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Pediculosis and resistance: the perennial problem

      Does resistance really exist among human ectoparasites? Do treatments for pediculosis become less effective over the years? Do the mites or lice actually develop resistance to the insecticides created by humans to do away with these parasitic arthropods?
      For the past three decades, we have carefully followed the literature and listened to discussions about resistance and human lice.
      • Kucirka S.A
      • Parish L.C
      • Witkowski J.A
      The story of lindane resistance and head lice.
      ,
      • Parish L.C
      • Witkowski J.A
      • Kucirka S.A
      Lindane resistance and pediculosis capitis.
      Periodically, the question of resistance surfaces, but it is never satisfactorily answered nor is it sufficiently dismissed.
      • Brainerd E
      From eradication to resistance five continuing concerns about pediculosis.
      ,
      • Clore E.R
      Lice an ancient pest with new resistance.
      ,
      • Goldsmid J.M
      Head louse treatment is there an insecticide resistance problem? Letter.
      ,
      • Helm K.F
      • Lane A.T
      • McPhilmy J
      Pseudoresistance to pediculicides in a case of pubic lice.
      ,
      • Rasmussen J.E
      Pediculosis treatment and resistance.

      Definitions

      A major problem in the discussion involves the definitions. While denotations should be appropriate to set apart the terms involved, many times, connotations of selected words become intermingled with the denotations.
      • Kucirka S.A
      • Parish L.C
      • Witkowski J.A
      The story of lindane resistance and head lice.
      ,
      • Parish L.C
      • Witkowski J.A
      • Kucirka S.A
      Lindane resistance and pediculosis capitis.
      ,
      • Brainerd E
      From eradication to resistance five continuing concerns about pediculosis.
      ,
      • Clore E.R
      Lice an ancient pest with new resistance.
      ,
      • Goldsmid J.M
      Head louse treatment is there an insecticide resistance problem? Letter.
      ,
      • Helm K.F
      • Lane A.T
      • McPhilmy J
      Pseudoresistance to pediculicides in a case of pubic lice.
      ,
      • Rasmussen J.E
      Pediculosis treatment and resistance.
      ,
      • Witkowski J.A
      • Parish L.C
      The usual and not so usual forms of pediculosis.
      ,
      • Meinking T.L
      • Taplin D
      Advances in pediculosis, scabies and other mite infestations.
      ,
      • Burgess I.F
      • Brown C.M
      • Burgess N.A
      Synergized pyrethrin mousse, a new approach to head lice iradication efficacy in field and laboratory studies.
      ,
      • Aydemir E.H
      • Unal G
      • Kutlar M
      • et al.
      Pediculosis capitis in Istanbul.
      ,
      • Sinniah B
      • Sinniah D
      Resistance of head louse (Pediculus humanus capitis de Geer) to DDT in Malaysia.
      ,
      • Maunder J.W
      Resistance to orgenochlorine insecticides in head lice and trials using alternative compounds.
      ,
      • Blommers L
      • van Lennep M
      Head lice in the Netherlands susceptibility to insecticides in field samples.
      ,
      • Taplin D
      • Meinking T.L
      • Castillero P.M
      • et al.
      Permethrin 1% cream rinse (NIX) for treatmentof Pediculus humanus var capitis infestation.
      ,
      • Brandenburg K
      • Deinard A.S
      • DiNapoli J
      • et al.
      1% Permethrin cream rinse vs 1% lindane shampoo in treating pediculosis capitis.
      ,
      • Bowerman J.G
      • Gomez M.P
      • Austin R.D
      • et al.
      Comparative study of permithrin 1% crème rinse and lindane shampoo for treatment of head lice.
      ,
      • Cole M.M
      • Clark P.H
      Development of resistance to synergized pyrethrins in body lice and cross resistance to DDT.
      ,
      • Blommers L
      Insecticidal tests on immature head lice, Pediculus capitis (Anoplura) a new technique.
      ,
      • Vander Stichele R.H
      • Dezeure E.M
      • Bogaert M.G
      Systematic review of clinical efficacy of topical treatments for head lice.
      ,
      • Chosidow O
      • Chastang C
      • Brue C
      • et al.
      Controlled study of malathion and Math Eq-phenothrin lotions for Pediculus humanus var capitis-infested schoolchildren.
      ,
      • Rupes V
      • Moravec J
      • Chmela J
      • et al.
      Resistance of head lice (Pediculus capitis) to permethrin in Czech Republic.
      ,
      • Mumcuoglu K.Y
      • Hemingway J
      • Miller J
      • et al.
      Permethrin resistance in the head louse Pediculus capitis from Israel.
      ,
      • Burgess I.F
      • Peock S
      • Brown C.M
      • et al.
      Head lice resistant to pyrethroid insecticides in Britain.
      ,
      • Picollo M.I
      • Vassena C.V
      • Casadio A.A
      • et al.
      Laboratory studies of susceptibility and resistance to insecticides in Pediculus capitis (Anoplura Pediculidae).
      ,
      • Silverton N
      Malathion resistant pediculosis capitis.
      ,
      • Izri M.A
      • Briere C
      Premiers cas de resistance de Pediculus capitis Linn 1758 au malathion en France.
      ,
      • Burgess I.F
      Human lice and their management.
      ,
      • Witkowski J
      • Parish L.C
      What’s new in the management of lice.
      ,
      • Meinking T.L
      Infestations.

