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Case Note

Case Note: Papular Eruption on the Back of the Knees


An 8-year-old female presents for management of itchy 'bumps' on the posterior knees.

Initial evaluation

  • Healthy appearing female
  • No significant past medical history except for atopic dermatitis for which the patient regularly uses topical corticosteroids and emollients
  • Review of systems is unremarkable
  • Presents with flesh-colored, umbilicated papules on the popliteal fossa bilaterally
    • Few scattered lesions on the posterior thighs
    • Lesions occur on a background of very mild eczematous plaques consistent with atopic dermatitis
  • Image 1
    Image 1

Enter your diagnosis


  • A diagnosis of molluscum contagiosum is rendered
    • In the setting of mild atopic dermatitis

Initial Treatment

  • Lesions are treated with 0.7% cantharidin for 4 hours' duration prior to washing
  • Follow up in 2 weeks

Follow-up evaluation strategy

2-week follow-up evaluation:

  • The patient and her family report that the lesions became moderately inflamed, then many resolved
  • Remaining lesions are treated with 0.7% cantharidin for 4 hours' duration prior to washing
  • Recommended that treatment for atopic dermatitis is resumed after the inflammatory lesions resolve
  • Follow up in 2 and 6 weeks
    • Resolved

Subsequent treatment steps:

  • Destruction of each lesion by pricking with a large (18-gauge) needle or a #11 blade is a therapeutic option
    • Removal of the core with a comedone extractor will also enhance resolution
  • Adults will usually tolerate curettage or electrocautery to destroy smaller lesions
    • Anesthesia with topical eutectic mixture of lidocaine and prilocaine (EMLA) cream (lidocaine 2.5% and prilocaine 2.5%) may allow this form of treatment without injected anesthesia
  • Whilst treatment of molluscum in children with imiquimod is not supported by evidence, there is emerging data that 10% potassium hydroxide solution, intralesional candida antigen, and oral cimetidine (40 mg/kg daily) may be effective treatments in the pediatric population
  • In severe cases, trichloroacetic acid peels, up to 50% concentration, or podophyllin may be considered
  • Surgical excision is the first-line therapy for mucosal (i.e., intraoral or ocular) molluscum

Treatment pitfalls:

  • All destructive methods for molluscum carry the risk of scarring
  • In adults, molluscum lesions, like flat warts, may be spread through shaving
    • Patient should be instructed to use only an electric razor (preferably with floating heads) to shave as this type of shaving may be less likely to spread the lesions
  • Again in adults, the most common error is diagnosing molluscum as genital warts
    • If there is any question consider a biopsy, as these treatment strategies are distinct
  • In immunosuppressed individuals, other infectious agents (e.g., herpes simplex, Cryptococcus neoformans, Penicillium marneffei) may produce lesions (umbilicated papules) mimicking molluscum contagiosum
    • If there is any question as to the correct diagnosis, a biopsy is indicated

General discussion

Molluscum contagiosum is a viral infection caused by molluscum contagiosum virus (MCV subtypes 1, 2, 3, 4) that results in flesh-to-pink colored umbilicated papules. It is highly contagious and is spread through direct contact with infected skin, vertical transmission during labor, and possibly also through fomite transmission. Lesions of molluscum are typically asymptomatic, though may develop erythema, pruritus, or tenderness in some individuals. They will often spontaneously resolve without therapy in healthy children and adults within 6-9 months. Persistent lesions, though not considered dangerous, may be challenging to completely treat, and scarring is a risk of aggressive therapies. Minimal evidence supports the use of any treatments for molluscum. The approach to the therapy of molluscum contagiosum is divided into three treatment groups: children, adults, and the immunosuppressed.


Children may have a few lesions or many lesions (50-100s). The prevalence of molluscum in children under the age of 16 is estimated between 5-16%. Atopic dermatitis has been proposed as a risk factor for increased susceptibility due to autoinoculation through an impaired skin barrier during scratching. Molluscum contagiosum limited to the genital area in children should raise the possibility of child abuse.


Molluscum contagiosum in normal adults is usually a sexually transmitted disease (STD) and is found in the genital area. As with condyloma acuminatum, use of male latex condoms may not completely eradicate the risk of transmission to sexual partners.

Immunosuppressed individuals

Molluscum infection has been reported to affect 5-18% of immunocompromised individuals and up to 30% of people with advanced acquired immune deficiency syndrome (AIDS). Adults with extensive lesions outside the genital area must be evaluated for immunosuppression, especially human immunodeficiency virus (HIV) infection. Molluscum contagiosum in the setting of HIV infection is a direct consequence of significant immunosuppression. In advanced AIDS, extensive facial or genital molluscum is very common. Total cure is almost impossible. Individual lesions in general do not spontaneously resolve as in healthy adults and children and may require treatment.

The treatment of choice of HIV-infected patients with extensive molluscum is the institution of highly active antiretroviral therapy (HAART). Once the helper T-cell count exceeds 100, the molluscum lesions begin to resolve, leaving no scars. This takes several months. Aggressive therapy that potentially would scar should not be undertaken until the full beneficial effects of immune reconstitution have been realized. Immune reconstitution syndromes, with marked inflammation of molluscum lesions, have been reported. Persistent lesions are treated for cosmesis at the patient's request.

Further reading

Moye VA, Cathcart S, Morrell DS. Safety of cantharidin: a retrospective review of cantharidin treatment in 405 children with molluscum contagiosum. Ped Derm 2014;31:450-4.

Nguyen HP, Franz E, Stiegel KR, Hsu S, Tyring SK. Treatment of molluscum contagiosum in adult, pediatric, and immunodeficient populations. J Cutan Med Surg 2014;18:299-306.

Olsen JR, Gallacher J, Piguet V, Francis NA. Epidemiology of molluscum contagiosum in children: a systematic review. Fam Pract 2014;31:130-6.

van der Wouden JC, van der Sande R, van Suijlekom-Smit LW, Berger M, Butler CC, Koning S. Interventions for cutaneous molluscum contagiosum. Cochrane Database Syst Rev 2009;7:CD004767.


Disclaimer: The material above has been adapted from Therapeutic Strategies prepared by It has not been reviewed by the DermQuest Editorial Board for its accuracy or reliability. Reference to any products, service, or other information does not constitute or imply endorsement, sponsorship, or recommendation by members of the Editorial Board.