Scabies

General Characteristics of Scabies

Basic features of scabies

• 8-legged mite
• Whitish-brown color
• Round body

Clinically relevant species

Sarcoptes scabiei (var. hominis) causes human scabies.

Epidemiology

• Approximately 200 million cases reported worldwide annually
• A relatively common infestation that can affect individuals of any age or socioeconomic status
• The highest incidence Incidence The number of new cases of a given disease during a given period in a specified population. It also is used for the rate at which new events occur in a defined population. It is differentiated from prevalence, which refers to all cases in the population at a given time. Measures of Disease Frequency occurs in countries with significant poverty.
• Most vulnerable groups:
  • Young children
  • Elderly
  • Immunocompromised immunocompromised A human or animal whose immunologic mechanism is deficient because of an immunodeficiency disorder or other disease or as the result of the administration of immunosuppressive drugs or radiation. Gastroenteritis

Pathogenesis

Reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli

Humans are the primary reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli of S. scabiei.

Transmission

• Direct: person-to-person transmission upon prolonged skin-to-skin contact, usually between family members or sexual partners; casual skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions contact unlikely to result in infestation
• Environmental: Crusted scabies can be transmitted via fomites Fomites Inanimate objects that carry pathogenic microorganisms and thus can serve as the source of infection. Microorganisms typically survive on fomites for minutes or hours. Common fomites include clothing, tissue paper, hairbrushes, and cooking and eating utensils. Adenovirus (scarves, hats, bedding, etc ETC The electron transport chain (ETC) sends electrons through a series of proteins, which generate an electrochemical proton gradient that produces energy in the form of adenosine triphosphate (ATP). Electron Transport Chain (ETC).).

Risk factors

• More common during instances of overcrowding, including:
  • Natural disasters
  • War
• Scabies can be easily transmitted in institutional settings, such as:
  • Psychiatric wards
  • Nursing homes
  • Day-care facilities
• In individuals with compromised cellular immunity Cellular immunity Manifestations of the immune response which are mediated by antigen-sensitized T-lymphocytes via lymphokines or direct cytotoxicity. This takes place in the absence of circulating antibody or where antibody plays a subordinate role. Squamous Cell Carcinoma (SCC), there is a high risk of developing hyperkeratotic/crusted scabies.
• Resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing to first-line treatments is an emerging concern, particularly with permethrin (documented in studies from 2000-2024)

Life cycle

• After mating, the female mites burrow into the stratum corneum Stratum corneum Skin: Structure and Functions.
• Lays 2–3 eggs per day before dying (life span, approximately 4–6 weeks)
• Larvae Larvae Wormlike or grublike stage, following the egg in the life cycle of insects, worms, and other metamorphosing animals. Ascaris/Ascariasis hatch in 3–4 days and reach adulthood in the burrow.
• Mite burden:
  • Low in classic scabies (10–15 mites)
  • High in crusted scabies (up to 2 million mites)
• Mites can survive without a host for approximately 24–36 hours; thus, fomite transmission may occur.

Pathophysiology

The infestation takes the form of:

• Initial infestation:
  • Sensitization occurs over 4–6 weeks.
  • This delay in symptom onset occurs because of a delayed-type hypersensitivity Delayed-type hypersensitivity Type IV hypersensitivity reaction, or delayed-type hypersensitivity, is a cell-mediated response to antigen exposure. The reaction involves T cells, not antibodies, and develops over several days. Presensitized T cells initiate the immune defense, leading to tissue damage. Type IV Hypersensitivity Reaction reaction.
• Reinfestation: occurs if the patient is already sensitized, so pruritus Pruritus An intense itching sensation that produces the urge to rub or scratch the skin to obtain relief. Atopic Dermatitis (Eczema) occurs within 1–3 days.