      Cure

      absence of live lice and/or viable nits when evaluated at 14 days.

      Insecticide or pediculicide

      a chemical that kills the insect. While the mechanism of action is often not known, respiratory paralysis is usually responsible for death.

      Louse

      the adult form of Pediculus capitis, P. corporis, and Pthirus pubis, the three members of the Anoplura order that infest humans.
      • Witkowski J.A
      • Parish L.C
      The usual and not so usual forms of pediculosis.

      Nit

      the encapsulated egg of a louse. (Some observers consider only the empty shell as a nit.) The viable nit is a small, whitish to semitranslucent object attached to the hair. It fluoresces under Wood’s light. The dead nit or hatched nit, which is also attached to the hair, is opaque and gray. Newly laid eggs take 9–12 days to hatch.

      Nit-free

      the patient must not have even one nit attached to a hair on the scalp whether or not the nit is dead. The nodule should be differentiated from hair casts (muffs), debris, and hair shaft abnormalities.

      Npa

      National Pediculosis Association founded in 1983 as Mothers Against Lice. An organization that fosters a nit-free environment.

      Nymph

      a developing louse. There are three instars of development, each of which is completed in 3 days. The maximum lifespan from hatching to death is approximately 30 days.
      • Witkowski J.A
      • Parish L.C
      The usual and not so usual forms of pediculosis.

      Ovicidal

      absence of nymphs at 7–10 days: can also be determined by in vitro studies.

      Ovicidal failure

      presence of first and second instar nymphs 1 week after treatment.

      Recurrence

      presence of newly hatched lice (first and second instar nymphs) 7–10 days after treatment.
      • Burgess I.F
      • Brown C.M
      • Burgess N.A
      Synergized pyrethrin mousse, a new approach to head lice iradication efficacy in field and laboratory studies.

      Reinfestation

      presence of live lice in all stages of development and viable nits in children previously considered cured.
      • Meinking T.L
      • Taplin D
      Advances in pediculosis, scabies and other mite infestations.

      Residual effect

      retention of pediculocide on the hair and scalp after washing. It kills emerging nymphs and prevents reinfestation for a limited time.

      Resistance

      abnormal strains of the same species that acquire immunity to a given insecticide through selective breeding. This immunity is acquired through exposure to an insecticide, which kills susceptible insects and leaves the hardier survivors to reproduce.
      • Aydemir E.H
      • Unal G
      • Kutlar M
      • et al.
      Pediculosis capitis in Istanbul.
      Presence of live lice 24 hours after treatment.
      • Burgess I.F
      • Brown C.M
      • Burgess N.A
      Synergized pyrethrin mousse, a new approach to head lice iradication efficacy in field and laboratory studies.

