Norwegian (crusted) scabies differs from ordinary scabies in that the mite burden is extremely high (millions vs. 10-15 in ordinary scabies). The reason affected patients do NOT exhibit pruritus despite massive infestation is:
- A Mites in crusted scabies produce an analgesic substance
- B Impaired cellular immunity (as in HIV, HTLV-1, Down syndrome) prevents mounting a sensitization response leading to reduced pruritus ✓
- C Thick hyperkeratotic crust physically prevents mite antigens from reaching sensory nerves
- D Host genetic variants conferring itch-insensitivity
Explanation
Ordinary scabies pruritus is caused by a delayed-type hypersensitivity (Type IV) immune response to mite antigens, developing after 4-6 weeks of primary sensitization. In Norwegian/crusted scabies, the host is severely immunocompromised (HIV, HTLV-1, organ transplant, Down syndrome, malnutrition, steroid use) — their deficient T-cell-mediated immune response fails to mount the normal sensitization response. Without this immunological response, pruritus is reduced or absent despite astronomical mite burdens. These patients are highly contagious. Treatment: oral ivermectin combined with topical permethrin.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.