Initial assessment
The diagnosis of scabies should be suspected in patients with compatible signs and symptoms, particularly pruritus that worsens at night. It is important to recognize that the intensity of pruritus does not always correlate with the extent of skin lesions. In infants younger than two to three months, pruritus may be absent. Clinical suspicion is strengthened by the presence of a generalized dermatosis and a suggestive epidemiological context, especially when other household members or close contacts have similar signs or symptoms.
Clinical manifestations and lesion distribution
The most typical skin lesions in scabies are disseminated erythematous papules, whose presentation may vary significantly between individuals according to skin type and the inflammatory response to the parasite. The characteristic distribution includes the periumbilical region, waistline, genitals, breasts, buttocks, axillary folds, fingers with involvement of the interdigital spaces, wrists and the extensor surfaces of the limbs. In adults, the head, palms and soles are usually spared, although these areas may be involved in infants and young children.
Burrows and other associated lesions
Burrows are the pathognomonic sign of scabies. They appear as greyish lines approximately half to one centimetre long, often excoriated and with a central haemorrhagic crust, although they are rarely observed in clinical practice. Vesicles, usually at the beginning of the burrows, and firm nodules up to half a centimetre in diameter may coexist, particularly on the genitals, groin and buttocks.
Diagnostic confirmation and empirical diagnosis
Empirical diagnosis is often sufficient to initiate treatment, provided that the clinical and epidemiological picture is compatible. Definitive confirmation may be obtained through positive microscopic examination of skin scrapings, with identification of mites, eggs or parasite fragments. In clinical practice, treatment should not be delayed because of the absence of laboratory confirmation when suspicion is high.
Differential diagnosis
Before initiating treatment, common differential diagnoses should be considered. Atopic dermatitis is characterized by pruritus without nocturnal predominance and absence of burrows. Papular urticaria or strophulus presents with pruritic excoriated papules, predominantly in exposed areas. Infantile acropustulosis presents with non-pruritic lesions confined to the palms and soles. Dermatitis herpetiformis presents as a chronic, symmetrical vesicopustular eruption with persistent pruritus throughout the day. Chickenpox is distinguished by lesions at multiple stages of evolution, associated enanthem and, sometimes, fever. In children with predominantly nodular scabies, diagnoses such as urticaria pigmentosa, Langerhans cell histiocytosis or cutaneous lymphoma should also be considered.
General principles of treatment
Once scabies is confirmed, topical treatment should be applied to the entire body, except the head and neck. In children younger than two years and in older adults, treatment should include the scalp, face and neck, avoiding the mouth and eyes. It is essential that treatment is carried out simultaneously in all household contacts, even if asymptomatic, to prevent reinfestation. Personal clothing and bed linen used in the previous seventy-two hours should be washed at temperatures above fifty to sixty degrees or kept in a sealed plastic bag for at least seventy-two hours. Nail trimming should be recommended. There is no indication to treat domestic animals. Children and adults may return to school or work twenty-four hours after the first treatment.
Treatment options
In neonates younger than one month, sterilized pure petrolatum may be used, applied alternately to the upper and lower halves of the body until clinical improvement. Sulfur six to ten percent in sterilized petrolatum is safe in children younger than two months and should be applied for three consecutive nights, with possible repetition after seven days. Benzyl benzoate, contraindicated in children younger than thirty months, is one of the reference treatments in adults, including pregnant and breastfeeding women, and in children above that age, applied at night on two consecutive days. Permethrin five percent is effective but contraindicated in children younger than two months. Crotamiton, which is less effective, may be useful in nodular scabies and post-scabetic pruritus and is contraindicated in children, pregnancy and breastfeeding. Oral ivermectin is reserved for severe or special cases and should not be used in children weighing less than fifteen kilograms, pregnant women or breastfeeding women.
Crusted scabies
Crusted scabies is a severe variant of scabies, observed mainly in patients with immunosuppression, malnutrition, physical or mental disability, or advanced age. It is characterized by hyperkeratosis with thick scales and crusts, which may affect any area of the body, predominantly the elbows, knees, hands, feet and scalp, sometimes mimicking psoriasis. These patients have a high parasite burden and increased risk of transmission and should be referred for hospital care.
Reassessment, complications and follow-up
At clinical reassessment, ideally after two to four weeks, symptom improvement allows discharge. In the absence of improvement, causes of treatment failure should be considered, such as inadequate application, absence of repeat application, incomplete treatment of contacts or failures in environmental measures. Pruritus may persist for two to four weeks after eradication of the parasite, characterizing post-scabetic pruritus, and should not be interpreted as persistence of infestation. In these cases, oral antihistamines, medium- to high-potency topical corticosteroids, moisturizers and calamine lotions are indicated, always assessing causes of treatment failure before restarting therapy. In cases of secondary bacterial infection, such as impetigo or furunculosis, topical or systemic antibiotics should be instituted according to severity. Scabetic nodules may be treated with potent topical corticosteroids, intralesional corticosteroids, crotamiton or topical calcineurin inhibitors.
