Scabies

Scabies is a contagious skin infestation caused by the mite Sarcoptes scabiei var. hominis, an obligate human parasite that burrows into the epidermis. The female mite tunnels through the stratum corneum at 2-3 mm per day, depositing eggs and fecal pellets (scybala) in the burrow. The intense pruritus that characterizes scabies is caused by a delayed-type (Type IV) hypersensitivity reaction to mite proteins, eggs, and feces — this is why itching begins 4-6 weeks after initial infestation (time for sensitization to develop) but occurs within 24-48 hours upon reinfestation. Classic scabies involves 10-15 mites on the body. Norwegian (crusted) scabies occurs in immunocompromised patients and involves millions of mites with thick, hyperkeratotic crusts — it is highly contagious. Transmission occurs through prolonged skin-to-skin contact (15-20 minutes of direct contact) or through fomites in crusted scabies. Mites cannot survive off the human host for more than 48-72 hours.