Pathophysiology of Scabies
The parasite is transmitted from human to human via direct body contact: playing children, bed warmth and intercourse are considered the main exposition risks. (All-clear signal for shared textiles like bedding: the infection risk is under 1/200!). After mating, the 0.3 – 0.4 mm large female mites dig into the horny layer of the epidermis, where they form tunnel-like ducts for the deposition of their eggs. A few days later, they die there. 3 weeks after egg deposition, the mite grubs emerge from the eggs and then drill back to the skin surface. The life-cycle repeats.
Symptoms and Diagnosis of Scabies
A severe nocturnal itching is typical for scabies. This itching is caused by the mite antigens, which are released during the decay of the mite bodies. As a clinical correlation of the mite ducts, convoluted and partially palpable skin efflorescences can be found. Later, papules and vesicles develop. Mainly, warm body regions with little horny skin are affected. This includes the mammilla region, the interdigital folds, the male genital, the axillary folds and the wrists. Via scratching, secondary efflorescences like excoriations, crusts, and eczema formation develop. Bacterial superinfections of the skin can occur as a complication.
Characteristic primary and secondary efflorescences at scabies
Primary efflorescences have a comma or dot shape. Straight efflorescences are mostly scratching marks!
|Primary Efflorescences||Secondary Efflorescences|
|Comma-like or irregularly convoluted mite ducts||Crusts|
Source: Robert Koch Institute
Latency between infection and the onset of the symptoms amounts to roughly 3 weeks (see above: grub hatching); thus, one should ask for possibly previous expositions within this period of time in medical history (e.g. vacation). The attack of several persons within a family or a social facility (e.g. kindergarten, hospital wards) is especially suspicious.
For detection of scabies, dermatoscopy (detection of the mite ducts) and microscopy are appropriate measures. Skin samples for microscopic examination are gained via tape pull-off or abrasion.
The massive mite infestation at immune suppressed patients (AIDS, long-term glucocorticoid/cytostatic therapy, leukemia, DM) is referred to as Scabies norvegica. Due to the decreased immune reaction, there is no or only a slight itching. With dirty-brown papules, bark formation and desquamation, the skin is ichthyosiform.
Psychiatric differential diagnosis delusional parasitosis
Especially in older patients with nocturnal pruritus, delusional parasitosis should be excluded. Also in the context of organic psychoses (e.g. at alcohol deprivation delirium or exsiccosis), this chronic tactile hallucinosis often occurs, whereas the patient is convinced that animals or insects live under his skin.
Therapy of Scabies: The Best Medication Has No Effect Without Compliance!
The three main pillars of successful therapy of scabies are hygienic measures, medicamentous therapy, and, most of all, the compliance of the affected person. The first-resort agent is the antiscabietic agent permethrin. As a crème, it is applied to the whole integument including fingernails and the tip of the toes. Further options are e.g. benzylbenzoate, ivermectin and allethrin. In order to avoid an auto-re-infection, clothes, bedding and towels have to be changed and washed daily during therapy.
Important Final Fact about Scabies
Outside of the human body, the mites survive no more than 2 – 4 days at room temperature.