Table of Contents
Overview of Scabies Mites
Scabies is a contagious skin disease caused by tiny parasitic mites. In humans, the typical causative agent is the itch mite Sarcoptes scabiei var. hominis. These mites belong to the arachnids (like spiders and ticks) and are specially adapted to living in the top layer of human skin.
Scabies is found worldwide and affects people of all ages and social backgrounds. Crowded living conditions and close skin contact favor its spread, but it can also occur in families, schools, hospitals, and nursing homes.
The Causative Agent: *Sarcoptes scabiei* var. *hominis*
Morphology and Life Form
Scabies mites are:
- Extremely small: about 0.3–0.5 mm long (barely visible as tiny dots).
- Oval to round, flattened body.
- 8-legged (typical of arachnids) in adults; larvae have 6 legs.
- Light-colored, with short bristles and special mouthparts adapted for burrowing and feeding on skin material and tissue fluid.
They live exclusively in the outermost skin layer, the epidermis (especially in the stratum corneum), and cannot penetrate into deeper tissues.
Life Cycle on and in the Skin
The entire life cycle usually takes place on one host:
- Fertilization
- Mating occurs on the skin surface.
- After fertilization, the male soon dies; the fertilized female is responsible for infestation.
- Burrowing and Egg Laying
- The female burrows tunnels in the stratum corneum.
- In these “burrows” she lays 2–3 eggs per day, up to a total of about 30–50 eggs.
- She can survive on the host for several weeks.
- Larvae and Nymphs
- After 2–4 days, 6‑legged larvae hatch from the eggs.
- The larvae move to the skin surface, find small depressions (e.g., hair follicles), and molt into 8‑legged nymphs.
- Through several molts they develop into sexually mature adult mites.
- Duration
- Entire development from egg to adult: roughly 2–3 weeks.
- Off the host, mites usually die within a few days (faster in heat and dryness, slower in cool, humid environments).
Transmission and Host Range
Human-to-Human Transmission
The main mode of transmission is prolonged, close skin‑to‑skin contact, for example:
- Sharing a bed (parents–children, partners).
- Caring activities (nursing, physical assistance).
- Close play among children.
Brief, casual contact (e.g., a handshake) usually does not suffice for transmission.
Role of the Environment and Objects
In classic scabies (normal infestation):
- Transmission via clothing, bedding, or furniture is possible but rare, because mites quickly lose viability away from the human body.
In severe forms with heavy mite load (e.g., crusted scabies, see below):
- Many mites can be shed into the environment.
- Contamination of bedding, couches, and clothing becomes more relevant.
- Transmission via contaminated textiles and surfaces is then much more likely.
Animal Mites and “Pseudo‑Scabies”
Mites that parasitize animals (e.g., dogs, pigs, horses) can also be transmitted to humans. This leads to:
- Pseudo‑scabies (animal scabies): an itchy skin reaction triggered when animal mites accidentally infest humans.
- These animal mites usually:
- Cannot complete their life cycle on humans.
- Survive only for a limited time.
- Do not produce typical long burrows.
Symptoms often appear on body parts in direct contact with the animal (e.g., forearms, legs). The infestation usually resolves once contact with the infested animal is broken and/or the animal is treated.
Pathogenesis: How Mites Cause Disease
Mechanical Damage
- Female mites burrow within the stratum corneum, forming fine tunnels.
- They ingest skin material and tissue fluid, weakening the skin barrier locally.
- Burrowing and feeding cause microinjuries but only minimal direct tissue destruction.
Immune Response and Allergic Reaction
The main symptoms are not due to the bite itself, but to the immune reaction against:
- Mite saliva.
- Fecal pellets.
- Mite body components (antigens).
After a first infestation:
- It can take 2–6 weeks for the immune system to become sensitized.
- Only then does intense itching typically develop.
Upon re‑infestation:
- The immune system reacts faster.
- Symptoms can appear within a few days.
The characteristic severe itching is thus largely an allergic‑type reaction of the host.
Crusted (Norwegian) Scabies
In individuals with weakened immune systems or severely reduced sensation (e.g., certain neurological diseases), a special form may develop:
- Crusted scabies:
- Very thick, crusty, scaly plaques on the skin.
- Infestation with millions of mites.
- Often reduced or absent itching.
Because many mites are present, this form is highly contagious and plays a major role in outbreaks in institutions.
Clinical Picture
Typical Symptoms
Key features of classic scabies include:
- Intense itching, often:
- Worse at night and in warmth (bed).
- Leading to scratching and secondary skin damage.
- Mite burrows:
- Thin, slightly raised, winding or straight lines.
- A few millimeters long.
- At the end of the burrow, a small vesicle or papule may be seen where the mite resides.
- Scratching lesions:
- Excoriations (scratch marks).
- Crusts, sometimes small bacterial infections due to skin barrier damage.
Typical Body Sites
Scabies mites prefer warm, thin, and sheltered skin areas. In adults, typical sites include:
- Between the fingers (finger webs).
- Flexor sides of the wrists.
- Elbows, armpits.
- Around the navel, waistline.
