Crusted Norwegian scabies is an extremely rare hyperkeratotic variant of scabies infestation. We report herein a case of crusted scabies in a woman with severe rheumatoid arthritis (RA) treated by tocilizumab (TCZ), a monoclonal antibody blocking the interleukin-6 receptor.
An 80-year-old female with severe RA developed erythroderma followed by hyperkeratosis, widespread scaling over the trunk, arms, hands and limbs, and crusted lesions on her scalp. She was on TCZ (8 mg/kg per month) with prednisone (20 mg/d). Several hypotheses were evoked (i.e., vasculitis, psoriasis or paraneoplasic syndrome) but a microscopic examination of the skin surprisingly demonstrated numerous scabies mites and eggs leading to the diagnosis of crusted (Norwegian) scabies. After repeated use of ivermectin and application of topical piperonyl butoxide cream, scabies disappeared. However, there had been outbreak of scabies among two individuals in the staff and two patients. TCZ was stopped after four infusions because of a severe infectious pneumonia and failure.
Crusted scabies should be evoked in case of pruritus or erythroderma occurring under TCZ therapy and it may be explained by the interleukin-6 blockade.