      Treatment failure

      the insecticide was applied in the appropriate manner, but live lice and viable nits are present 24 hours after treatment.

      Treatment

      Effective treatment
      • Witkowski J
      • Parish L.C
      What’s new in the management of lice.
      can be achieved if the pediculocide is applied carefully and of sufficient quantity on two occasions 1 week apart. A nit comb should be used during that interval, and the patient should be checked for surviving lice and nymphs between treatments and for the following week.
      • Burgess I.F
      Human lice and their management.

      Pediculicides

      Ddt

      an organochlorine compound. It has been used as a dusting powder and lotion.

      Lindane

      an organochlorine compound used as a 1% shampoo or cream. Lindane has low to medium ovicidal activity and no residual effect.
      • Meinking T.L
      Infestations.
      The shampoo is applied undiluted to dry hair. After 5 minutes water is added, creating a lather, and the hair is rinsed. A second application is not recommended because of possible neurological damage
      • Taplin D
      • Meinking T
      Infestations.
      that may be caused by excessive exposure to the medication. Blood levels increase with repeated use and accumulate in the body.
      • Ginsberg C.M
      • Lowry W
      Absorption of gamma benzene hexachloride following application of Kwell shampoo.
      The FDA recommended labeling the product that encourages the use of lindane only for patients who have either failed to respond to adequate doses or are intolerant of other approved therapies.

      Pyrethrin

      a derivative of the Chrysanthemum cinerariaefolium (pyrethrum) flower heads. It is usually combined (synergized) with piperonyl butoxide (Butacide). The latter compound has insecticidal activity and antioxidant properties, which prevent or decrease the development of resistance. It prevents oxidative breakdown of pyrethrins by the insect’s detoxification system.
      • Rasmussen J.E
      Pediculosis treatment and resistance.
      The combination is available as a gel, lotion, and shampoo. Commercial products in the USA include RID shampoo: R&C shampoo; A200 pyrinate shampoo, liquid and gel; Triple-X; Para-nit; Pronto; Paratrol; Clear Lice System; Basc, and NoLice. Synergized pyrethrins have low to medium ovicidal activity and no residual effect.
      • Meinking T.L
      Infestations.
      These products are applied to dry hair for 10 minutes and then rinsed off with water. Patients with allergy to ragweed or turpentine should avoid their use.
      • Elston D.M
      What’s eating you? Pediculus humanus (head louse and body louse).
      A second application is recommended at 7–10 days. Nit removal is necessary.

      Pyrethroid

      a synthetic pyrethrin. It is available as 1% permethrin creme rinse (NIX) and as 5% permethrin cream (Elimite). Pyrethroids have high ovicidal activity and residual activity for 2-3 weeks.
      • Meinking T.L
      Infestations.
      Permethrin is applied to dry hair after shampooing with an anionic shampoo without a conditioner (Prell concentrate). Dry the hair thoroughly with a towel or hair dryer. Then saturate the hair with permethrin creme rinse and massage into scalp, especially behind the ears and the base of the neck. Rinse off after 10-20 minutes. Do not use a regular shampoo again for 24 hours. Use a nit comb and repeat after 7 days.
      • Meinking T.L
      Infestations.

      Malathion

      an organophosphate compound, an irreversible cholinesterase inhibitor. It is used as a 0.5% lotion (Ovide lotion). Malathion has moderate to high ovicidal activity and some residual activity.
      • Meinking T.L
      Infestations.
      This is applied to dry hair in an amount to thoroughly wet the hair and scalp. Wash hands after applying. Allow hair to air dry, and do not use a towel or hair dryer. A regular shampoo can be used after 8-12 hours. Use a nit comb and repeat application after 7–10 days.
      • Meinking T.L
      Infestations.
      Malathion has an objectional odor due to release of sulfhydryl compounds through hydrolysis.
      • Rasmussen J.E
      Pediculosis treatment and resistance.

