Pediculosis

Meet the patient

A mother brings her eight-year-old daughter in with an itchy scalp of four weeks' duration, worst at the back of her head and behind her ears. On parting the hair you find tiny grey-white specks glued fast to the hair shafts close to the scalp — they do not slide when you pinch and pull the hair. The school has had several cases this term.[1][3]

Three questions now frame the whole topic: which louse is this? (the scalp site and the cemented nit say head louse), is there a vector-borne disease risk? (head lice carry none — only body lice do), and what is the treatment principle? (two applications 7 days apart, plus treat every household contact at once). The nit that will not slide is the diagnosis.[1][9]

Three lice, three sites, three routes

The single most useful concept is that the three lice are distinguished by site and route — scalp hair is capitis, the clothing seam is corporis, coarse body hair is pubis — and only the body louse transmits systemic disease. Site plus habitat equals species.[1][8]

[1] [8] [9]

The head and body louse are ecological variants of one species — a head louse moved onto clothing becomes a body louse within generations — but they are kept apart in life by hair-shaft diameter, microclimate and feeding frequency. The pubic louse is the short, broad, crab-like insect with large claws on its second and third legs, adapted to grip coarse body hair rather than fine scalp hair.[1][6]

The body louse is the vector — three killer diseases

This is the fact examiners test most: only the body louse transmits systemic bacterial disease, and it carries three historically devastating infections. Head and pubic lice do not.[1][11]

[1] [8] [11]

The preventable-death rule: a febrile homeless or displaced patient with body lice gets empirical doxycycline 100 mg BD for 5 to 7 days before the serology returns — typhus, relapsing fever and trench fever are all doxycycline-responsive, and waiting for confirmation costs lives. For relapsing fever, stand ready for the Jarisch-Herxheimer reaction (rigors, hypotension within 2 hours) with IV access and fluids at the bedside.[8][11]

The nit — "if it slides, it is not a nit"

The single bedside rule that wins the viva: a louse nit is cemented to the hair shaft by chitin glue and does not slide; dandruff, hair casts and lint slide freely. Pinch and pull the hair — if the speck moves, it is not a nit.[1][9]

The female cements each egg to a hair shaft or clothing fibre with a chitinous glue from her accessory gland, insoluble in water and most shampoos. Eggs hatch in 7 to 10 days; the three nymphal instars each take 4 to 7 days; the adult lives 30 to 40 days and feeds every 4 to 6 hours. Off-host survival is short — the head louse dies within 36 to 48 hours, the body louse survives 7 to 10 days in clothing.[1][3]

The viable-nit rule: hair grows about 0.3 to 0.4 mm per day, so a nit within 6 mm of the scalp was laid in the last 2 to 3 weeks and is likely viable, while a nit more than 1 cm out is an empty old shell. That single threshold tells an active infestation from a residual dead one.[3][4][9]

Etymology for viva gold: pediculus is Latin for "a little louse"; Phthirus comes from the Greek phtheir, "a louse", giving us phthiriasis. Maculae caeruleae — the blue-grey macules of pubic louse bites — are Latin for "blue spots", the haemosiderin signature of the crab louse's feeding.[1][6]

Detection combing and the itch

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Detection combing with a fine-toothed comb (0.2 to 0.3 mm) is the diagnostic gold standard, and visual inspection alone is inadequate. A single combing session detects about 65 to 75 percent of infestations; three sessions reach about 90 percent; a single visual inspection catches only about 30 percent.[1][3]

Comb conditioned or damp hair systematically from root to tip, parting the hair in 1 cm sections at the occiput and behind the ears where the louse prefers the warm, well-perfused skin. Wood's lamp is an adjunct — live nits fluoresce pale blue, empty nits dull white. Dermoscopy (entomodermoscopy) at 10 to 30x confirms the species directly.[1][9]

The itch is not the bite — it is a delayed type I and type IV hypersensitivity to louse saliva, which is why a first infestation is silent for 4 to 6 weeks (sensitisation) and a re-infestation itches within 1 to 4 days (memory). Asymptomatic carriers — 5 to 20 percent of infested children — are the reservoir that keeps outbreaks alive, which is why you treat every contact, symptomatic or not.[1][9]

Resistance — why permethrin is no longer king

Permethrin resistance is the most important change in pediculosis in 30 years, and it is the reason dimeticone has displaced permethrin as first-line in many regions. The mechanism is kdr (knockdown-resistance) mutations in the louse voltage-gated sodium channel — T929I and L920F confer low-level resistance, and the super-kdr double mutation M827I plus T929I confers high-level resistance.[3][12]

In regions of high resistance, permethrin 1 percent now cures only 60 to 80 percent of cases — down from near-complete cure in the 1990s. Resistance to malathion (esterase gene amplification) and ivermectin (glutamate-gated chloride channel mutations) also occurs but is less common. The clinical answer is to switch class when an agent fails — permethrin failure to dimeticone, malathion or oral ivermectin — and to favour the physical, non-neurotoxic agents (dimeticone, benzyl alcohol) where resistance is biologically implausible.[12][13]