- Genital area (penis, scrotum), buttocks.
- Areas under tight clothing or jewelry.
In infants and small children, additional sites may be affected:
- Palms and soles.
- Face and scalp (which are usually spared in adults).
Complications
Untreated scabies can lead to:
- Secondary bacterial infections:
- Impetigo, abscesses, cellulitis from scratching.
- In some regions: post‑infectious complications due to certain bacteria (e.g., glomerulonephritis after streptococcal infections).
- Sleep disturbances because of severe night‑time itching.
- Psychological stress and social stigma.
Crusted scabies carries an especially high risk of bacterial superinfection and rapid spread in communal settings.
Diagnosis
Clinical Assessment
Doctors often suspect scabies based on:
- Typical symptoms (night‑time itching, distribution of lesions, affected contacts).
- Visible signs such as burrows, papules, and scratch marks.
A precise medical history (anamnesis) is important, especially:
- Onset and duration of itching.
- Other family members or close contacts with similar complaints.
- Residence in communal facilities or recent stays in hospitals, shelters, etc.
- Animal contacts in suspected pseudo‑scabies.
Direct Detection of Mites
To confirm the diagnosis, mites or their products can be identified:
- Skin scrapings:
- The skin is gently scraped at the tip of a suspected burrow.
- Material is placed on a glass slide and examined under a microscope for mites, eggs, or fecal pellets.
- Adhesive tape method:
- Transparent tape is applied to a lesion and then examined microscopically.
- Dermatoscopy:
- With a special magnifying device, the burrow and the mite at its end (often described as a small “triangle” or “delta”) can be visualized.
A negative test does not exclude scabies, particularly in mild cases. Therefore, diagnosis is often primarily clinical.
Treatment of Scabies
Principles of Therapy
Effective control of scabies requires:
- Killing the mites (and as far as possible their developing stages).
- Treating all affected individuals and close contacts at the same time to prevent re‑infestation.
- Environmental hygiene measures adapted to the severity of the infestation.
- Symptom relief, especially relief of itching and treatment of secondary infections.
Antiparasitic Medications
Commonly used drugs (exact preparations and dosing are determined by medical professionals):
- Topical agents (applied to the skin):
- Usually creams or lotions containing agents that are toxic to mites.
- Applied to the entire body (in infants, including scalp, as advised by a physician), often from the jawline downward.
- Left on for a certain number of hours, then washed off.
- Sometimes repeated after about a week to catch newly hatched mites.
- Systemic therapy (oral medication):
- Used in some regions especially for:
- Crusted scabies.
- Large outbreaks.
- Patients in whom topical treatment is difficult.
- The drug is taken by mouth and distributed via the bloodstream to the skin.
Treatment of Contacts and Outbreak Control
- All close contacts (household members, regular sexual partners, close caregivers) should be treated simultaneously, even if they have no symptoms yet, because of the long incubation period.
- In communal institutions (e.g., nursing homes, shelters):
- Coordinated treatment of residents and staff may be necessary.
- Temporary isolation measures can be considered for patients with crusted scabies.
Environmental Measures
In classic scabies:
- Normal cleaning is usually sufficient:
- Wash worn clothing, bed linen, and towels on the treatment day at sufficiently high temperatures.
- Items that cannot be washed may be stored in sealed bags for several days (until mites die off).
In crusted scabies:
- More extensive measures are needed because of the high mite burden:
- Thorough vacuuming of furniture and mattresses.
- Frequent change and hot washing of textiles.
- Disinfection of frequently touched surfaces as recommended by health authorities.
Symptom Relief and Care of the Skin
- Itching can persist for some time even after successful mite eradication (post‑scabietic itch) because the immune system still reacts to mite antigens remaining in the skin.
- Measures:
- Skin‑soothing lotions and creams.
- Treatment of eczema‑like changes and secondary infections if present.
- In severe cases, anti‑itch medications may be used under medical supervision.
Prevention and Public Health Aspects
Individual Prevention
- Avoid long, close skin contact with individuals known to have untreated scabies.
- Avoid sharing clothes and bedding with individuals who are infected or suspected to be infected.
- Seek medical evaluation early in case of persistent nocturnal itching, particularly if other household members have similar symptoms.
Institutional and Community Measures
Because scabies mites are highly adapted to spreading via close contact, they are important in:
- Nursing homes.
- Hospitals.
- Shelters and refugee accommodations.
- Childcare facilities and schools.
Important strategies include:
- Early recognition and reporting of suspected cases.
- Systematic examination of contact persons.
- Coordinated treatment campaigns for all affected and close contacts.
- Clear information to staff, patients, and families to reduce fear and stigma.
In the special case of crusted scabies:
- Rapid isolation of the affected person.
- Intensive environmental decontamination.
- Often repeated systemic and topical treatments under medical supervision.
Scabies is thus a parasitic skin disease in which mites are the direct causative agents, but the characteristic disease picture is mainly determined by the human immune response to these mites. Knowledge of the mite’s biology and transmission routes is crucial for effective diagnosis, therapy, and prevention, especially in settings where many people live or are cared for together.