Scabies (from Latin: scabere, “to scratch”) is a common ectoparasitic infestation caused by the mite Sarcoptes scabiei. Colloquially it is also known as the seven-year itch.

Cause

Scabies is classified by the WHO as a water-related disease. It is a contagious infection caused by mite Sarcoptes scabiei and is most commonly transmitted by direct skin-to-skin contact. The disease may also be transmitted via objects sharing like clothing, towel, bed sheets and other personal items.

Mites are microscopic in size (about 300 microns) but may be visible as pinpoints of white.Transfer of fertilized (pregnant) female mite is necessary for transmission of infection. Pregnant female mites create tunnels into the outermost layer of skin (stratum corneum) and deposits eggs into shallow burrows. Eggs turn into larvae in 3-10 days, which turns into nymphal stages before becoming adults in 10-14 days. Adults live for 3-4 weeks on the skin of the host. Males roam on the skin occasionally  burrows.

The movement of mites on and within skin causes an intense itch. Itch is a result of delayed cell-mediated inflammatory response to the allergens (mite themselves, their saliva, eggs or feces). The allergic reaction is both delayed (cell-mediated) and immediate (antibody mediated) type. The allergic response involves IgE type of antibodies.

Symptoms

Itching (pruritus) is the main symptom of scabies. There is an incubation period of 1 month between infection and development of itching, due to the development of hypersensitivity to mite antigens. Symptoms might appear within few day in case of re-infection. In some cases, symptoms might appear months or years after infection. The condition affects children more commonly than young adults and is less common in the elderly.

Itching is classical symptoms which are made worse my warmth and it worsens at night. It is commonly seen in more than one person in the family due to close contact between individuals, especially in countries like India where many individuals of the same family stay in the same house or room.

Skin lesions have uncharacteristic morphology but characteristic distribution. The rash is commonly at web spaces, wrists, ulnar border of forearm and arm, anterior axillary fold, nipple and areola in women, periumbilical region and in males, over genitalia, anterior thighs, buttocks and natal cleft. Face, scalp, back, palms and soles are spared in adults but are involved in infants, young children, and immunosuppressed individuals.

Variations

These are variants from typical scabies.

• Genital scabies appears in sexually active adult males and lesions are seen on glans, the shaft of the penis, scrotum, and inner thighs.
• Scabies in “clean”: seen in individuals with good personal hygiene.
• Nodular scabies
• Keratotic scabies

Complications

• Bacterial infection (Infected scabies): If left untreated scabies develops secondary bacterial infection by streptococci or staphylococci. Scabies lesions turn into pustules or bullae filled with pus.
• Eczematized scabies: Seen in infants and children
• Paraphimosis: Inability to retract foreskin due to edema
• Immunological sequelae: If associated with streptococcal infection, glomerulonephritis, and rheumatic fever are potential complications in children.

Diagnosis

In countries like India which are endemic to scabies and diagnosis is made based on family history of itching and typical findings of scabies in the patient.Mites can be demonstrated by scraping the skin lesions or by extracting the mite from a burrow. It can also be diagnosed by finding mite eggs or fecal pellets.

Differential diagnosis (conditions which resemble scabies) includes:

• Dermatitis
• Syphilis
• Urticaria
• Allergic reactions
• Ectoparasite infestations

Treatment

Along with the infected individual all members of the family, at least all children and spouse of the infected individual should be treated simultaneously even if they do not have any symptoms.

Treatment consists of the overnight topical application of scabicidal drug solutions from neck to toe, carefully covering all parts, after a scrub bath. Application of drug should be repeated after  days if the drug is not ovicidal.

Topical therapy of scabies is as follows:

• Permethrin (5%): It is most effective treatment for scabies and is the treatment of choice. The only  single application is required as it is miticidal and ovicidal.
• Gamma benzene hexachloride (GBH – 1%): This also requires an only single application but it is relatively contraindicated in infants and pregnant women. It can cause neurotoxicity in infants if overused.
• Benzyl benzoate (25%): 3 applications are required, it is contraindicated in children, infected and eczematized scabies. It causes severe burning.
• Sulfur (5%): 3 applications required and is moderately effective, contraindicated in infected or eczematized scabies, it stains, stinks and is sticky.
• Crotamiton (10%): 3 applications required and has an additional anti-itching effect.
• Lindane
• Malathion

For itching, antihistaminic drugs can be given.

Usually, it takes 1-2 weeks for symptoms like itching to disappear. Failure of treatment may be due to non-compliance with the method of application, reinfestation from the family member or due to drug resistance.

If there are complications then they have to be treated before application of anti scabietic agents. bacterial infections can be treated with oral or topical antibiotics. Eczematization and nodular scabies can be treated with steroids.

Prevention

No vaccine is available for scabies. mass treatment can be used in high prevalence areas. Maintaining good personal hygiene is also important.