      Nit removal

      complete nit removal plays an essential role in the management of infestation. Because no pediculicide is 100% ovicidal, grooming and nit removal in the period between treatments are essential. Combs that are sold with the pediculicides are often made of plastic and are not as good as a metal comb. The teeth of a plastic comb often bend and break, especially when used on coarse, thick hair. A metal comb, LiceMeister, can be purchased through the NPA or through the Internet. Applying a conditioner, a creme rinse, a detangler, or a light oil can facilitate combing the hair. The cement protein that binds the nit to the hair can be loosened by wetting the hair with water or by application of a mixture of 50% distilled white vinegar and 50% water or by use of a product called Step 2, which contains 8% formic acid. The latter product is sold with a metal nit comb.
      • DeFelice J
      • Rumsfield J
      • Bemstein J.E
      • et al.
      Clinical evaluation of an after-pediculicide nit removal system.

      Education

      Upon discovery of an infestation, parents should notify the school and the school in turn should notify the other parents. Teachers, parents, and the school nurse should be instructed in screening techniques, about the available parasitocides, methods of controlling fomites, and how to prevent reinfestation.

      Fomite and environmental control

      Sharing combs, brushes, washcloths, towels, and headgear should be avoided. While at school, clothing should be stored under the desk or seat
      • Elston D.M
      What’s eating you? Pediculus humanus (head louse and body louse).
      or in a large backpack or plastic bag and not on closely set pegs or shelves. Combs and brushes may be coated with the parasitocide for 15 minutes and then washed in hot water. Towels, wash cloths, headgear, clothes, and bed linens should be washed in hot water and dried on the hot cycle. Nonwashable items should be dry cleaned or sealed in a plastic bag and stored in a warm place for 2 weeks.
      • Meinking T.L
      Infestations.
      Rugs, upholstered furniture, mattresses, and car seats should be vacuumed to remove lice and hair with attached nits that might have been shed.

      Contact control

      All family members and close contacts over the preceding month should be examined for evidence of infestation.
      • Taplin D
      • Meinking T
      Infestations.
      Screening with a nit comb is more effective than when done by hand.
      • Meinking T.L
      Infestations.
      It is often difficult to detect an early infestation because these patients are usually asymptomatic, and lice and nits are hard to find. The majority of infestations go undetected for several weeks after they begin. When in doubt, it is better to treat all close contacts that may be carriers than to treat the same child repeatedly.
      • Meinking T.L
      Infestations.

      Ancillary measures

      Treatment of the associated skin irritation and infection with topical steroids and systemic antibiotics is an important part of management.

      Reasons for treatment failure

      Most treatment failure can be attributed to causes other than resistance.
      • Burgess I.F
      Human lice and their management.
      These include poor compliance or improper use of the antilice product, lack of nit removal, new exposure to lice, and use of an out-of-date preparation. Applying the insecticide to wet hair reduces its activity. When immersed in water, lice grasp hairs reflexly and close their spiracles, or breathing apparatus, thereby reducing penetration of the pediculicide into the louse.
      • Burgess I.F
      • Brown C.M
      • Burgess N.A
      Synergized pyrethrin mousse, a new approach to head lice iradication efficacy in field and laboratory studies.
      Shampoos, creme rinses, and lotions should therefore by applied to dry hair for most effective use. Applying the antilice medication to hair that has been wet with water also dilutes the insecticide, decreasing its concentration. Because of the short contact time and the dilution factor that occurs when water is added to a shampoo preparation, insufficient amount of concentrated insecticide is present for too short a time to kill lice and viable nits, and in many instances, both survive.
      • Burgess I.F
      Human lice and their management.
      For this reason, although shampoos are convenient, they are considered less ovicidal
      • Sexton C
      • Miller A.J
      A comparison of a single occasion treatment of head louse infestation with phenothrin liquid shampoo or a carbaryl lotion.
      and have been blamed for fostering the survival of the head louse.
      • Burgess I.F
      • Brown C.M
      • Burgess N.A
      Synergized pyrethrin mousse, a new approach to head lice iradication efficacy in field and laboratory studies.
      Using an insufficient amount of the pediculicide can also result in treatment failure. This is especially common when using lotion preparations. Thick and long hair requires more pediculicide to completely wet the hair. A sufficient amount of lotion should be used to thoroughly wet all of the hair, especially behind the ears and at the nape of the neck. Often, these sites of high prevalence are never exposed to the insecticide. Avoid using a shampoo with a built-in conditioner before applying a pediculicide. The conditioner in the shampoo can coat the hair, interfering with bonding of the pediculicide to the hair and scalp.
      • Meinking T.L
      Infestations.
      When bonding is prevented, the residual effect of the antilouse preparation is diminished, allowing emerging nymphs to survive and also permitting reinfestation to occur. Because there are no pediculicides that can kill all viable nits, lack of nit removal and retreatment at 7 days will result in treatment failure. Reinfestation is always possible but depends on the presence of untreated close contacts who may be carriers and the duration of the residual effect on the treated hair. Finally, pyrethrins are unstable when exposed to heat and light: therefore, old products may be ineffective.