Clinical Presentation

Key characteristic features

• Severe nocturnal itching 
• Characteristic rash Rash Rocky Mountain Spotted Fever distribution
• Commonly involved areas:
  • Interdigital folds
  • Flexor aspects of the wrists
  • Axillary folds
  • Extensor aspects of the elbows and knees
  • Male genitalia
  • Periareolar skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions (especially in women) 
  • Periumbilical skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions
  • Waist
  • Inferior portions of buttocks and adjacent thighs
  • Lateral and posterior aspects of the feet
• Areas usually spared in classic scabies: head (except in young children) and back
• Scratching may result in:
  • Excoriations Excoriations Excoriation is a linear abrasion produced by mechanical means (scratching, rubbing, or picking) that usually involves only the epidermis but can reach the papillary dermis. Secondary Skin Lesions
  • Crusts
  • Eczema Eczema Atopic dermatitis, also known as eczema, is a chronic, relapsing, pruritic, inflammatory skin disease that occurs more frequently in children, although adults can also be affected. The condition is often associated with elevated serum levels of IgE and a personal or family history of atopy. Skin dryness, erythema, oozing, crusting, and lichenification are present. Atopic Dermatitis (Eczema)
  • Bacterial superinfections of the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions can occur as a complication.

Classic scabies

• Pruritus Pruritus An intense itching sensation that produces the urge to rub or scratch the skin to obtain relief. Atopic Dermatitis (Eczema), usually worse at night
  • Delayed-type hypersensitivity Delayed-type hypersensitivity Type IV hypersensitivity reaction, or delayed-type hypersensitivity, is a cell-mediated response to antigen exposure. The reaction involves T cells, not antibodies, and develops over several days. Presensitized T cells initiate the immune defense, leading to tissue damage. Type IV Hypersensitivity Reaction reaction to the mite, mite feces, and mite eggs
  • For primary infestation, symptoms begin in 3–6 weeks.
  • For previously infested individuals, symptoms begin within 1–3 days.
• Cutaneous findings:
  • Multiple small, erythematous papules that are often excoriated 
  • Distribution is rarely localized to a single area.
  • Burrows Burrows Dermatologic Examination (2–15 mm; thin, gray, or brown lines) are often not visible due to excoriation Excoriation Mastitis or secondary infection.
  • Miniature wheals, vesicles Vesicles Female Genitourinary Examination, pustules, and, rarely, bullae Bullae Erythema Multiforme also may be present.
• Nodular scabies:
  • A less common manifestation of classic scabies
  • Present as pruritic, firm, and erythematous papules measuring 5–6 mm in diameter
  • Usual sites involved: axillary folds, groin Groin The external junctural region between the lower part of the abdomen and the thigh. Male Genitourinary Examination, genitalia, buttocks
  • May represent a focally heightened hypersensitivity reaction

Crusted scabies

• Also called scabies crustosa or Norwegian scabies:
  • Occurs in conditions that compromise cellular immunity Cellular immunity Manifestations of the immune response which are mediated by antigen-sensitized T-lymphocytes via lymphokines or direct cytotoxicity. This takes place in the absence of circulating antibody or where antibody plays a subordinate role. Squamous Cell Carcinoma (SCC) (AIDS, cytostatic therapy, leukemia), and with long-term topical glucocorticoids Glucocorticoids Glucocorticoids are a class within the corticosteroid family. Glucocorticoids are chemically and functionally similar to endogenous cortisol. There are a wide array of indications, which primarily benefit from the antiinflammatory and immunosuppressive effects of this class of drugs. Glucocorticoids; also seen in older adults and in patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship with Down syndrome Down syndrome Down syndrome, or trisomy 21, is the most common chromosomal aberration and the most frequent genetic cause of developmental delay. Both boys and girls are affected and have characteristic craniofacial and musculoskeletal features, as well as multiple medical anomalies involving the cardiac, gastrointestinal, ocular, and auditory systems. Down syndrome (Trisomy 21)
  • Massive mite infestation
  • Poorly defined, erythematous patches Patches Vitiligo that quickly develop prominent scaling
  • Scales Scales Dry or greasy masses of keratin that represent thickened stratum corneum. Secondary Skin Lesions may become warty; crusts and fissures may also appear.
  • Occurs most prominently on the scalp, hands, and feet 
  • Nails are often thickened, discolored, and dystrophic.
• Pruritus Pruritus An intense itching sensation that produces the urge to rub or scratch the skin to obtain relief. Atopic Dermatitis (Eczema) may be minimal or absent.

