Scabies


Scabies is a cutaneous parasitosis due to the presence of the mite Sarcoptes scabiei hominis within the epidermis. It exists in two forms: ordinary scabies, relatively benign and moderately contagious; and crusted scabies, favoured by immune deficiency, extremely contagious and refractory to conventional treatment. Person to person transmission takes place chiefly through direct skin contact, and sometimes by indirect contact (sharing clothing, bedding). The challenge in management is that it must include simultaneous treatment of both the patient and close contacts, and at the same time, decontamination of clothing and bedding of all persons undergoing treatment, in order to break the transmission cycle.

Clinical features

Ordinary scabies

In older children and adults

– Itching, worse at night, very suggestive of scabies if close contacts have the same symptom 
and
– Typical skin lesions:
• Scabies burrows (common): fine wavy lines of 5 to 15 mm, corresponding to the tunnels made by the parasite within the skin. Burrows are most often seen in the interdigital spaces of the hand and flexor aspect of the wrist, but may be present on the areolae, buttocks, elbows, axillae. The back and the face are spared. Burrows may be associated with vesicles, corresponding to the entry point of the parasite in the skin.
• Scabies nodules (less common): reddish-brown nodules, measuring 2 to 20 mm, on the genitals in men, persisting after effective treatment (they are not necessarily indicative of active infection).
and/or
– Secondary skin lesions: resulting from scratching (excoriations, crusts) or super-infection (impetigo).

Typical lesions and secondary lesions may co-exist, or specific lesions may be entirely masked by secondary lesions.

In infants and young children

– Vesicular eruption; often involving palms and soles, back, face, and limbs. Secondary infection or eczematisation is frequent. Isolated scabies nodules in the axillae may be the only manifestation.
– Examination of the mother’s hands may support the diagnosis. 

Crusted (Norwegian) scabies

Thick, scaly, erythematous plaques, generalised or localised, resembling psoriasis, with or without itching (50% of cases). Delay in diagnosis may lead to a scabies epidemic.

Treatment

In all cases

– Close contacts of the patient are treated simultaneously, even in the absence of symptoms.
– Clothing and bedding (including that of contacts) are changed after each treatment. They are washed at ≥ 60 °C then dried in the sun, or exposed to sunlight for 72 hours, or sealed in a plastic bag for 72 hours.

Ordinary scabies

Topical treatment

Topical scabicides are applied over the entire body (including the scalp, post-auricular areas, umbilicus, palms and soles), avoiding mucous membranes and face, and the breasts in breastfeeding women. Particular attention should be paid to common infestation sites. The recommended contact time should not be shortened or exceeded; the patient must not wash his hands while the product is in use (or the product should be reapplied if the hands are washed). In infants, the hands must be wrapped to prevent accidental ingestion of the product. Topical scabicides should not be applied to broken or inflamed skin. Treatment of secondary bacterial infection, if present, should be initiated 24 to 48 hours before use of topical scabicides (see Impetigo).

The preferred treatment is 5% permethrin (lotion or cream):
Children > 2 months and adults: one application, with a contact time of 8 hours, then rinse off. Permethrin is easier to use (no dilution required), and preferred over benzyl benzoate in children, and pregnant/lactating women. One application may be sufficient, but a second application 7 days later reduces the risk of treatment failure.

or, if not available, benzyl benzoate 25% lotion:


Children < 2 years

Children 2-12 years

Children > 12 years and adults


Dilution

Lotion must be diluted before use:


Use undiluted 25% lotion

1 part 25% lotion
+ 3 parts water

1 part 25% lotion
+ 1 part water

Contact time

12 hours (6 hours for infants
< 6 months), then rinse off

24 hours, then rinse off

24 hours, then rinse off

A second application of benzyl benzoate (e.g. after 24 hours, with a rinse between the 2 applications; or 2 successive applications, 10 minutes apart, when the first application has dried, with a rinse after 24 hours) reduces the risk of treatment failure.
The second application is not recommended for children < 2 years and pregnant women.

Oral treatment

Treatment with ivermectin PO (200 micrograms single dose) is an alternative: it is more practical than topical treatment (e.g. in the case of an epidemic or for treating contacts) and can be started right away in the case of secondary infection. A single dose may be sufficient; a second dose 7 days later reduces the risk of treatment failure.

Ivermectin is not recommended for children < 15 kg or pregnant women (safety not established)1 .
Administration of ivermectin to patients with loiasis carries a risk of severe neurological complications when significant Loa loa microfilaraemia is present (see Filariasis, Chapter 6)2 .

Weight

15 to 24 kg

25 to 35 kg

36 to 50 kg

51 to 65 kg

Ivermectin 3 mg tab

1 tab

2 tab

3 tab

4 tab

Ivermectin 6 mg tab

½ tab

1 tab

tab

2 tab

Treatment effectiveness is judged on clinical grounds. Itching may persist for 1 to 3 weeks after elimination of the parasite.

Persistence of typical burrows beyond 3 weeks should lead to suspicion of treatment failure (insufficient treatment, e.g. the scalp was not included in topical treatment or the patient washed his hands during the treatment period), or early re-infestation (contacts and environment not treated). In these cases, patient and contacts should be retreated.

Persistent itching may be due to another condition, initially masked by scabies.

Crusted scabies

Treatment combines simultaneous administration of oral ivermectin and topical scabicide at regular intervals, e.g. every week for 2 to 3 weeks or more, according to severity and clinical response.

Crusts should be softened (salicylic acid ointment) and removed before applying local treatment (otherwise, local treatment is ineffective).

As exfoliated skin scales may spread the parasite, the patient should be isolated during the treatment, staff should use protection (gloves, gowns and hand washing after contact), and environment (bedding, floors and surfaces) should be decontaminated.



Footnotes
Ref Notes
1 Treatment with ivermectin in these patients is reserved for severe cases for which no alternative exists (see Crusted scabies).
2 In areas where loiasis is endemic, certain precautions are recommended before administering ivermectin: e.g. measure the Loa loa microfilaraemia, if possible, or ensure that the patient has no history of loiasis (migration of an adult worm under the conjunctiva or transient « Calabar » swellings), nor history of severe adverse reactions following a previous treatment with ivermectin, or if in doubt, use topical treatment in preference to oral.