      Resistance—scope of the problem

      Shortly after the introduction of DDT in the 1940s, resistance to body lice appeared in Korea at the onset of the conflict in 1950. By the mid 1950s, at least 37 countries had reported DDT resistance of P. corporis.
      • Kucirka S.A
      • Parish L.C
      • Witkowski J.A
      The story of lindane resistance and head lice.
      Subsequently, reports appeared in the 1970s and 1980s, suggesting resistance of both head lice and pubic lice.
      • Kucirka S.A
      • Parish L.C
      • Witkowski J.A
      The story of lindane resistance and head lice.
      ,
      • Clore E.R
      Lice an ancient pest with new resistance.
      ,
      • Goldsmid J.M
      Head louse treatment is there an insecticide resistance problem? Letter.
      ,
      • Aydemir E.H
      • Unal G
      • Kutlar M
      • et al.
      Pediculosis capitis in Istanbul.
      ,
      • Sinniah B
      • Sinniah D
      Resistance of head louse (Pediculus humanus capitis de Geer) to DDT in Malaysia.
      Even pseudoresistance was reported.
      • Helm K.F
      • Lane A.T
      • McPhilmy J
      Pseudoresistance to pediculicides in a case of pubic lice.
      Within the past few years, there has been a resurgence of interest in the subject because of treatment failure after use of currently available pediculicides. Resistance to lindane in head lice was reported in the United Kingdom in 1971
      • Maunder J.W
      Resistance to orgenochlorine insecticides in head lice and trials using alternative compounds.
      and the Netherlands in 1978.
      • Blommers L
      • van Lennep M
      Head lice in the Netherlands susceptibility to insecticides in field samples.
      Treatment failure was reported in Panama in 1986,
      • Taplin D
      • Meinking T.L
      • Castillero P.M
      • et al.
      Permethrin 1% cream rinse (NIX) for treatmentof Pediculus humanus var capitis infestation.
      in Arizona in 1986,
      • Brandenburg K
      • Deinard A.S
      • DiNapoli J
      • et al.
      1% Permethrin cream rinse vs 1% lindane shampoo in treating pediculosis capitis.
      and in Mexico in 1987.
      • Bowerman J.G
      • Gomez M.P
      • Austin R.D
      • et al.
      Comparative study of permithrin 1% crème rinse and lindane shampoo for treatment of head lice.
      Resistance of body and head lice to synergized pyrethrins was reported in 1961
      • Cole M.M
      • Clark P.H
      Development of resistance to synergized pyrethrins in body lice and cross resistance to DDT.
      and of head lice in the Netherlands in 1979.
      • Blommers L
      Insecticidal tests on immature head lice, Pediculus capitis (Anoplura) a new technique.
      A study reported in 1995 concluded that the natural pyrethrins are not sufficiently effective to justify their use.
      • Vander Stichele R.H
      • Dezeure E.M
      • Bogaert M.G
      Systematic review of clinical efficacy of topical treatments for head lice.
      Resistance to pyrethroids was first reported in Tasmania in 1990.
      • Goldsmid J.M
      Head louse treatment is there an insecticide resistance problem? Letter.
      Since then, resistance has been reported in France in 1994,
      • Chosidow O
      • Chastang C
      • Brue C
      • et al.
      Controlled study of malathion and Math Eq-phenothrin lotions for Pediculus humanus var capitis-infested schoolchildren.
      the Czech Republic in 1995,
      • Rupes V
      • Moravec J
      • Chmela J
      • et al.
      Resistance of head lice (Pediculus capitis) to permethrin in Czech Republic.
      Israel in 1995,
      • Mumcuoglu K.Y
      • Hemingway J
      • Miller J
      • et al.
      Permethrin resistance in the head louse Pediculus capitis from Israel.
      Britain in 1995,
      • Burgess I.F
      • Peock S
      • Brown C.M
      • et al.
      Head lice resistant to pyrethroid insecticides in Britain.
      and Argentina in 1998.
      • Picollo M.I
      • Vassena C.V
      • Casadio A.A
      • et al.
      Laboratory studies of susceptibility and resistance to insecticides in Pediculus capitis (Anoplura Pediculidae).
      Some studies showed cross-resistance to other members of the pyrethroid family of drugs.
      • Picollo M.I
      • Vassena C.V
      • Casadio A.A
      • et al.
      Laboratory studies of susceptibility and resistance to insecticides in Pediculus capitis (Anoplura Pediculidae).
      Clinical evidence of resistance to malathion appeared in England in 1972,
      • Silverton N
      Malathion resistant pediculosis capitis.
      Tasmania in 1990,
      • Goldsmid J.M
      Head louse treatment is there an insecticide resistance problem? Letter.
      in France in 1995,
      • Izri M.A
      • Briere C
      Premiers cas de resistance de Pediculus capitis Linn 1758 au malathion en France.
      and again in southern England in 1995.
      • Burgess I.F
      Human lice and their management.
      Clinical treatment failures in many instances were correlated with results of laboratory tests on pediculicides. There was also a relationship between previous extensive use of the pediculicide and treatment failure.
      • Taplin D
      • Meinking T.L
      • Castillero P.M
      • et al.
      Permethrin 1% cream rinse (NIX) for treatmentof Pediculus humanus var capitis infestation.

