may have an inhibitory effect on the organism, there is no truth to the rumor that leaving the hair unwashed causes the lice to slip and fall to their deaths. Kerosene and mayonnaise have both been advocated as being pediculicidal. Kerosene is commonly used in many parts of the world, but presents a very real risk of injury and should be avoided. Mayonnaise, on the other hand, may smell a bit odd, but is less likely to cause harm.

Chick-Chack is another "natural" remedy that contains coconut oil, anise oil, and ylang ylang oil. Dermatologists may recognize ylang ylang oil as one of the ingredients used for patch testing for botanical and fragrance allergy. Although the published data regarding this agent are sparse, there is one study of 119 children treated either with Chick-Chack or a spray containing permethrin, malathion, piperonyl butoxide, isododecane, and propellant. Treatment was successful in over 90% of children in both treatment groups. (32) Although no serious side effects were reported in this study, caution is advised until more data are available. Aliphatic alcohols also deserve evaluation for use in resistant lice. (33)

Crotamiton (Eurax[R]) is a relatively poor anti-scabetic agent that has some efficacy as an antipruritic. In one small study, it eradicated head lice in 47 of 49 patients. (34)

In general, louse treatments are marketed for head lice and not labeled for other use. Body lice can be eradicated by proper hygiene and laundering of clothing. Unfortunately, those who acquire body lice are generally not in a position to launder clothing. A pediculicide applied to the clothing can be of benefit, although none are labeled for use for body lice. They may also be helpful to treat any lice adherent to body hairs and to treat concurrent infestation with head lice, public lice or scabies. Permethrin treated clothing may help to prevent infestation. (35)

Pubic lice are often susceptible to agents used for head lice, such as malathion, permethrin, pyrethrins, and even lindane. (36) Some in vitro data suggest that crotamiton can be pediculicidal against Pthirus pubis. (37) Resistance to pyrethrins has been documented among pubic lice. In some cases, 5% permethrin will be effective. (38) Eyelash infestation can be treated with an occlusive agent such as Vaseline. (39) Fluorescein dye strips have also been used, but controlled studies are lacking. Yellow oxide of mercury ointment is best avoided because of the potential for systemic toxicity.

Dr. Elston has been a consultant to Merck Pharmaceuticals, manufacturer of ivermectin.

References

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Dirk M. Elston MD

Department of Dermatology, Geisinger Medical Center, Danville, PA

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