Management — the two-dose 7-day rule

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Four principles govern head-lice treatment, and each is examined: two applications 7 days apart, simultaneous treatment of all contacts, no school exclusion, and combing after each application. Miss any one and the infestation returns.[1][3]

Why two doses? No pediculicide is reliably 100 percent ovicidal, so eggs that survive the first application hatch over the next week. The second application at day 7 kills the hatchlings before they themselves lay eggs — omit it and you have re-infestation within a fortnight. Treat every household contact on the same day, symptomatic or not, because one untreated carrier re-infects everyone.[1][5]

The pediculicide ladder for head lice:[1][3]

• Dimeticone 4 percent lotion — first-line in resistance regions; a physical silicone that suffocates the louse by occluding its spiracles, so resistance is biologically implausible. Apply to dry hair, scalp to ends, leave 8 hours or overnight, repeat at day 7. Safe from 6 months and in pregnancy.
• Permethrin 1 percent cream rinse — the pyrethroid workhorse, opening the sodium channel to paralyse the louse; apply to washed, towel-dried hair for 10 minutes, repeat day 7. First-line only where resistance is low.
• Malathion 0.5 percent — an organophosphate that inhibits acetylcholinesterase; apply to dry hair for 12 hours, repeat day 7. Higher ovicidal activity but flammable, strong-smelling, and avoided under 6 months and in pregnancy.
• Benzyl alcohol 5 percent, spinosad 0.9 percent, isopropyl myristate 50 percent — physical or nicotinic-receptor alternatives; spinosad often needs only one application but is costly.
• Oral ivermectin 200 to 400 mcg/kg, repeated at day 7 to 10 — for refractory or resistant disease; avoid under 15 kg and in pregnancy.

[1] [3] [5] [1] [3] [13]

Body lice — treat the wardrobe, not just the patient

Body lice live in the clothing seams, not on the body, so environmental decontamination is the treatment and the patient's wash is almost incidental. The lice and nits are found in the seams of underclothing at the waist, collar, cuffs and axillae.[1][8]

Bathe the patient, then remove the infested clothing and bedding and either hot machine-wash at 60 degrees Celsius or above and tumble-dry, dry-clean, seal in a plastic bag for 14 days (off-host survival is 7 to 10 days), or freeze for 48 hours. Add topical permethrin 1 or 5 percent to the body in severe infestation, and address the social determinants — housing, displacement, poverty — because they are the root cause.[5][8]

In outbreaks among displaced populations, the WHO recommends permethrin 0.5 or 1 percent dusting powder or, more effectively, permethrin 2 percent impregnation of underwear and bedding, which cuts body-louse burden by over 90 percent within 4 to 6 weeks. Any febrile case gets empirical doxycycline before serology.[8][11]

Pubic lice — the STI, and the eyelash exception

Pubic lice are sexually transmitted in adults, so every adult case carries a 20 to 30 percent chance of a co-existing STI and warrants a full screen. Treat with permethrin 1 percent or malathion 0.5 percent to all affected coarse hair, repeated at day 7, and treat every sexual contact from the previous month simultaneously.[1][6]

The pathognomonic maculae caeruleae — blue-grey macules on the trunk and thighs from bite haemosiderin — clinch the diagnosis when present. Examine all coarse body hair, including the eyelashes (phthiriasis palpebrarum), present in 5 to 10 percent.[6][7]

Pubic lice in a child is a safeguarding flag. Sexual transmission is the usual route, though non-sexual spread via shared towels or bedding with an infested adult is documented — the safeguarding assessment is mandatory either way, and the local child-protection team should be involved.[6][7]

Traps, confessions and preventable harm

Everyone treats the patient and forgets the contacts. That is how head lice "come back" within a fortnight. Re-infestation from an untreated asymptomatic carrier — not resistance — is the commonest cause of apparent treatment failure. Treat every household member on the same day.[1][5]

Everyone gives a single application. No pediculicide is 100 percent ovicidal; the second dose at day 7 kills the hatchlings and is non-negotiable. A single application is the most common reason for "treatment failure".[1][3]

Consultant confession: the highest-yield question in a head-lice clinic is not "is the scalp itchy?" but "who else in the house has been scratching?". The asymptomatic sibling or parent is the reservoir, and treating only the index child guarantees a return visit.[1][9]

The preventable-harm list:[1]

• Treating the patient but not the contacts — re-infestation is the rule.
• A single pediculicide application — the day-7 second dose is non-negotiable.
• Continuing permethrin despite failure — switch to dimeticone, malathion or oral ivermectin for kdr resistance.
• Using pediculicide on the eyelashes — petrolatum only.
• Missing louse-borne disease in a febrile homeless patient with body lice — empirical doxycycline before serology.
• Missing safeguarding when pubic lice appear in a child.
• Excluding children from school under "no-nit" policies — not evidence-based; the child returns after one completed treatment.

