Scabies is a contact infectious disease caused by human itch mites. Scabies is easily spread through close contacts, and transmission through animals and contaminants is also present. The main risk factor is crowded environments, such as schools, shelters, barracks, and some farmhouses, but there is no clear link with poor sanitation.
Scabies is caused by male and female itch mites or fertilized female itch mites parasitizing human skin. Itch mites are flat, oval, yellowish white, with four pairs of feet on the ventral side. Male itch mites are about 0.2 - 0.25mm long, and females are about 0.3 - 0.4mm long. Itch mites are eggs, larvae, nymphs, and adults during their different life stages. Males often die shortly after mating, and females have a long life span of up to 2 months. After conception, the females burrow into the epidermal stratum corneum, ovulate while migrating, and dig a tunnel. they can dig 0.5 - 5mm long per day. After the females finished ovulation in the tunnel, they die at the end of the tunnel. It takes about 3 - 4 days for the eggs to hatch into larvae, and it takes about 10 days for the larvae to transform into adults. The life span of adults is about 3 - 5 weeks. Adults are more active at night and can survive for 2 - 3 days after leaving the human body.
Figure 1 itch mite
Signs and Symptoms
The characteristic manifestations of typical scabies are sinus tracts, red papules, and generalized pruritus, including uninfected skin, predominantly at night. The sinus tracts are creeping, few millimeters in length in the epidermis, mostly in the interdigital areas, flexion side of the wrist and elbow, genitals, armpits, umbilical cord, nipples, and buttocks. Patients usually present with secondary papules, pustules, vesicles, and decrustation.
Figure 2 typical scabies
Pruritus is caused by delayed hypersensitivity of the skin to itch mites and their saliva, eggs, or feces. Therefore, symptoms can be delayed for up to 3 weeks after infection, and last for several days after recovery. The secondary lesions are more pronounced than the sinus tracts.
Infant scabies occurs mostly on the armpits, head, face, and diaper area, occasionally on the palms and soles, and can be manifested by vesicles, pustules, and nodules. The nodules in some infants resemble urticaria pigmentosa. Secondary group A streptococcus or Staphylococcus aureus infection may occur, and erosions and yellow scabs can be present.
Figure 3 infant scabies
Nodular scabies is characterized by nodules with a diameter of 2 - 20 mm, with severe pruritus, mostly in the genitals, buttocks, groin, and armpits. The nodules are reddish brown, do not contain itch mites, are caused by strong hypersensitivity reactions of local skin to the products of itch mites, and can last for several weeks after treatment. Some patients need glucocorticoid injections.
Figure 4 nodular scabies
Norwegian scabies, also known as crusted scabies, is more contagious, more common in patients with immunosuppressive treatments or mental retardation.
Norwegian scabies is manifested by psoriatic dermatitis with varying degrees of white desquamation, mainly on the extremities, but also on the scalp, face, neck, and buttocks. Subungual areas may be involved, and thickening and onychodystrophy may occur. About half of patients do not present obvious pruritus.
Figure 5 Norwegian scabies
Bullous scabies is less common, and the lesions are similar to bullous pemphigoid, mostly in the arms, calves, and trunk, occasionally in the genitals, groin, thighs, buttocks, neck, and feet.
Figure 6 bullous scabies
Pityriasis rosea-like scabies
Scaly plaques and annular plaques are on the chest, abdomen, and back, resembling pityriasis rosea in morphology and distribution. Some skin lesions are distributed along the skin tension lines.
Figure 7 pityriasis rosea-like scabies
Subcorneal pustular scabies
Red, annular, creeping, vesicular, pustular lesions, pruritic scars, and desquamative papules occur in the armpits and trunk, clinically resembling subcorneal pustular dermatosis, and desquamation can also be seen in other areas.
Figure 8 subcorneal pustular scabies
If there are clinical manifestations, specifically severe pruritus at night, sinus tracts, and itch mites found in microscopy, the disease can be diagnosed.
Topical permethrin or lindane can be applied to the old children and adults, and shower is necessary in 8 - 14 hours. Permethrin is often the first choice because lindane is neurotoxic. This treatment lasts for 7 days.
Topical permethrin can also be applied to infants and young children, but periorbital and perioral areas should be avoided.
6% - 10% sulfur cream is safe and effective for continuous use for 24 hours to 3 days.
Ivermectin is used for patients who do not respond to topical treatment or who cannot insist topical treatment or who are immunosuppressed against Norwegian scabies.
Close contacts also need treatment. Personal items, such as towels, clothes, bedclothes, should be dried in a heated dryer.
Pruritus can be treated with topical hormone ointment or oral antihistamines.
Patients with exudations and yellow scabs should be treated with systemic or topical anti-staphylococcal and anti-streptococcal antibiotics.
Although the itch mites are eliminated, the symptoms and lesions subside within 3 weeks.