      Reasons for louse resistance

      Resistance both clinical (treatment failure) and in the laboratory has been reported from many parts of the world to all commonly used pediculicides. This is not surprising, in view of the adaptability of insects.
      • Chosidow O
      • Chastang C
      • Brue C
      • et al.
      Controlled study of malathion and Math Eq-phenothrin lotions for Pediculus humanus var capitis-infested schoolchildren.
      The evolution of genetically selected tolerance or resistance when an insect is repeatedly exposed to nonkilling (sublethal) concentrations of a pediculicide is inevitable
      • Stallbaumer M
      • Ibarra J
      Clinical efficacy of treatment for head lice.
      and will result in the emergence of a population of insects that are immune to the product.
      • Chosidow O
      • Chastang C
      • Brue C
      • et al.
      Controlled study of malathion and Math Eq-phenothrin lotions for Pediculus humanus var capitis-infested schoolchildren.
      This can be accomplished, theoretically, by use of products with residual activity.
      • Vander Stichele R.H
      • Dezeure E.M
      • Bogaert M.G
      Systematic review of clinical efficacy of topical treatments for head lice.
      While residual activity may supplement a poorly ovicidal product and temporarily help prevent infestation, as the level of pesticide wanes, sublethal concentrations prevail on the hair and scalp.
      • Vander Stichele R.H
      • Dezeure E.M
      • Bogaert M.G
      Systematic review of clinical efficacy of topical treatments for head lice.
      If lice are present, they are usually nymphs, unless there had been a reinfestation. Resistance develops more easily when nymphs rather than adults are exposed, possibly because nymphs are exposed for a longer time.
      • Kucirka S.A
      • Parish L.C
      • Witkowski J.A
      The story of lindane resistance and head lice.
      Exposing lice to nonkilling concentrations can also occur because of use of inadequate quantities of the pediculicide, by dilution of the product when applied to wet hair, or by using a shampoo formulation.
      • Meinking T.L
      Infestations.
      Shampoos often result in dilution and short contact time. Resistance is always due to partial treatment failure.
      • Maunder J.W
      Strategic aspects of insecticide resistance in head lice.
      Clinical and laboratory observations have demonstrated that repeated and prolonged use of the pediculicide on a louse population is likely to select out resistant lice.
      • Chosidow O
      • Chastang C
      • Brue C
      • et al.
      Controlled study of malathion and Math Eq-phenothrin lotions for Pediculus humanus var capitis-infested schoolchildren.
      Previous extensive use of lindane-containing products is related to lindane resistance.
      • Meinking T.L
      Infestations.
      Exclusive use of pyrethroids in Israel resulted in a lack of efficacy of permethrin.
      • Mumcuoglu K.Y
      • Hemingway J
      • Miller J
      • et al.
      Permethrin resistance in the head louse Pediculus capitis from Israel.
      Head lice in the United States are less susceptible to permethrin than are lice in an area where the product was never used.
      • Pollack R.J
      • Kiszewski A
      • Armstrong P
      • et al.
      Differential permethrin susceptibility of head lice sampled in the United States and Bomeo.
      Cross-resistance to other insecticides with a similar chemical structure or mode of action may also occur.
      • Meinking T.L
      Infestations.
      Some believe that the previous use of DDT in addition to a pyrethroid with minimal residual activity and malathion set the stage for the rapid development of resistance to permethrin in Israel.
      • Mumcuoglu K.Y
      • Hemingway J
      • Miller J
      • et al.
      Permethrin resistance in the head louse Pediculus capitis from Israel.
      DDT, lindane, and pyrethroids affect the sodium channels in nerve cell membranes, thus affecting repolarization, while malathion affects acetylcholinesterase in nerve synapses. Detoxification enzymes that prevent the insecticide from reaching the site of action cause resistance of some lice. Resistance to most insecticides is produced by the oxidase mechanism (mixed function oxidase), while the esterase mechanism is often associated with resistance to malathion. Lice therefore can become resistant by one or more mechanisms.
      • Meinking T.L
      Infestations.
      ,
      • Brogdon W.G
      • McAllister J.C
      Insecticide resistance and vector control.

