eMedicine Specialties > Dermatology > Parasitic Infections

Lice: Differential Diagnoses & Workup

Author: Dirk M Elston, MD, Director, Department of Dermatology, Geisinger Medical Center
Contributor Information and Disclosures

Updated: Sep 4, 2009

Differential Diagnoses

Impetigo
Scabies

Other Problems to Be Considered

True nit infestation must be distinguished from hair casts (pseudonits). Hair casts are ringlike remnants of the inner root sheath of the hair follicle. They are amorphous and freely moveable along the hair fiber.

Many scalp conditions can cause pruritus. Seborrheic dermatitis presents with erythema and scale. It affects the scalp, eyebrows, nasolabial folds, and central chest. Acne necrotica presents with folliculitis with superficial pustules within scattered hair follicles. It is extremely pruritic, and patients pick at the lesions. Secondary follicular excoriations typically are noted on examination.

Free-living primitive psocid lice feed on decaying matter in leaves, old books, and animal habitats. They may cause human scalp infestation when children visit a library or doghouse that is infested. Psocids have large heads with massive jaws and are distinguished easily from Anoplura lice.

Workup

Laboratory Studies

  • The identification of adult lice or nits is diagnostic. The presence of bruiselike bites or dark specks of louse dung suggest the diagnosis of louse infestation.
  • Lice move rapidly. A helpful technique is to fasten a piece of transparent adhesive tape to the infested areas. Lice stick to the tape. The tape then becomes a convenient coverslip for a microscopic slide.
  • Mature lice are approximately the size of a sesame seed (3-4 mm) with an elongated body, 3 pairs of legs, and narrow anterior mouthparts. Wide crablike bodies and claws distinguish pubic lice. Nits are approximately 1 mm in length, transparent, and flasklike in appearance.
  • Nits and lice fluoresce with a Wood light.
  • Properly evaluating persons who have been raped is essential. Evidence should be collected in such a way as to avoid contamination and to ensure a legal chain of custody. Human DNA can now be identified in the amount of blood present in a pubic louse. Lice recovered after an attack have the potential to provide evidence valuable in securing a conviction.

Other Tests

  • Evaluate individuals with pubic lice for other sexually transmitted diseases (STDs). Evaluations may include serologic testing for syphilis and AIDS, culture for gonorrhea, and antigen testing for chlamydia.

Histologic Findings

Louse bites demonstrate intradermal hemorrhage and a polymorphous wedge-shaped infiltrate rich in eosinophils.

More on Lice

Overview: Lice
Differential Diagnoses & Workup: Lice
Treatment & Medication: Lice
Follow-up: Lice
Multimedia: Lice
References
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Further Reading

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Keywords

lice, lice infestation, pediculosis, louse infestation, nits

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Contributor Information and Disclosures

Author

Dirk M Elston, MD, Director, Department of Dermatology, Geisinger Medical Center
Dirk M Elston, MD is a member of the following medical societies: American Academy of Dermatology
Disclosure: Nothing to disclose.

Medical Editor

Daniel J Hogan, MD, Clinical Professor of Internal Medicine (Dermatology), NOVA Southeastern University; Investigator, Hill Top Research, Florida Research Center
Daniel J Hogan, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Dermatology, American Contact Dermatitis Society, and Canadian Dermatology Association
Disclosure: Nothing to disclose.

Pharmacy Editor

David F Butler, MD, Professor of Dermatology, Texas A&M University College of Medicine; Chair, Department of Dermatology, Director, Dermatology Residency Training Program, Scott and White Clinic, Northside Clinic
David F Butler, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Dermatology, American Medical Association, American Society for Dermatologic Surgery, American Society for MOHS Surgery, Association of Military Dermatologists, and Phi Beta Kappa
Disclosure: Nothing to disclose.

Managing Editor

Edward F Chan, MD, Clinical Assistant Professor, Department of Dermatology, University of Pennsylvania School of Medicine
Edward F Chan, MD is a member of the following medical societies: American Academy of Dermatology, American Society of Dermatopathology, and Society for Investigative Dermatology
Disclosure: Nothing to disclose.

CME Editor

Catherine Quirk, MD, Clinical Assistant Professor, Department of Dermatology, Brown University
Catherine Quirk, MD is a member of the following medical societies: Alpha Omega Alpha and American Academy of Dermatology
Disclosure: Nothing to disclose.

Chief Editor

Dirk M Elston, MD, Director, Department of Dermatology, Geisinger Medical Center
Dirk M Elston, MD is a member of the following medical societies: American Academy of Dermatology
Disclosure: Nothing to disclose.

 
 
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