USMLE

Pediculus humanis and Phthirus pubis (Lice)

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Parasites
  1. Giardia lamblia
  2. Toxoplasma gondii
  3. Entamoeba histolytica
  4. Cryptosporidium
  5. Naegleria fowleri
  6. Trypanosoma brucei
  7. Plasmodium Overview
  8. Plasmodium Disease (Malaria)
  9. Babesia
  10. Trypanosoma cruzi
  11. Leishmania
  12. Trichomonas vaginalis
  13. Enterobius vermicularis (pinworm)
  14. Ascaris lumbricoides (giant roundworm)
  15. Strongyloides stercoralis (threadworm)
  16. Ancyclostoma and Necator
  17. Trichinella spiralis
  18. Trichuris trichiura (whipworm)
  19. Toxocara canis
  20. Onchocerca volvulus
  21. Loa loa
  22. Wuchereria bancrofti
  23. Taenia solium
  24. Diphyllobothrium latum
  25. Echinococcus granulosus
  26. Schistosoma
  27. Clonorchis sinensis
  28. Sarcoptes scabiei (Scabies)
  29. Pediculus humanis and Phthirus pubis (Lice)

Summary

Lice are blood-sucking parasites that burrow into the skin to cause itchy excoriations. Lice affecting people can be broken down into two species: pediculus humanus and pthirus pubis. 

Pediculus primarily affects the head and scalp, along with the eyelashes, while pthirus primarily affects the pubic region. Head lice or pediculus can be transmitted through hair-to-hair contact, which is why they are commonly seen in crowded households or in spaces with infected children. On the other hand, pubic lice or pthirus is transmitted through sexual or other physical contact involving the pubic area. Infections with lice generally present as intense pruritus with skin excoriations. Lice infections are diagnosed by wet combing the hair with a fine-toothed comb to reveal the eggs, called nits. Finally, the first-line treatment for lice is permethrin, usually administered as a shampoo. 

Key Points

  • Pediculus humanus (head lice)/Phthirus pubis (pubic lice)
    • Characteristics
      • Blood-sucking lice
    • Transmission
      • Head lice
        • Hair-to-hair contact
        • Common among households with  infected children, crowded living quarters
      • Pubic lice
        • Close physical or sexual contact
    • Presentation
      • Intense pruritus with excoriations
        • Crawling sensation
      • Eyelash infection
        • Can cause conjunctivitis
        • Can be caused by both species
      • Pediculus humanus infests scalp and neck
      • Phthirus pubis infests pubic and perianal regions
    • Pathogenesis
      • Females lay nits (eggs) that are attached to the hair
        • Nits > 1 cm away from hair shaft may indicate an old, not active, infection
    • Diagnosis
      • Examine by wet combing hair with a fine-tooth comb
        • Visual inspection of base of hair for nits or (less commonly) live bugs
    • Treatment
      • Permethrin
      • Nit combing (after shampooing with permethrin)