Dermatology · Infectious Dermatology
The facts most likely to be tested
Pediculosis capitis presents with intense pruritus and nits (eggs) firmly attached to the hair shafts, most commonly at the occipital and postauricular regions.
The first-line treatment for head lice is topical permethrin 1% or pyrethrins, requiring a second application 7–9 days later to kill newly hatched nymphs.
Pediculosis corporis is associated with poor hygiene and crowded living conditions, serving as a vector for Rickettsia prowazekii (epidemic typhus), Bartonella quintana (trench fever), and Borrelia recurrentis (relapsing fever).
The hallmark of pediculosis corporis is excoriations and pruritic papules on the trunk and shoulders, with lice and nits found primarily in the seams of clothing rather than on the skin.
Pediculosis pubis (crabs) is a sexually transmitted infection that presents with pruritus and blue-gray macules known as maculae caeruleae at the site of bites.
Treatment for pediculosis pubis includes topical permethrin 1% or pyrethrins, with mandatory screening for other sexually transmitted infections and treatment of all sexual partners.
Resistance to topical pediculicides is increasing, making oral ivermectin or topical spinosad effective second-line alternatives for refractory cases.
Vignette unlocked
A 24-year-old male presents to the clinic complaining of persistent, intense itching in his groin area for the past two weeks. He reports having multiple sexual partners recently. Physical examination reveals excoriations in the pubic region and several blue-gray macules on his lower abdomen. Close inspection of the pubic hair reveals small, oval-shaped nits firmly attached to the hair shafts.
What is the most appropriate next step in management?
Topical permethrin 1% application and screening for other sexually transmitted infections.
The patient's presentation of pruritus, maculae caeruleae, and nits is pathognomonic for pediculosis pubis; because it is an STI, screening for other infections is mandatory.
Full handout
High yield triage
Etiology / Epidemiology
Common in school-aged children via direct contact or fomites. Highly contagious in crowded environments.
Clinical Manifestations
Intense pruritus is the hallmark. Nits (eggs) attached to hair shafts are pathognomonic.
Diagnosis
Clinical diagnosis via visual inspection with a fine-toothed comb. Wood's lamp may show fluorescence.
Treatment
Permethrin 1% is the first-line topical treatment. Avoid lindane due to neurotoxicity.
Prognosis
Excellent with proper hygiene. Retreatment at day 9 is mandatory to kill newly hatched nymphs.
Full handout
Epidemiology & Etiology
Pediculosis is caused by obligate ectoparasites: Pediculus humanus capitis (head), P. humanus corporis (body), and Pthirus pubis (pubic). Transmission occurs via direct contact or shared personal items like hats and combs. It is most prevalent in school-aged children and those in close-contact living conditions.
Pertinent Anatomy
Lice are wingless insects that require human blood for survival. Nits are firmly cemented to the hair shaft near the scalp, distinguishing them from dandruff which is easily dislodged. Pthirus pubis has a predilection for coarse hair, specifically the pubic region, axilla, and eyelashes.
Pathophysiology
Lice feed on human blood, injecting saliva that triggers a Type I hypersensitivity reaction. This immune response causes the characteristic intense pruritus. The life cycle involves eggs (nits), nymphs, and adults, with the entire cycle lasting approximately 30 days.
Clinical Manifestations
Patients present with intense pruritus, often worse at night. Excoriations and secondary bacterial infections (e.g., impetigo) may occur from scratching. Look for nits on the hair shaft or adult lice near the scalp. Red flag: secondary lymphadenopathy suggests severe bacterial superinfection.
Diagnosis
Diagnosis is confirmed by visual inspection using a fine-toothed nit comb under bright light. A Wood's lamp examination may reveal live nits as fluorescent. No laboratory testing is required; the presence of live lice or viable nits confirms the diagnosis.
Treatment
Permethrin 1% lotion is the first-line therapy. Apply to damp hair, leave for 10 minutes, then rinse. Lindane is contraindicated due to risk of seizures and neurotoxicity. Wet combing is a critical adjunct to remove nits.
Prognosis
Prognosis is excellent with adherence to treatment protocols. Retreatment at day 9 is essential to eliminate nymphs that hatched after the initial application. Failure to treat contacts leads to high rates of reinfestation.
Differential Diagnosis
Seborrheic dermatitis: greasy scales that flake off easily
Tinea capitis: associated with alopecia and broken hair shafts
Scabies: burrows in interdigital spaces, not hair shafts
Contact dermatitis: history of new hair product exposure
Psoriasis: silvery scales on erythematous plaques