Dermatology, Venereology and Leprosy
Scabies
Scabies for MBBS and NEET-PG: Sarcoptes scabiei, burrows and nocturnal itch, distribution, crusted scabies, permethrin and ivermectin with contact treatment, mapped to NMC codes DR5.
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Scabies
Scabies for MBBS and NEET-PG: Sarcoptes scabiei, burrows and nocturnal itch, distribution, crusted scabies, permethrin and ivermectin with contact treatment, mapped to NMC codes DR5.
Scabies is a common, intensely itchy infestation and a frequent public health problem. This chapter covers the biology of Sarcoptes scabiei, the burrow and its typical distribution, the delayed hypersensitivity that explains the itch, the crusted variant, and the correct use of permethrin and ivermectin with simultaneous contact treatment.
High-yield: Scabies
- Scabies is an infestation by the mite Sarcoptes scabiei var. hominis, whose female burrows into the stratum corneum to lay eggs.
- The cardinal symptom is intense itching that is worse at night and often affects several household members.
- The pathognomonic lesion is the burrow, a short serpiginous grey line, and the diagnostic sign is a mite at its end.
- Classic sites are the finger webs, wrists, axillae, periumbilical skin, genitalia in men and the areolae in women.
- In infants the palms, soles, face and scalp are involved, which are usually spared in adults.
- The itch and rash result from a delayed hypersensitivity reaction, which is why symptoms may take weeks to appear in a first infestation.
- Diagnosis is confirmed by demonstrating the mite, eggs or faecal pellets on skin scraping under the microscope.
- Crusted or Norwegian scabies is a hyperinfestation with thick scale and thousands of mites, seen in immunosuppressed or debilitated patients and highly contagious.
- Permethrin 5% cream is the first-line treatment, applied to the whole body from the neck down and washed off after 8 to 12 hours.
- In infants and young children the application must include the head and neck.
- Oral ivermectin is useful for crusted scabies, institutional outbreaks and where topical treatment is difficult.
- All close contacts and household members must be treated at the same time, whether or not they itch, to prevent reinfestation.
- Clothing and bed linen should be washed in hot water and dried, as the mite does not survive long off the host.
- Itching can persist for two to four weeks after successful treatment because of ongoing hypersensitivity, and this does not mean treatment failure.
Scabies essentials
- **Organism:** Sarcoptes scabiei var. hominis; female burrows in stratum corneum.
- **Hallmark:** Nocturnal itch, burrows in finger webs and wrists, several affected contacts.
- **First-line treatment:** Permethrin 5% cream neck-down (whole body in infants), repeat after a week.
- **Crusted scabies:** Thick scale, thousands of mites, immunosuppressed; oral ivermectin.
NMC competencies in this chapter
- **DR5.1:** Aetiology, microbiology, pathogenesis and natural history of scabies
- **DR5.2:** Identifying and differentiating scabies from other skin conditions
- **DR5.3:** Pharmacology and administration of drugs used to treat scabies
Frequently Asked Questions
Why is scabies itch worse at night?
The itch is a delayed hypersensitivity response to the mite and its products, and it is characteristically worse at night. It is often shared among household members, which is a strong diagnostic clue.
How is permethrin applied?
As a 5% cream over the whole body from the neck down in adults, left on for 8 to 12 hours and then washed off, with a repeat application after about a week. In infants the head and neck must also be treated.
Why must contacts be treated together?
Untreated close contacts and household members re-infest the patient, so everyone is treated at the same time whether or not they itch, along with washing of clothing and bedding.
What is crusted scabies?
A hyperinfestation with thick scale containing thousands of mites, seen in immunosuppressed or debilitated people. It is very contagious and is treated with oral ivermectin alongside topical therapy.
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