      Strategies for control of suspected louse resistance

      During the past few years, anecdotal reports have surfaced of suspected head lice resistance (treatment failures) in some communities in the United States. This resistance is to all commonly used pediculicides, including synergized pyrethrins, 1% permethrin, and even 5% permethrin cream. At the present time, this resistance to recommended drugs remains poorly defined
      • Pollack R.J
      • Kiszewski A
      • Armstrong P
      • et al.
      Differential permethrin susceptibility of head lice sampled in the United States and Bomeo.
      due to insufficient data.
      Drugs for head lice.
      Because exposure to a pediculicide may be different, reports of resistance in one community may not be valid in another.
      • Burgess I.F
      Clinical efficacy of treatment for head lice.
      Therefore, the relatively safe over-the-counter pediculicides remain the products of choice for newly recognized infestations.
      • Pollack R.J
      • Kiszewski A
      • Armstrong P
      • et al.
      Differential permethrin susceptibility of head lice sampled in the United States and Bomeo.
      Parents should use the standard preparation as initial treatment, carefully following the manufacturer’s instructions. If lice remain alive 24 hours after treatment, they are probably resistant and further treatment with the same or related agent will not be effective, and will only expose the child to more insecticide. The parent should then switch to a product that contains a different chemical and make every effort to follow the package instructions for use.
      Overkill, using the product according to instructions, is the best way to avoid development of the population of head lice that includes individual insects with varying gradations of resistance.
      • Taplin D
      • Castillero P.M
      • Spiegel J
      • et al.
      Malathion for treatment of Pediculus humanus var capitis infestation.
      When choosing this alternative product, it should be borne in mind that lotions remain undiluted after application and deliver a higher concentration of the pediculicide to the lice and viable nits for a longer period of time.
      • Doss S
      • Powell C.A
      • Miller A.J
      Phenothrin lotion, the latest recruit in the battle against head lice; the results of two controlled comparative studies.
      Furthermore, alcoholic vehicles penetrate the aeropyles (breathing pores on the opercelum) of nits and the louse spiracles better than aqueous vehicles.
      • Burgess I.F
      • Brown C.M
      • Burgess N.A
      Synergized pyrethrin mousse, a new approach to head lice iradication efficacy in field and laboratory studies.
      Finally, before deciding that the lice are resistant, reinfestation should be excluded.
      The best choice of pediculicide will depend on local resistance patterns:
      • 1.
        Permethrin cream 5% (Elimite)—apply to dry hair for 10 minutes
        • Elston D.M
        What’s eating you? Pediculus humanus (head louse and body louse).
        or leave on overnight with a shower cap.
        • Meinking T.L
