Pediculosis Pubis
Pubic lice infestation (pediculosis pubis or phthiriasis pubis) is a parasitic skin condition caused by the crab louse Pthirus pubis. It spreads mainly through sexual contact or direct skin-to-skin contact. At Haute-Derma, managing pubic lice means accurately recognizing the characteristic skin findings, screening for other sexually transmitted infections, and treating both the patient and their partners thoroughly.
Pathophysiology & Transmission Dynamics
The Parasite and Its Life Cycle
- Cause: the crab louse, Pthirus pubis, is a small wingless insect 1–3 mm long with a short, broad, oval body that looks like a tiny crab.
- Adaptations: its two back pairs of legs end in large claws that grip tightly onto widely spaced hairs such as pubic, perianal, underarm and chest hair, beard hair and eyelashes.
- Life span and reproduction: the female lays 1–3 eggs a day. The eggs (nits) are cemented to the base of the hair shaft with a chitin-based glue. They hatch into nymphs after 7–10 days, which become adult lice in about 2 weeks.
How It Spreads and How the Skin Reacts
- Sexual contact: by far the most common route in adults, through direct skin-to-skin contact of the genital and groin area.
- Indirect spread: through shared underwear, towels, bedding or toilet seats. This is less common, because lice that leave the host die without a blood meal within 24–48 hours.
- Allergic and blood reactions: the louse pierces the skin with its mouthparts to feed on blood several times a day. Its saliva contains anticoagulants and enzymes that trigger a hypersensitivity reaction, causing intense itching and blue-gray spots on the skin.
Clinical Features
Symptoms in the Pubic Area and Other Body Hair
- Intense itching: in the groin, genitals, armpits or chest. It tends to flare at night or when body temperature rises with exercise and sweating.
- Blue-gray spots (maculae caeruleae): flat, pale gray or light blue spots 0.5–1 cm across that do not blanch under glass pressure. They appear on the thighs, buttocks, lower abdomen or groin as a result of a chemical reaction between louse saliva and hemoglobin in human blood.
- Other skin signs: red papules, scratch marks, and louse droppings seen as tiny dark brown specks on the skin or underwear.
Eyelash Infestation (Phthiriasis Palpebrarum)
- Most often seen in children, who catch it indirectly from an infested parent or caregiver.
- Symptoms: itchy eyelid margins, cloudy brown crusts around the lash roots, watery eyes, redness and secondary conjunctivitis. On close inspection, lice and nits can be seen clinging to the base of the lashes.
Differential Diagnosis of Itchy Groin and Genital Rashes
| Pubic Lice (Pediculosis Pubis) | Jock Itch (Tinea Cruris) | Contact Dermatitis |
|---|---|---|
| The parasite Pthirus pubis | Dermatophyte fungi | Contact allergens such as condoms or chemicals |
| Nits on hair roots, blue-gray spots | Red patches with a raised, sometimes vesicular border | Sharply defined red patches that may weep |
| Lice and nits found on the hairs | KOH microscopy shows septate hyphae | No specific organism |
| Blue-gray spots present (characteristic sign) | No blue-gray spots | No blue-gray spots |
| Partners usually infested too | Rarely spread by direct contact | Not contagious |
How We Diagnose It at Haute-Derma
History and Exposure: We ask about recent sexual contact, genital itching in partners, and shared personal items.
Clinical Examination and Dermoscopy:
- All hair-bearing areas are examined: the pubic area, armpits, chest, abdomen, beard and eyelashes.
- Dermoscopy: shows lice gripping the base of the hairs and oval, capped nits firmly attached to the hair shaft.
Screening for Other Sexually Transmitted Infections (STIs): Because pubic lice are mainly sexually transmitted, every patient should be offered screening for other STIs such as syphilis, HIV, chlamydia, gonorrhea and genital herpes.
Treatment and Contact Management at Haute-Derma
Topical Pediculicides
- Permethrin (cream or lotion):
- Apply evenly to all pubic hair, the groin, the perianal area, and underarm and chest hair.
- Leave it on for the time stated by the manufacturer, then wash off. A second application 7–10 days later is essential to kill newly hatched lice.
- Dimethicone / malathion: effective alternatives that kill lice and nits by coating them, disrupting their water balance and suffocating them.
Systemic Treatment
- Oral ivermectin: used for extensive infestation, failure of topical treatment, or patients unable to follow the topical routine. A second dose 7–14 days later is essential.
Treating Eyelash Infestation (Phthiriasis Palpebrarum)
- Do not apply standard lice creams around the eyes.
- Apply plain petroleum jelly to the lid margins twice a day for 10 days to smother the lice and loosen the glue holding the nits.
- Lice and nits are removed directly from the lashes with fine medical forceps under magnification.
Essential Partner and Environmental Measures
- Treat partners: all sexual partners from the past month must be examined and treated at the same time.
- Clean personal items: wash all underwear, towels and bedding in hot water and dry on a high heat setting. Items that cannot be washed should be sealed in a tightly tied plastic bag for 2 weeks.
