Module 35: Dermoscopy in General Dermatology -- Infectious Diseases
1. Learning Objectives
After completing this module, the learner should be able to:
- Define the term entomodermoscopy and explain how dermoscopy extends beyond tumor diagnosis to the identification of infectious and parasitic skin conditions.
- Recognize the pathognomonic "jet with contrail" sign (delta-wing jet sign) of scabies and describe the dermoscopic anatomy of the mite burrow.
- Identify the dermoscopic hallmarks of molluscum contagiosum -- polylobular white-to-yellow amorphous structures with surrounding crown (corona) vessels -- and distinguish additional vascular patterns (radial, punctiform).
- Differentiate cutaneous warts from calluses, corns, and melanoma using dermoscopy, including the frog spawn/mosaic pattern (thrombosed capillaries), and describe how dermoscopic features vary across common warts, palmo-plantar warts, facial-plane warts, and genital warts.
- Distinguish vital nits from empty nits, dead nits, and pseudonits on dermoscopy in cases of pediculosis (head lice and pubic lice).
- Describe the dermoscopic features of tick bites (Ixodes ricinus), tungiasis (Tunga penetrans), cutaneous larva migrans, and myiasis, and explain how dermoscopy aids in confirming diagnosis and guiding management for each.
- Recognize the dermoscopic patterns of tinea nigra (pigmented spicules/wispy lines not respecting dermatoglyphics) and tinea corporis with vellus hair involvement (brown spots with white-yellowish halos), and explain their clinical significance in avoiding misdiagnosis.
- Apply the dermoscopic features to construct a systematic differential diagnosis across viral, bacterial, fungal, and ectoparasitic skin infections.
2. Prerequisites
| Module | Title | Relevance |
|---|---|---|
| Module 01 | Introduction and Principles of Dermoscopy | Equipment handling, polarized vs nonpolarized dermoscopy, contact vs noncontact technique, image capture fundamentals |
| Module 02 | Histopathologic Correlations of Dermoscopic Structures | Understanding color-structure correlations, vascular pattern recognition, and dermoscopic-histopathologic concordance relevant to infectious processes |
Recommended additional background: Basic clinical dermatology knowledge of common viral exanthems, ectoparasitic infestations, and superficial fungal infections.
3. Key Concepts
3.1 Entomodermoscopy: Dermoscopy Beyond Tumors
The term entomodermoscopy is a portmanteau of "entomology" and "dermoscopy," introduced by Zalaudek et al. (2008) to describe the use of dermoscopy in the diagnosis of infectious skin disorders. This concept reflects a paradigm shift: the dermatoscope, originally developed and widely adopted for the evaluation of pigmented skin lesions and tumors, is equally valuable as a rapid, noninvasive diagnostic tool for infections and infestations.
Entomodermoscopy enables clinicians to:
- Directly visualize parasites, mites, larvae, lice, and nits in vivo
- Identify characteristic morphologic patterns linked to specific infectious agents
- Reduce the need for invasive procedures such as skin scrapings, biopsies, or microscopic preparations
- Monitor treatment effectiveness by confirming eradication of organisms
3.2 Diagnostic Framework for Infectious Dermoscopy
Dermoscopic findings in infectious diseases must always be integrated with:
- Clinical history (exposure, travel, contacts, immunosuppression)
- Clinical morphology (distribution, primary lesion type)
- Histopathologic findings (when biopsy is performed)
- Laboratory results (KOH preparation, culture, PCR)
Dermoscopic patterns in infections are organized by etiologic category:
| Category | Conditions |
|---|---|
| Viral | Molluscum contagiosum, common warts, palmo-plantar warts, facial-plane warts, genital warts |
| Ectoparasitic | Scabies, pediculosis (head lice, pubic lice), tick bites, tungiasis, cutaneous larva migrans, myiasis |
| Fungal | Tinea nigra, tinea corporis (with vellus hair involvement) |
| Bacterial | Folliculitis, lupus vulgaris, trichomycosis palmellina |
3.3 Why Dermoscopy Matters in Infectious Dermatology
Several infectious conditions are notorious clinical mimics of tumors:
- Tinea nigra can mimic junctional nevi or acral lentiginous melanoma, potentially leading to unnecessary biopsies
- Molluscum contagiosum may be confused with basal cell carcinoma or keratoacanthoma in adults
- Viral warts (especially plantar) may be confused with callus, corn, or even melanoma
- Scabies may be clinically indistinguishable from eczema without dermoscopic confirmation
Dermoscopy provides a rapid, point-of-care diagnostic layer that prevents misdiagnosis and guides appropriate treatment.
