Ink test
The “ink test” is another adjunct to help identify burrows. A nontoxic, watersoluble felt-tip marker is rubbed over an area suspected of having burrows. After waiting a few moments for the ink to sink into the disrupted stratum corneum overlying burrows, the ink is washed off, leaving an ink-demarcated burrow to examine.4 This can be performed as an adjunct to dermoscopy.5
The course of scabies
The mite’s life cycle
There are 4 stages in the mite’s life cycle: egg, larva, nymph, and adult. Female mites deposit 2 to 3 eggs per day as they create their burrows. The eggs are oval, 0.1 to 0.15 mm in length, and hatch in 3 to 8 days. The resultant larvae migrate to the skin surface and burrow into the intact stratum corneum to construct almost invisible, short burrows called molting pouches.
The larvae progress through 2 nymphal stages before a final molt to the adult stage. Larvae and nymphs live in molting pouches or in hair follicles. They appear similar to adults except for smaller size and, during the larval stage, 3 pair of legs. Adult female mites are 0.3 to 0.4 mm long and about 0.25 to 0.35 mm wide, about twice the size of males. Mating occurs when a male penetrates the molting pouch of the adult female. Impregnated females then extend their molting pouches to form the characteristic serpentine burrows, laying eggs in the process. The total period to progress from egg to the gravid female stage takes 10 to 14 days. The impregnated females spend the remaining 2 months of their lives in burrows.3,4
The mites live in and on the stratum corneum, burrowing into but never below the stratum corneum. The burrows appear as raised, serpentine lines varying from a few millimeters up to several centimeters long. Transmission occurs by the transfer of ova-bearing females.3,4
Cause of the rash and itch
The mites do not “bite.” Instead, the hallmark of scabies, when found, are the burrows created by the mites. However, it is common to see a papular urticarial type response as an allergic reaction to antigens associated with the mite itself, its scybala, and eggs. In fact, after acquiring scabies for the first time, itch does not appear for 2 to 6 weeks (average, 3 to 4 weeks) because the host needs to be sensitized to these antigens.4
It is not until the immunologic reactivity or sensitization develops that the host becomes symptomatic and aware of a problem. This requirement for sensitization explains the often gradual onset of itch. The incubation period is important in transmission to other individuals during the asymptomatic phase.3 However, a previously sensitized host may experience itch within hours to days after reinfestation.4
Epidemiology of scabies
Scabies infestations occur in all geographic areas and climates, and affects people of all ages and socioeconomic strata.7 For unexplained reasons, those with African ancestry rarely acquire scabies.7
It is most common in those who have close physical contact with others and, therefore, disproportionately affects children, mothers of young children, sexually active young adults, nursing home populations, and those in crowded living situations. Scabies is commonplace in developing countries. It is possible to acquire scabies after sleeping in unsanitary bedding. The scabies mite does not carry other diseases.7
Crusted scabies
Crusted scabies, a rare form of scabies also known as Norwegian scabies, is an aggressive infestation that usually occurs in immunodeficient, debilitated, or malnourished persons. Crusted scabies, because of the huge mite burden, is associated with greater transmissibility than scabies.7 Interestingly, because of impaired allergic response or indifference to itch, some of these patients may exhibit little pruritus.7