Dermatologic Manifestations of Scabies Clinical Presentation

The historical aspects of scabies infestations are quite reliable in suggesting the diagnosis. Lesion distribution, intractable pruritus that is worse at night, and similar symptoms in close contacts should immediately rank scabies at the top of the clinical differential diagnosis.

Lesion distribution differs in adults and children. Adults manifest lesions primarily on the flexor aspects of the wrists, the interdigital web spaces of the hands, the dorsal feet, axillae, elbows, waist, buttocks, and genitalia. Pruritic papules and vesicles on the scrotum and penis in men and areolae in women are highly characteristic.

Infants and small children may develop lesions diffusely, but unlike adults, lesions are common on the face, scalp, neck, palms, and soles. All cutaneous sites are susceptible in immunocompromised and elderly patients, who often have a history of a widespread, pruritic eczematous eruption.

Consider the diagnosis of scabies in any patient presenting with a recent onset of intense itching that is accentuated at night.

Clinical findings include both primary and secondary lesions. Primary lesions are the first manifestation of the infestation, and these typically include small papules, vesicles, and burrows. Secondary lesions are the result of rubbing and scratching, and they may be the only clinical manifestation of the disease. If so, the diagnosis must be inferred by the history, lesion distribution, and accompanying symptoms.

Primary scabies lesions

The distribution is highly characteristic in typical cases.

Burrows are a pathognomonic sign and represent the intraepidermal tunnel created by the moving female mite. They appear as serpiginous, grayish, threadlike elevations ranging from 2-10 millimeters long, as seen in the image below.

They are not readily apparent and must be actively sought. A black dot may be seen at one end of the burrow, indicating the presence of a mite. High-yield locations for burrows include the webbed spaces of the fingers, flexor surfaces of the wrists, elbows, axillae, belt line, feet, scrotum in men, and areolae in women. In infants, burrows are commonly located on the palms and soles, as in the image below. The actual mites are microscopic and cannot be visualized with the unaided human eye.

One- to 3-mm erythematous papules and vesicles are seen in typical distributions in adults. The vesicles are discrete lesions filled with clear fluid, although the fluid may appear cloudy if the vesicle is more than a few days old, as in the image below.

Papules rarely contain mites and most likely represent a hypersensitivity reaction. Papules are common on the shaft of the penis in men and on the areolae in women, as in the images below.

Unlike adults, who rarely present with facial and neck involvement, this presentation is fairly typical in children.

In very young children and infants, a widespread eczematous eruption primarily on the trunk is common, as in the image below.

In neonates unable to scratch, pinkish-brown nodules may develop and range in size from 2-20 mm, as demonstrated in the images below. Mites are rarely found within the nodules. Infants may have 1 to 3-mm papules, vesicles, and pustules on the palms and soles.

Crusted scabies, previously referred to as Norwegian scabies, manifests with marked thickening and crusting of the skin. Lesions are often hyperkeratotic, crusted, and cover large areas. Marked scaling is common, and pruritus may be minimal or absent. Nail dystrophy and scalp lesions may be prominent. The hands and arms are usual locations, but all sites are vulnerable, as demonstrated in the images below. Mites can number in the thousands to millions in this form. Predominantly affected are those with immunosuppression, neurological disorders, or institutionalization. Possible risk factors for profound infestation in these specific populations include an inability to mount an immune response, perceive pruritus, and/or physically scratch the skin (a mechanism to rid the body of mites).

Nodular scabies occurs in 7-10% of patients with scabies, particularly young children as noted above. Pink, tan, brown, or red nodules may be present, ranging from 2-20 millimeters in diameter.

Secondary scabies lesions

These are the result of scratching, secondary infection, and/or the host immune response against the mites and their products.

Characteristic findings include excoriations, widespread eczema, honey-colored crusting, postinflammatory hyperpigmentation, erythroderma, prurigo nodules, and frank pyoderma.^{[3, 4]}

Human scabies is caused by the host-specific mite, S scabiei var hominis, an obligate human parasite. It is a member of the class Arachnida, subclass Acari, order Astigmata, and family Sarcoptidae.

Human infestation with S scabiei varieties of animal origin can occur. Both domestic and wild animals worldwide are susceptible to infestation with S scabiei, and the resultant disease is referred to as sarcoptic mange. Mange due to S scabiei varieties other than hominis has been reported in dogs, pigs, horses, camels, black bears, monkeys, dingoes, and wild fox, among others. Although reports have described transfer to humans from animals, experimental studies have demonstrated limited cross-infectivity between different host species. Further, genotyping studies have revealed that the Sarcoptes mites segregate into separate host-associated populations, thus limiting the transmission across host species.

In the rare instance of transmission of nonhuman scabies from animals to humans, the clinical manifestations differ in many respects. The incubation period is shorter, the symptoms are transient, the infestation is self-limiting, no burrows are formed, and the distribution is atypical compared with infestation caused by S scabiei var hominis. Contacts of patients with scabies contracted from an animal source require no treatment.