4. Core Content
4.1 Viral Diseases
4.1.1 Molluscum Contagiosum
Etiology and clinical context: Molluscum contagiosum (MC) is a common viral infection caused by a contagious human-specific poxvirus. It presents as small, dome-shaped, umbilicated papules, most commonly in children, sexually active adults, and immunocompromised individuals.
Dermoscopic features:
| Feature | Description | Significance |
|---|---|---|
| Central pore/umbilication | Visible opening at the apex of the papule | Corresponds to the central crater through which molluscum bodies are extruded |
| Polylobular white-to-yellowish amorphous structures | Multilobed structureless areas occupying the center of the lesion | Represent lobules of infected epidermal cells containing viral inclusion bodies (Henderson-Paterson bodies) |
| Crown vessels (corona vessels) | Linear, fine, blurred telangiectasias arranged peripherally around the central structures | Surround the lesion in a "corona"-like pattern; most characteristic vascular pattern |
| Radial vascular pattern | Vessels radiating outward from the center | Alternative vascular arrangement; may be seen instead of or alongside crown vessels |
| Punctiform vascular pattern | Tiny dot-like vessels | Can present in lesions with inflammation or excoriations |
Clinical Pearl: In a study of 211 histopathologically confirmed MC lesions (Ianhez et al., 2011), in addition to the classic crown vessel pattern, radial and punctiform vascular patterns were also identified, particularly in inflamed or excoriated lesions.
Differential diagnosis considerations: The combination of central polylobular structureless areas surrounded by crown vessels is highly characteristic. "Dermoscopy leaves little doubt about the diagnosis" when these features are present.
4.1.2 Cutaneous Warts
Etiology and clinical context: Cutaneous warts are caused by human papillomavirus (HPV). On close clinical examination, disruption of dermatoglyphics (skin markings) and thrombosed capillaries may be appreciated. Warts can be difficult to distinguish clinically from calluses, corns, or even melanoma.
Dermoscopic features by wart subtype:
Common Warts (Verruca Vulgaris)
Pathognomonic pattern -- "Frog spawn" / Mosaic pattern:
- Definition: Multiple densely packed papillae, each containing a central red or black dot or loop, surrounded by a whitish halo
- Red dots/loops: Represent normal, non-thrombosed capillaries within the elongated dermal papillae
- Black dots/loops: Represent thrombosed capillaries
- Whitish halo: Surrounds each papilla, corresponding to the acanthotic and hyperkeratotic epidermis
This characteristic pattern of thrombosed capillaries within individual papillae creates the "frog spawn" appearance that is virtually diagnostic.
Palmo-Plantar Warts
| Feature | Description |
|---|---|
| Verrucous, yellowish, structureless areas | Background color corresponding to hyperkeratotic stratum corneum |
| Black dots or streaks | Irregularly distributed; correspond to thrombosed vessels and microhemorrhages caused by pressure effects on the soles |
| Brownish to reddish lines or dots | Correspond to hemorrhages within the verrucous surface |
Key differential diagnosis -- Palmo-plantar warts vs mimics:
| Condition | Dermoscopic Appearance |
|---|---|
| Plantar wart | Yellowish structureless areas + irregularly distributed black dots/streaks (thrombosed vessels) |
| Callus | Homogeneous opacities without vascular dots |
| Corn | Translucent central core without vascular dots |
| Acral melanoma | Parallel ridge pattern, fibrillar pattern -- acral melanocytic patterns (see Module on acral lesions) |
Clinical pitfall: The distinction between plantar wart and acral melanoma is critical. Warts disrupt dermatoglyphics and show thrombosed capillaries; melanoma follows the sulci and crista patterns of the acral skin.