Diagnosis

Diagnosis is confirmed through the detection of scabies mites, eggs, or fecal pellets (scybala) through microscopic examination. However, diagnosis can be clinically based on history and physical examination findings.

• History and physical exam
  • Widespread itching that is worse at night and spares the head and back
  • Characteristic pruritic lesions and distribution
  • Other household members or sexual contacts with similar symptoms
• Scabies preparation
  • Scraping: Apply mineral oil to the skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions and vigorously scrape the surface with a number-15 scalpel blade. Add potassium Potassium An element in the alkali group of metals with an atomic symbol k, atomic number 19, and atomic weight 39. 10. It is the chief cation in the intracellular fluid of muscle and other cells. Potassium ion is a strong electrolyte that plays a significant role in the regulation of fluid volume and maintenance of the water-electrolyte balance. Hyperkalemia hydroxide (KOH) if crusted scabies present, then view under the microscope.
  • Adhesive tape test: Use strong transparent (packing-type tape), remove rapidly, then apply to a microscope slide and view under the microscope.
  • Classic scabies has a low mite count, so a negative scabies preparation does not rule out infestation. Sensitivity of scabies preparations: 40%–90%; specificity: 100%
• Burrow ink test:
  • Blue or black ink is applied to a suspected lesion.
  • Alcohol is used to remove surface ink.
  • A characteristic zigzag line that runs across and away from the lesion indicates a mite burrow.
• Dermoscopic examination:
  • Delta wing sign: a dark triangular shape that represents the head of the mite within a burrow
  • It is less specific than scabies preparations.
• Labs: Blood tests are not indicated. However, eosinophilia Eosinophilia Abnormal increase of eosinophils in the blood, tissues or organs. Autosomal Dominant Hyperimmunoglobulin E Syndrome has been reported.

Occasionally, scabies may be misdiagnosed as atopic dermatitis Dermatitis Any inflammation of the skin. Atopic Dermatitis (Eczema). This often occurs due to the appearance of the erythematous papules. Patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship who are misdiagnosed will have initial symptom alleviation with topical glucocorticoids Glucocorticoids Glucocorticoids are a class within the corticosteroid family. Glucocorticoids are chemically and functionally similar to endogenous cortisol. There are a wide array of indications, which primarily benefit from the antiinflammatory and immunosuppressive effects of this class of drugs. Glucocorticoids; however, this will not treat the underlying cause (i.e., mite infestation).

Management

General approach

2 pillars of successful therapy in scabies:

• Hygienic measures
• Medication therapy and compliance Compliance Distensibility measure of a chamber such as the lungs (lung compliance) or bladder. Compliance is expressed as a change in volume per unit change in pressure. Veins: Histology

Hygienic measures

Decontaminate bedding, towels, and clothing via:

• Washing in hot water
• Dry cleaning
• Sealing in a plastic bag for ≥ 72 hours

Medication

• 1st-line treatment in adults:
  • Classic scabies: topical permethrin 5%, though resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing is increasingly reported
  • Crusted scabies: combination therapy with topical permethrin 5% and oral ivermectin Ivermectin A mixture of mostly avermectin h2b1a (rn 71827-03-7) with some avermectin h2b1b (rn 70209-81-3), which are macrolides from streptomyces avermitilis. It binds glutamate-gated chloride channel to cause increased permeability and hyperpolarization of nerve and muscle cells. It also interacts with other chloride channels. It is a broad spectrum antiparasitic that is active against microfilariae of onchocerca volvulus but not the adult form. Anthelmintic Drugs (often requiring multiple doses of ivermectin Ivermectin A mixture of mostly avermectin h2b1a (rn 71827-03-7) with some avermectin h2b1b (rn 70209-81-3), which are macrolides from streptomyces avermitilis. It binds glutamate-gated chloride channel to cause increased permeability and hyperpolarization of nerve and muscle cells. It also interacts with other chloride channels. It is a broad spectrum antiparasitic that is active against microfilariae of onchocerca volvulus but not the adult form. Anthelmintic Drugs)
• 1st-line treatment in children: topical permethrin and antibiotic therapy (if secondary scabies lesions become infected)
• Second application of permethrin 1–2 weeks later may be necessary to eliminate mites.
• Alternative treatments for resistant cases: benzyl benzoate 25% lotion and sulfur 5-10% ointment (which has seen renewed use due to permethrin resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing)
• Nodules that persist after eradication of mites can be treated with potent topical corticosteroids Corticosteroids Chorioretinitis or intralesional corticosteroid injections.
• Pruritus Pruritus An intense itching sensation that produces the urge to rub or scratch the skin to obtain relief. Atopic Dermatitis (Eczema):
  • May increase after the introduction of a scabicidal agent.
  • Oral antihistamines Antihistamines Antihistamines are drugs that target histamine receptors, particularly H1 and H2 receptors. H1 antagonists are competitive and reversible inhibitors of H1 receptors. First-generation antihistamines cross the blood-brain barrier and can cause sedation. Antihistamines such as hydroxyzine Hydroxyzine A histamine h1 receptor antagonist that is effective in the treatment of chronic urticaria, dermatitis, and histamine-mediated pruritus. Unlike its major metabolite cetirizine, it does cause drowsiness. It is also effective as an antiemetic, for relief of anxiety and tension, and as a sedative. Antihistamines hydrochloride or diphenhydramine Diphenhydramine A histamine h1 antagonist used as an antiemetic, antitussive, for dermatoses and pruritus, for hypersensitivity reactions, as a hypnotic, an antiparkinson, and as an ingredient in common cold preparations. It has some undesired antimuscarinic and sedative effects. Antihistamines hydrochloride help with severe pruritus Pruritus An intense itching sensation that produces the urge to rub or scratch the skin to obtain relief. Atopic Dermatitis (Eczema).

Prevention

• Changing and washing clothes, bedding, and towels on a daily basis
• Topical permethrin for family and close personal contacts, including cohabitants or those with prolonged skin-to-skin contact in the preceding 6 weeks
• Thorough cleaning and vacuuming of rooms used by the affected individual(s)
• Those with classic scabies can return to school or work after the proper treatment has been given.
• In endemic communities with prevalence Prevalence The total number of cases of a given disease in a specified population at a designated time. It is differentiated from incidence, which refers to the number of new cases in the population at a given time. Measures of Disease Frequency ≥10%, WHO recommends consideration of mass Mass Three-dimensional lesion that occupies a space within the breast Imaging of the Breast drug administration (MDA) strategies using 2 doses of ivermectin Ivermectin A mixture of mostly avermectin h2b1a (rn 71827-03-7) with some avermectin h2b1b (rn 70209-81-3), which are macrolides from streptomyces avermitilis. It binds glutamate-gated chloride channel to cause increased permeability and hyperpolarization of nerve and muscle cells. It also interacts with other chloride channels. It is a broad spectrum antiparasitic that is active against microfilariae of onchocerca volvulus but not the adult form. Anthelmintic Drugs (200 μg/kg) for population-level control, and permethrin 5% cream for those with contraindications Contraindications A condition or factor associated with a recipient that makes the use of a drug, procedure, or physical agent improper or inadvisable. Contraindications may be absolute (life threatening) or relative (higher risk of complications in which benefits may outweigh risks). Noninvasive Ventilation to ivermectin Ivermectin A mixture of mostly avermectin h2b1a (rn 71827-03-7) with some avermectin h2b1b (rn 70209-81-3), which are macrolides from streptomyces avermitilis. It binds glutamate-gated chloride channel to cause increased permeability and hyperpolarization of nerve and muscle cells. It also interacts with other chloride channels. It is a broad spectrum antiparasitic that is active against microfilariae of onchocerca volvulus but not the adult form. Anthelmintic Drugs.