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The mantra: two doses a week apart, treat every contact, treat the wardrobe for body lice, screen for STIs in pubic lice, and petrolatum for the eyelashes.[1]

Prognosis, special populations and disposition

Head lice are benign and self-limiting once treated; recurrence almost always means re-infestation from an untreated contact or unrecognised resistance. Body-louse prognosis is dominated by the louse-borne disease — untreated typhus kills 10 to 40 percent, relapsing fever up to 70 percent, both falling below 5 percent with early doxycycline. Pubic lice are cured easily; the prognostic determinant is the missed STI.[1][11]

In children, dimeticone 4 percent is first-line (safe from 6 months, no resistance); permethrin from 2 months; oral ivermectin reserved for refractory disease over 15 kg. In pregnancy, dimeticone is first-line, permethrin is acceptable, malathion and oral ivermectin are avoided. The homeless or displaced patient with body lice needs decontamination plus empirical doxycycline for any fever plus social-work input on housing.[3][5]

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The ward-round test

Click to reveal the stems — answer before you open each one

Stem 1. An eight-year-old has an itchy scalp and white specks on her hair shafts that will not slide when pinched. Name the diagnosis, the bedside sign, and the treatment principle.[1]

Answer: Head lice (pediculosis capitis). The sign is a nit cemented to the shaft that does not slide (chitin glue). Treat with dimeticone 4 percent, two applications 7 days apart, plus combing and simultaneous treatment of all household contacts.[1][3]

Stem 2. A homeless man with body lice is febrile and confused. What three diseases must you consider, and what do you start before the serology returns?[8][11]

Answer: Louse-borne typhus (Rickettsia prowazekii), louse-borne relapsing fever (Borrelia recurrentis) and trench fever (Bartonella quintana). Start empirical doxycycline 100 mg BD for 5 to 7 days before serology, and watch for Jarisch-Herxheimer in relapsing fever.[8][11]

Stem 3. A sexually active adult has itchy pubic hair and crab-like insects on the shafts. What is the diagnosis, and what two things must you do beyond treating the lice?[6]

Answer: Pubic lice (Phthirus pubis). Beyond topical permethrin 1 percent repeated at day 7, do a full STI screen (HIV, syphilis, hepatitis B, gonorrhoea, chlamydia — 20 to 30 percent co-infection) and treat all sexual contacts from the previous month simultaneously.[1][6]

Stem 4. A child has itchy red eyes with lice and nits at the lash bases. What is the diagnosis, and what is the one treatment you must NOT use?[7]

Answer: Phthiriasis palpebrarum (eyelash lice). Do NOT apply permethrin or any pediculicide near the eye — use petrolatum occlusion to the lash bases for 8 to 10 days. Exclude pubic lice elsewhere, screen for STIs in adults, and initiate safeguarding assessment in a child.[6][7]

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References

1. [1]Coates SJ, Thomas C, Chosidow O, et al. Ectoparasites: Pediculosis and tungiasis J Am Acad Dermatol, 2020.PMID 31306729
2. [2]Do-Pham G, Monsel G, Chosidow O. Lice Semin Cutan Med Surg, 2014.PMID 25577849
3. [3]Meister L, Ochsendorf F. Head Lice Dtsch Arztebl Int, 2016.PMID 27974145
4. [4]Burgess IF, Silverston P. Head lice BMJ Clin Evid, 2015.PMID 25587918
5. [5]Gunning K, Kiraly B, Pippitt K. Lice and Scabies: Treatment Update Am Fam Physician, 2019.PMID 31083883
6. [6]Creighton-Smith M, Sloan SB. Pediculosis Pubis JAMA Dermatol, 2019.PMID 31553414
7. [7]Gurnani B, Badri T, Hafsi W. Phthiriasis Palpebrarum 2026.PMID 29083779
8. [8]Powers J, Badri T, Syed HA. Pediculosis Corporis 2026.PMID 29489282
9. [9]Ko CJ, Elston DM. Pediculosis J Am Acad Dermatol, 2004.PMID 14699358
10. [10]Flinders DC, De Schweinitz P. Pediculosis and scabies Am Fam Physician, 2004.PMID 14765774
11. [11]Chosidow O. Scabies and pediculosis Lancet, 2000.PMID 10711939
12. [12]Meinking TL. Clinical update on resistance and treatment of Pediculosis capitis Am J Manag Care, 2004.PMID 15515630
13. [13]Lamassiaude N, Toubate B, Neveu C, et al. The molecular targets of ivermectin and lotilaner in the human louse Pediculus humanus humanus: New prospects for the treatment of pediculosis PLoS Pathog, 2021.PMID 33600484