Cantharidin for molluscum contagiosum

Treatment choices

Multiple treatments exist, including destructive, immunomodulatory, and antiviral methods. Curettage is a widely used destructive therapy. In a prospective study of curettage, Hanna et al described cure in 81% of patients aged 1 to 18 years.³ In a retrospective study with nearly 2000 patients (aged 1 to 18 years), curettage after applying a eutectic mixture of lidocaine and prilocaine resulted in cure in 70% after the first treatment and in another 26% after a second treatment; the remaining 4% needed a third treatment.¹⁰ Patient and parent satisfaction was very high, at 87% in both studies.

For younger children and those concerned about the pain of curettage, imiquimod can be considered. Imiquimod is a toll-like receptor 7 agonist that activates interferon-α, inducing an antiviral response. Among 13 children, applying 5% imiquimod cream 3 times a week for 16 weeks resulted in total lesion clearance in 15%, substantial reduction in lesion number in 54%, and progressive disease or no change in 31%.¹¹ Despite local side effects, which included erythema (85%), itching (75%), burning sensation (23%), and pain (11%), imiquimod was well tolerated. In a later study, 5% imiquimod cream had a 55% clearance rate after one visit, 41% after a second, and 3% after a third, with a 45% parental satisfaction rate and a 36% incidence of adverse effects.³

Silver nitrate paste was used in another regimen in 389 children to treat widespread, small (1 mm), erythematous papules and plaques, which are often observed in children with MC.¹² After one application all lesions were cleared in 90% of patients; second and third applications resulted in clearance in an additional 7% and 2%, respectively. With a very high overall success rate and no scar formation, silver nitrate might be an ideal therapy. Of note, 17% of patients experienced adverse effects. Further studies are needed, as this was the only study evaluating silver nitrate.¹²

Cantharidin

Cantharidin is a fatty substance of the terpenoid class produced by beetles belonging to the order Coleoptera and the family Meloidae, otherwise known as blister beetles.⁹ When epidermal cells absorb cantharidin, serine proteases are released, which cause the breakdown of the desmosomal plaque, part of the anchorage structure of cells; loss of intracellular attachments; and the formation of intraepidermal blisters.¹³ Cantharidin is on Health Canada’s restricted substance list and is classified as a natural health product.¹⁴

In a chart review of 300 US children (mean age 4.7 years) with MC who were treated with cantharidin crystals (52.5 mg) in a flexible collodion (7.5 mL) on up to 20 nonfacial lesions, 2.1 visits on average resulted in clearing of all lesions in 90% of children and meaningful overall improvement in an additional 8%.¹⁵ Most (92%) experienced mild to moderate blistering—as expected with the use of cantharidin—and about one-third (37%) experienced erythema at the site. Other side effects included mild pain (14%), burning sensation (10%), itching (6%), and postinflammatory hyperpigmentation or hypopigmentation (8%). Parents were then called for a follow-up interview and 95% reported they were satisfied. In another retrospective study from North Carolina, 405 children (mean age 5.8 years) were treated with cantharidin on a total of 9688 lesions for an average of 2.6 visits per child.¹⁶ Side effects included pain (7%) and severe blistering (2.5%); rare side effects included itching, mild infection, irritation, id reaction, and bleeding. Most parents (86%) were satisfied and others were concerned about pain and irritation.

In a prospective randomized trial from Quebec, 124 children (median age of 5 years) with 10 to 100 nonfacial and nongenital lesions were treated with curettage, cantharidin, salicylic acid and lactic acid, and imiquimod.³ Curettage was found to be the most efficacious, with side effects in only 5% of cases. Cantharidin was a useful alternative in the office setting but required more visits, and 19% of cases had moderate complications due to blisters.

More recently, a prospective, randomized, double-blind, placebo-controlled pilot study followed 94 children (mean [SD] age 4.9 [2.7] years) who were treated with cantharidin or placebo, with or without occlusion (4 groups in total).¹⁷ The clearance rate with cantharidin and occlusion was 42%, and was 30% with cantharidin only. In the placebo group, 11% had clearance (P = .0065). Minimal blistering and hyperpigmentation were observed with cantharidin.