Pediculosis — Viva

Q1: Definition and the three human lice (2 min)

What is pediculosis? Name the three species of sucking lice that infest humans, and for each give the site, the route of transmission, and whether it transmits systemic bacterial disease to humans.

Model answer: Pediculosis is infestation by obligate blood-feeding sucking lice of the suborder Anoplura. The three species are Pediculus humanus capitis (head louse, on scalp hair, transmitted by direct head-to-head contact, NOT a vector of systemic disease), Pediculus humanus humanus (body louse, lives in clothing seams, transmitted by shared infested clothing/bedding, IS the vector of three louse-borne bacterial diseases: epidemic typhus Rickettsia prowazekii, louse-borne relapsing fever Borrelia recurrentis, trench fever Bartonella quintana), and Phthirus pubis (pubic/crab louse, on coarse body hair, transmitted sexually, NOT a vector).

^[4]

Q2: How do you distinguish nits from dandruff and hair casts, and how do you confirm a viable infestation? (3 min)

Model answer: Nits are louse eggs firmly cemented to the hair shaft by chitinous glue from the female's accessory gland; they do NOT slide when the hair is pinched and pulled. Dandruff (seborrhoeic dermatitis) and hair casts (keratin cylinders, pseudo-nits) slide freely along the hair. The diagnosis of active infestation requires either a live louse or a viable nit within 6 mm of the scalp (nits > 1 cm from the scalp are almost always old empty shells; scalp hair grows ~ 0.3-0.4 mm/day). Detection combing on damp, conditioned hair is the gold standard: 3 sessions on 3 different days reaches ~ 90% sensitivity (visual inspection alone ~ 30%). Wood's lamp shows pale blue fluorescence of live nits (adjunct). Dermoscopy at 10-30x visualises the louse body, claws, and gut. Microscopy of a plucked hair with the nit attached in 10% KOH shows the embryo (viable) or an empty shell.

^{[1] [3]}

Q3: Investigations, and the role of an STI screen (2 min)

A 25-year-old man presents with pubic itch and visible crab lice. What additional investigations are required, and why?

Model answer: In any adult with pubic lice, the sexual contact is the route of transmission, so the patient and all sexual contacts within the previous month need examination and treatment simultaneously. A full STI screen is mandatory because the co-infection rate with other STIs is 20-30% in published case series. The screen should include: HIV (fourth-generation Ag/Ab), syphilis (RPR/VDRL with TPHA confirmation), hepatitis B (sAg/sAb/cAb), hepatitis C, NAAT for Chlamydia trachomatis (first-pass urine or urethral swab) and Neisseria gonorrhoeae (urethral, pharyngeal, rectal as appropriate), and consideration of Mycoplasma genitalium and trichomoniasis. Microscopy of a plucked pubic hair with the attached nit in 10% KOH confirms the species and viability. Examination of all coarse body hair (pubic, axillary, beard, moustache, eyelashes) is needed because the pubic louse can infest any coarse hair; in children, pubic lice trigger a safeguarding assessment for possible sexual abuse.

^{[1] [2]}

Q4: Management — first-line, second-line, and refractory disease (3 min)

Outline the stepwise management of head lice, including first-line, second-line, and refractory-disease options. Justify the rationale for the two-application 7-day regimen.

Model answer: First-line in most current guidelines is dimeticone 4% lotion (physical mode of action — suffocates the louse by occluding the spiracles; no neurotoxic resistance; safe from 6 months, in pregnancy, and in atopic skin). It is applied to dry hair, left for 8 hours, combed out with a fine-toothed comb, and repeated at day 7. Where permethrin resistance is low, permethrin 1% cream rinse (washed/towel-dried hair, 10 minutes) is acceptable. Alternatives include malathion 0.5% in isopropyl alcohol (12 hours, repeat day 7; higher ovicidal activity; flammable, strong odour), benzyl alcohol 5% lotion (10 minutes, repeat day 7; physical, safe from 6 months), spinosad 0.9% suspension (single application, expensive), and wet combing (conditioner + fine-toothed comb every 3-4 days for 14 days; lower efficacy ~ 50-60%, but the only option in < 6 months and in topical allergy). For refractory or multiply-resistant disease, oral ivermectin 200-400 mcg/kg as a single dose, repeated on day 7-10 (Chosidow 2010 NEJM: 97.1% vs 67.6% cure vs malathion at day 15). The rationale for the two-application 7-day regimen is that no agent is 100% ovicidal; a proportion of eggs survive the first application and the second application 7 days later kills the nymphs that have hatched from these surviving eggs before they can mature and reproduce.

^[1]

Q5: Complications, body lice as a vector, and public health (2 min)

What is the most important complication of pediculosis, and what public-health action is required?

Model answer: The most important complication is louse-borne systemic disease in a homeless or displaced patient with body lice — specifically louse-borne (epidemic) typhus (Rickettsia prowazekii), louse-borne relapsing fever (Borrelia recurrentis), and trench fever (Bartonella quintana). In a febrile homeless or displaced patient with body-lice-infested clothing, urgent assessment and specific treatment of the suspected louse-borne disease is required before serology returns — see the local guideline for the agent, dose and duration.^[4] Louse-borne relapsing fever carries a Jarisch-Herxheimer reaction within 2 hours of the first dose — have IV access, fluids, and resuscitation equipment at the bedside. Public-health action in outbreaks: (1) active case-finding by examining clothing seams, (2) environmental decontam…

References4ShowHide

1. [1]Burgess IF, Brown CM, Lee PN. Treatment of head louse infestation with 4 percent dimeticone lotion: randomised controlled equivalence trial BMJ, 2005.PMID 15951310
2. [2]Burgess IF, Brown CM, Lee PN. Treatment of head louse infestation with 4 percent dimeticone lotion: randomised controlled equivalence trial BMJ, 2005.PMID 15951310
3. [3]Burgess IF, Brown CM, Lee PN. Treatment of head louse infestation with 4 percent dimeticone lotion: randomised controlled equivalence trial BMJ, 2005.PMID 15951310
4. [4]Perine PL, Krause DW, Awoke S, et al. Single-dose doxycycline treatment of louse-borne relapsing fever and epidemic typhus Lancet, 1974.PMID 4143011