        Infestations.
      • 2.
        Apply a pediculicide with a different chemical structure, such as lindane or malathion (resistance to malathion has not been reported in the United States in recent years).
      • 3.
        Physical modalities such as mechanical louse and nit removal.
      • 4.
        Apply an occlusive agent to asphyxiate the lice. There are several choices but no controlled studies. Vaseline petroleum jelly, hair pomade, olive oil, mayonnaise, Crisco vegetable shortening, mineral oil, or essential oils purchased from health food stores have been used. Vaseline petroleum jelly is applied overnight under a shower cap. This requires repeated overnight treatments followed by combing.
        • Meinking T.L
        Infestations.
        The Vaseline is difficult to wash out. To remove the Vaseline, apply “Goop,” a mechanic’s hand cleaner, all over the scalp. Rinse after 10 minutes. Next apply “Gojo,” an orange pumice hand cleaner, to the hair when dry. Leave on for a few minutes and then rinse off. This is followed by use of a regular shampoo.
        • Ewers J
        How I did it, conquered head lice.
      • 5.
        HairClean 1-2-3 consists of anise, ylang-ylang, coconut oils, and isopropyl alcohol. This product is available in health food stores. It is applied to dry hair and shampooed off after 15 minutes. The treatment is repeated 1 week later.
        • Meinking T.L
        Infestations.
      • 6.
        Cotrimoxozole (sulfamethoxozole-trimethoprim) (Septra, Bactrim). Antibiotic present in the blood meal kills the louse’s symbiotic bacteria necessary for survival. This medication is administered orally in a therapeutic dose for 3 days followed by a second course after 1 week.
        • Witkowski J
        • Parish L.C
        What’s new in the management of lice.
      • 7.
        Ivermectin, a macrocyclic lactone (Mectizan), is an antihelminthic agent. Ivermectin is given orally in a dose of 200 μg/kg. Some children will require a second dose on the next day.
        • Meinking T.L
        Infestations.
        Another recommendation is to give the second dose 7–10 days later, especially if nit removal is incomplete.
        • Elston D.M
        What’s eating you? Pediculus humanus (head louse and body louse).
        Ivermectin is not ovicidal.
        • Bell T.A
        Treatment of Pediculus humanus var. capitis infestation in Cowlitz County, Washington, with ivermectin and the LiceMeister comb.
      • 8.
        Lice Guard Spray to prevent infestation and reinfestation. No studies have been done to prove its effectiveness.

      Conclusions

      Useful terms related to the head louse and its treatment are defined. Successful treatment depends on using the pediculicide according to the manufacturer’s instructions in sufficient quantity to wet all the hair, use of a nit comb, followed by repeated observation. Reasons for treatment failure are discussed. Resistance to pediculicides is worldwide; possible reasons for this are discussed. Finally, strategies for management of suspected louse resistance are outlined.

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