Complications

• Secondary bacterial skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease including  impetigo Impetigo Impetigo is a highly contagious superficial bacterial infection typically caused by Staphylococcus aureus (most common) and Streptococcus pyogenes. Impetigo most commonly presents in children aged 2 to 5 years with lesions that evolve from papules to vesicles to pustules, which eventually break down to form characteristic “honey-colored” crusts. Impetigo, ecthyma Ecthyma An ulcerative pyoderma usually caused by group a beta-hemolytic streptococcal infection at the site of minor trauma. Impetigo, and furunculosis
• Psychological issues including embarrassment and delusion Delusion A false belief regarding the self or persons or objects outside the self that persists despite the facts, and is not considered tenable by one’s associates. Delusional Disorder of parasitosis

Differential Diagnosis

• Atopic dermatitis Dermatitis Any inflammation of the skin. Atopic Dermatitis (Eczema): sometimes called allergic eczema Eczema Atopic dermatitis, also known as eczema, is a chronic, relapsing, pruritic, inflammatory skin disease that occurs more frequently in children, although adults can also be affected. The condition is often associated with elevated serum levels of IgE and a personal or family history of atopy. Skin dryness, erythema, oozing, crusting, and lichenification are present. Atopic Dermatitis (Eczema); chronic pruritic inflammatory skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions disease that occurs most frequently in children, but also in adults. Often associated with elevated IgE IgE An immunoglobulin associated with mast cells. Overexpression has been associated with allergic hypersensitivity. Immunoglobulins: Types and Functions and a personal or family history Family History Adult Health Maintenance of atopy Atopy Atopic Dermatitis (Eczema)
• Insect bites: It is important to rule out insect bites as an underlying etiology. Take into consideration the patient’s history and physical exam, symptom onset, and affected close contacts. 
• Dermatitis herpetiformis Dermatitis herpetiformis Rare, chronic, papulo-vesicular disease characterized by an intensely pruritic eruption consisting of various combinations of symmetrical, erythematous, papular, vesicular, or bullous lesions. The disease is strongly associated with the presence of hla-b8 and hla-dr3 antigens. A variety of different autoantibodies has been detected in small numbers in patients with dermatitis herpetiformis. Celiac Disease: autoimmune cutaneous eruption associated with gluten Gluten Prolamins in the endosperm of seeds from the triticeae tribe which includes species of wheat; barley; and rye. Celiac Disease sensitivity. Affected patients Patients Individuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures. Clinician–Patient Relationship typically develop intensely pruritic inflammatory papules and vesicles Vesicles Female Genitourinary Examination on the forearms Forearms Part of the upper extremity in humans and primates extending from the elbow to the wrist. Bowen Disease and Erythroplasia of Queyrat, knees, scalp, or buttocks. Skin biopsy Skin Biopsy Secondary Skin Lesions shows granular deposits of IgA IgA Represents 15-20% of the human serum immunoglobulins, mostly as the 4-chain polymer in humans or dimer in other mammals. Secretory iga is the main immunoglobulin in secretions. Immunoglobulins: Types and Functions in the dermal papillae Papillae Lips and Tongue: Anatomy.
• Bullous pemphigoid Bullous pemphigoid Bullous pemphigoid and pemphigus vulgaris are two different blistering autoimmune diseases. In bullous pemphigoid, autoantibodies attack the hemidesmosomes, which connect epidermal keratinocytes to the basement membrane. This attack results in large, tense subepidermal blisters. Bullous Pemphigoid and Pemphigus Vulgaris: autoimmune blistering disorder usually seen in older adults. Associated with a prodrome Prodrome Symptoms that appear 24–48 hours prior to migraine onset. Migraine Headache of pruritic, urticaria-like skin Skin The skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue. Skin: Structure and Functions lesions followed by tense bullae Bullae Erythema Multiforme on an erythematous urticarial base. Skin biopsy Skin Biopsy Secondary Skin Lesions shows linear deposition of IgG IgG The major immunoglobulin isotype class in normal human serum. There are several isotype subclasses of igg, for example, igg1, igg2a, and igg2b. Hypersensitivity Pneumonitis and/or C3 along the basement membrane Basement membrane A darkly stained mat-like extracellular matrix (ecm) that separates cell layers, such as epithelium from endothelium or a layer of connective tissue. The ecm layer that supports an overlying epithelium or endothelium is called basal lamina. Basement membrane (bm) can be formed by the fusion of either two adjacent basal laminae or a basal lamina with an adjacent reticular lamina of connective tissue. Bm, composed mainly of type IV collagen; glycoprotein laminin; and proteoglycan, provides barriers as well as channels between interacting cell layers. Thin Basement Membrane Nephropathy (TBMN).