Facial-Plane Warts
- Dotted vessels regularly distributed over a brown to flesh-colored background
- Often intermingled with whitish lines
Genital Warts (Condylomata Acuminata)
- Whitish reticulation is the characteristic dermoscopic pattern
Clinical Scenario
A 7-year-old boy presents with multiple verrucous papules on his hands. Dermoscopy of the largest papule reveals a central area with irregularly distributed red-to-black dots and globules surrounded by a whitish halo. The dots and globules correspond to thrombosed capillaries, and the whitish halo represents hyperkeratosis. No pigment network is observed.
What is the diagnosis, and what dermoscopic feature distinguishes viral warts from other papular lesions?
Common warts (verruca vulgaris)
The hallmark dermoscopic pattern of common warts is the presence of irregularly distributed red-to-black dots and globules (frog spawn pattern) corresponding to thrombosed capillary loops within the dermal papillae of the verrucous proliferation. These thrombosed vessels are surrounded by a whitish halo representing the hyperkeratotic epidermis. This pattern is critical for distinguishing warts from other papular lesions: seborrheic keratoses show comedo-like openings and milia-like cysts rather than thrombosed capillaries, and squamous cell carcinoma shows polymorphic atypical vessels. The absence of a pigment network excludes melanocytic lesions. In children, multiple acral warts are common and typically self-resolve, though treatment may be indicated for persistent or symptomatic lesions.
4.2 Ectoparasitic Diseases
4.2.1 Scabies
Etiology and clinical context: Scabies is caused by the mite Sarcoptes scabiei var. hominis. Dermoscopic findings can rapidly provide the diagnosis without the need for multiple skin scrapings. The dermoscopic patterns are particularly well observable on the lateral aspects of the fingers, toes, palms, soles, and around the mamilla, umbilicus, or genital areas.
Pathognomonic dermoscopic sign -- "Jet with contrail":
The diagnostic technique involves placing the dermatoscope over the end of a suspected burrow.
| Structure | Dermoscopic Appearance | Anatomic Correlate |
|---|---|---|
| Delta-wing jet sign (triangle) | Small, dark-brown, delta-shaped (triangular) structure | Anterior body of the adult female mite -- her mouth and two front legs |
| Contrail (burrow) | Thin tract of whitish scale trailing the delta-shaped structure; may assume an S-shaped (S-italic) configuration | The mite's burrow through the stratum corneum |
| Brown dots within the contrail | Small brown dots scattered along the whitish trail | Mite feces (scybala) deposited within the burrow |
| Posterior mite body | Usually translucent and often not visible | The posterior part of the mite is thin-walled and translucent |
The metaphor explained:
- The dark triangular structure = the "delta-wing jet airplane"
- The whitish burrow = the "condensation trail (contrail)" following the jet
- Together = "jet with contrail"
Technique tip: The delta-wing jet sign is best observed at the terminal end of a burrow. The S-shaped (italic-S) configuration of the burrow is characteristic. Linear burrows may show a delta-shaped structure at each end.
Clinical Pearl: Dermoscopy for scabies provides a rapid, noninvasive diagnosis that is particularly valuable in:
- Atypical presentations (crusted/Norwegian scabies, nodular scabies)
- Infants and young children (where skin scraping is difficult)
- Institutional outbreaks (rapid screening of multiple patients)
- Monitoring treatment response (confirming mite eradication)
4.2.2 Lice Infestations (Pediculosis)
Etiology and clinical context: Pediculus humanus capitis (head lice) and Phthirus pubis (pubic lice / "crabs") are worldwide ectoparasites causing thousands of infestations per year. Dermoscopy allows rapid and reliable diagnosis by identifying the louse or nits fixed to the hair shaft, and is useful in monitoring treatment effectiveness.
Key diagnostic principle: Identification of cemented nits (eggs) located within 1-2 mm from the scalp surface suggests active pediculosis infestation. Although the adult louse can be elusive and difficult to find, diagnosis can be confirmed by simply identifying nits on the hair shaft.
Dermoscopic classification of nits:
| Nit Type | Dermoscopic Appearance | Clinical Significance |
|---|---|---|
| Vital nits (containing viable nymphs) | Brown color, ovoid/roundish shape with a brownish center | Active infestation; treatment required |
| Empty nits | Translucent, oval shape with a flattened/flat free end (disfigured shape) | Hatched; may indicate treated or resolved infestation |
| Dead nits | Focal brown area (collapsed nymph) plus a translucent area (air pocket) within the nit casing | Nymph has died but not hatched; may indicate partially effective treatment |
Differentiating nits from pseudonits:
Dermoscopy is critical for distinguishing true nits from pseudonits, which include:
- Hair casts (peripilar keratin cylinders)
- Hair spray debris
- Scales from seborrheic dermatitis
True nits are cemented firmly to the hair shaft, have a consistent ovoid morphology, and sit at a fixed angle. Pseudonits are typically irregular, loosely adherent, and slide along the shaft.
