A nine-year-old boy with atopic dermatitis presents with a one-month history of discrete, flesh-coloured to pink papules in the ante-cubital fossae and on the lower abdomen. The lesions are asymptomatic. On close inspection, the lesions are dome-shaped, and some have a central umbilication. His four-year-old sister has several similar appearing lesions. What’s your diagnosis?

Molluscum contagiosum (MC) is a benign viral infection of the skin that often affects children. Lesions are self-resolving, lasting from months to years. It affects all ages and races. Up to 5% of youngsters may be affected. It’s also more common in sexually active adults and patients who are immunocompromised, especially those with HIV. MC can be more extensive and with a more prolonged course in immunocompromised people and in those with atopic dermatitis.

Pathogenesis

MC virus (MCV) is a poxvirus that only affects humans. It acts on replicating keratinocytes in the epidermis or epithelium. There are four subtypes, with MCV-1 implicated in the vast majority of cases and with MCV-2 most commonly seen in patients with HIV. The incubation period ranges from 2-7 weeks.

Transmission

• direct contact: skin, mucous membranes
• can be sexually transmitted in adults
• close contact sports — e.g. wrestling
• fomites — less often
• bath towels, swimming pools
• autoinoculation
• koebnerization — lesions occur at sites of minor trauma, such as scratching

Clinical findings

Characteristic lesions are small, flesh-coloured to pink, pearly papules with a waxy, smooth surface. They’re usually 3-5 mm in diameter and rarely can enlarge up to 2 cm. As they enlarge, lesions become dome-shaped and may develop central umbilication — a classic finding. A whitish substance can often be expressed from the centre with pressure.

Children can sometimes have more than 100 lesions. In adults there are usually less than 20, and they favour the lower trunk, genitalia and upper thighs. Mucosal lesions are rare.

Lesions can be arranged in clusters or linear (inoculated along scratches). They can occur anywhere with the most common locations being the skin folds, axillae, groin, trunk, face and extremities. Genital lesions appear in 10% of children. Genital involvement presenting without lesions elsewhere may signify sexual transmission. In atopic dermatitis, lesions are more common in dermatitic areas.

There can sometimes be redness and scaling around molluscum lesions — called molluscum dermatitis — that often precedes resolution.

Associated symptoms include:

• pruritus
• chronic conjunctivitis, punctate keratitis — eyelid lesions
• secondary bacterial infections — if lesions are scratched

 

Investigations

• clinical diagnosis — dome-shaped pearly papule with central umbilication
  – cryotherapy can freeze papules with sparing of central dell
• skin biopsy — rarely necessary
  – findings are diagnostic — Henderson-Patterson bodies are seen in keratinocyte

 

Treatment

Patients should be reassured MC is a benign and self-resolving viral infection. They should avoid sharing towels, or other possible fomites, and avoid trauma to sites such as scratching, or shaving if on the face. Antihistamines can help if there’s pruritus. Avoid irritating treatments on atopic skin. Consider an oral antibiotic treatment if there’s superinfection.

Patients will often request treatment, especially if lesions are in cosmetically sensitive areas. Treat them all to help reduce autoinoculation.

Proceed with caution — many of the treatments can be traumatic in young children.

Cantharin is an office-performed topical agent. It’s an extract from a blister beetle that causes blistering at the dermo-epidermal junction. It’s available as cantharidin 0.7% or 1% with salicylic acid 30%. Cantharidin is applied carefully to lesions, with the wooden end of an applicator stick or toothpick, then washed off 2-6 hours later. Treatment can be repeated in 1-3 weeks, as needed. Avoid the face and genital areas. There’s a small risk of scarring or severe reactions. Overall, cantharidin tends to be the least painful treatment, and is generally well tolerated.

Podophyllin is another office-based procedure. Podophyllotoxin is an antimitotic agent obtained as a 10% to 25% resin. It’s applied to lesions, often after cryotherapy and washed off that day. It can cause burning, erythema and erosions. It’s contraindicated in pregnancy.

Other topicals:

• salicylic acid
• trichloroacetic acid
• retinoid creams
  – topical tretinoin
• imiquimod (off-label use)
  – 5% cream applied nightly to lesions 5 days per week for 4-16 weeks
  – can be used in combination with cantharidin
• silver nitrate paste
• tape stripping
  – apply surgical tape to each lesion after bathing

 

Destructive therapies

These therapies can be painful. In kids, consider applying a topical anaesthetic before treating — e.g. EMLA cream applied under occlusion for 1 hour prior to procedure.

• manual expression
  – pierce lesion with a blade, express the core (with or without comedo extractor instrument)
• curettage
• cryotherapy
  – liquid nitrogen
• electrodessication

For severe cases, systemic therapy such as cimetidine is sometimes used.

For genital molluscum, destructive methods can be very effective. Podophyllin and imiquimod aren’t usually effective, unlike with genital warts. Examine sexual partners and screen for other sexually transmitted infections.