Pedikulosis Pubis

01Pathophysiology

Pthirus pubis is an obligate human ectoparasite that cannot survive away from the human body for more than 24 to 48 hours. Unlike Pediculus humanus capitis (head louse) or Pediculus humanus corporis (body louse), the pubic louse has a short, broad body with large anterior claw-like appendages that are specifically adapted to grasp the widely spaced, coarse hair found in the pubic region, perianal area, axillae, chest, and even eyelashes. This anatomical specificity is why Pthirus pubis preferentially colonizes these sites rather than the fine hair of the scalp.

The louse feeds on human blood by inserting its mouthparts into the dermis. During feeding, it injects salivary antigens into the skin. These antigens provoke a type IV (delayed) hypersensitivity reaction, which is the primary driver of the hallmark symptom: intense pruritus. Pruritus tends to worsen at night because lice are more active during nocturnal hours. The repeated feeding also causes small dermal hemorrhages, which, combined with the breakdown of hemoglobin by salivary secretions, produce the characteristic maculae ceruleae (also called "taches bleues"). These blue-gray, non-blanching macules, typically measuring 0.5 to 1 cm, are virtually pathognomonic for pediculosis pubis and result from the conversion of bilirubin to biliverdin by louse saliva.

The female louse lays eggs (nits) that are cemented to the base of the hair shaft by a chitin-like substance. These nits hatch in approximately 6 to 10 days, and the nymphs reach maturity in about 2 to 3 weeks. Understanding this life cycle is critical because it explains why a second treatment application at 7 to 10 days is necessary: the first application kills live lice but does not reliably penetrate the nit casing, so the second dose targets newly hatched nymphs before they can mature and reproduce.

Chronic scratching leads to excoriations, lichenification, and secondary bacterial superinfection (often with Staphylococcus aureus or Streptococcus pyogenes), which can complicate the clinical picture and make the underlying infestation harder to recognize.

02Classification and Clinical Manifestation

Key clinical pearl: The distribution of Pthirus pubis is limited to areas with widely spaced, coarse hair. It does not infest the scalp because the hair density and diameter there are suited to Pediculus humanus capitis, not to the pubic louse's claw morphology.

03Diagnostic Workup

The diagnosis of pediculosis pubis is fundamentally clinical. There is no serologic marker, blood test, or culture involved. The best initial step is thorough direct visual inspection of the pubic hair and other coarse hair-bearing areas under good lighting. Lice appear as small (1 to 2 mm), flat, translucent organisms that darken to rust-brown after a blood meal. Nits are oval, whitish-yellow structures cemented to the proximal hair shaft, distinguishable from hair casts or debris by the fact that they cannot be easily slid along the shaft.

When the diagnosis is uncertain or the infestation is light, dermoscopy provides the most accurate confirmation. Under magnification, the characteristic broad, crab-shaped body with thick claws becomes apparent, and dermoscopy can also differentiate living nits (which appear plump and translucent) from empty/non-viable nit casings (which appear flattened and opaque). This distinction matters because residual empty nits after treatment do not indicate treatment failure.

A frequently tested point is the mandatory STI screening that should accompany any diagnosis of pubic lice. Because pediculosis pubis is almost exclusively transmitted through intimate contact, its presence is a sentinel marker for other co-existing sexually transmitted infections. Failing to order an STI panel after diagnosing pubic lice is a common exam pitfall.

For eyelash involvement, slit-lamp examination can confirm the presence of nits and lice at the lash base and also evaluate for secondary conjunctivitis or keratitis.

04Management and Treatment

Acute treatment begins with permethrin 1% cream rinse, applied to the affected pubic and perianal hair (and any other involved sites such as axillae, thighs, or trunk) for 10 minutes, then thoroughly rinsed off. A second application at day 7 to 10 is necessary to kill nymphs that hatch from surviving nits after the first treatment. Patients should be instructed to avoid sexual contact until both they and their partners have completed treatment and are confirmed lice-free.

For patients who fail permethrin or in areas where permethrin resistance has been documented, malathion 0.5% lotion is applied overnight (8 to 12 hours) and washed off. Malathion is an organophosphate cholinesterase inhibitor and is both pediculicidal and partially ovicidal. It should be used with caution due to its flammability (alcohol-based vehicle) and is contraindicated in children under 6 years and during pregnancy.

Oral ivermectin at a dose of 200 mcg/kg as a single oral dose, repeated in 7 to 10 days, is a systemic alternative for resistant or widespread cases. It works by binding to glutamate-gated chloride channels in the parasite's nervous system, causing paralysis and death. Ivermectin is contraindicated in pregnancy (Category C) and in children weighing less than 15 kg.

Eyelash involvement is treated differently because topical pediculicides cannot safely be applied near the eyes. The standard approach is petrolatum (petroleum jelly) ointment applied thickly to the lash margins 2 to 4 times daily for 10 days. This works by physically suffocating the lice. Nits should be manually removed with fine-tipped forceps. If petrolatum fails, oral ivermectin can be used.

Partner treatment and environmental decontamination are non-negotiable components of management. All sexual contacts within the preceding 30 days must be treated simultaneously. Clothing and bedding should be machine washed in hot water and dried on high heat. Items that cannot be washed should be placed in a sealed plastic bag for at least 72 hours, as lice cannot survive away from the human host beyond 24 to 48 hours and nits cannot hatch at room temperature without body warmth.

Follow-up: Patients should be re-evaluated at 1 week after the second treatment. Persistent pruritus after successful eradication may continue for 1 to 2 weeks due to residual hypersensitivity to louse antigens and does not necessarily indicate treatment failure. The presence of only empty nit casings without live lice is considered a cure.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students lose points on pediculosis pubis questions is by failing to recognize the mandatory STI screening component. A vignette may present a straightforward case of pubic lice pruritus with visible nits, and the student correctly identifies the diagnosis and selects permethrin as treatment, but then the "next best step" answer choice is to order an STI panel, not to discharge the patient. Pediculosis pubis is a sentinel STI, and the question is testing whether you understand that its presence demands evaluation for chlamydia, gonorrhea, syphilis, and HIV.

The second major trap involves eyelash involvement in a child. When a vignette describes a prepubertal child with blepharitis and nits on the eyelashes, the correct answer is not just to treat with petrolatum. The question is testing whether you recognize this as a potential indicator of sexual abuse and initiate appropriate reporting and evaluation. Students who focus only on the dermatologic management and miss the child safeguarding dimension will select an incomplete answer.

A third tested concept is the difference between treatment failure and residual pruritus. After successful treatment, pruritus may persist for 1 to 2 weeks due to ongoing hypersensitivity to residual louse antigens in the skin. The presence of only empty nit casings (no live lice) on follow-up does not warrant re-treatment. Students who reflexively escalate therapy for persistent itch without confirming active infestation will choose the wrong answer.

Finally, remember the distinguishing morphology: Pthirus pubis has a broad, short body with prominent claws (resembling a crab), adapted to coarse, widely spaced hair. This is in contrast to Pediculus humanus, which is elongated and adapted to fine, densely packed scalp or body hair. A question showing a photomicrograph of a broad, crab-shaped organism on coarse hair is pointing you toward pubic lice, not head or body lice.