Pubic lice (Phthirus pubis):
Dermoscopy readily identifies pubic lice ("crabs") by directly visualizing Phthirus pubis attached to terminal hair shafts within hair-bearing areas, including:
- Genitalia (most common)
- Eyelashes (phthiriasis palpebrarum)
- Scalp
- Other body sites containing vellus hair
4.2.3 Tick Bites
Etiology and clinical context: Ixodes ricinus is a small human parasite (less than 1 mm in the nymphal stage) that can transmit various infectious diseases, most notably Lyme borreliosis. Dermoscopy provides convenient magnification and confirmation of the diagnosis.
Dermoscopic features of embedded tick:
| Feature | Description |
|---|---|
| Anterior legs | A pair (double pair) of anterior legs protruding from the surface of the skin |
| Chitinous body ("shield") | Seen as a brown to gray, translucent "shield" with pigmented streaks |
| Background | Erythematous background resulting from the inflammatory reaction to the tick bite |
Clinical utility of dermoscopy in tick bites:
- Diagnosis confirmation: Dermoscopy confirms the presence of an embedded tick, particularly when the tick is very small (nymphal stage) and difficult to see with the naked eye
- Post-extraction verification: Dermoscopy is useful in determining whether the tick has been completely extracted (i.e., whether mouthparts remain embedded in the skin)
4.2.4 Tungiasis
Etiology and clinical context: Tungiasis is an infectious disease caused by the flea Tunga penetrans, which lives in humid sand contaminated by feces of pigs and cows. The parasite penetrates the epidermis, causing reactive hyperplasia and subsequent light brown, itchy papules. Typical locations include the soles and periungual areas. It is endemic to tropical regions of Central and South America, sub-Saharan Africa, and the Indian subcontinent.
Dermoscopic features:
| Feature | Description |
|---|---|
| Central pigmented ring | A pigmented ring structure in the center of the lesion |
| Central pore | A pore in the middle of the pigmented ring (respiratory/egg-laying orifice of the flea) |
| Eccentric gray-bluish blotch | An off-center gray to bluish pigmented area |
| Whitish background | Overall whitish hue surrounding the structures |
| "Whitish chains" | A remarkable in vivo dermoscopic finding described by Bakos and Bakos (2008), possibly representing egg chains |
Clinical Pearl: Eggs may also be visible on dermoscopy, as documented by Cabrera and Daza (2008). The combination of a central pigmented ring with pore, eccentric gray-bluish blotch, and whitish background is characteristic of tungiasis.
4.2.5 Cutaneous Larva Migrans
Etiology and clinical context: Cutaneous larva migrans (CLM) is a hookworm-related infection acquired by skin contact with soil contaminated by feces of cats and dogs. Causative helminths include Ancylostoma braziliense, Ascaris suum, and Bunostomum phlebotomum. The larva penetrates through the skin, most frequently the foot.
Dermoscopic features:
| Feature | Description | Correlate |
|---|---|---|
| Translucent brownish/yellowish linear structure | A serpiginous (snake-like) linear structure in a segmental arrangement | Corresponds to the body of the larva itself |
| Red dotted vessels in the empty burrow | The empty portion of the burrow (behind the advancing larva) displays red dotted vessels | Represents the inflammatory trail left after the larva has migrated through |
Clinical Pearl: The segmental arrangement of the translucent structure is key -- it shows the advancing front of the larva, while the empty burrow behind it shows only the reactive vascular pattern. This allows the clinician to identify the leading edge of the larval track for targeted treatment (e.g., cryotherapy at the advancing front).
4.2.6 Myiasis
Etiology and clinical context: Myiasis is the infestation of living tissue by fly larvae (maggots). Dermoscopy can aid in diagnosis before extraction.
Dermoscopic features:
| Feature | Description |
|---|---|
| Central opening | A pore or opening in the skin surface |
| Dilated blood vessels | Surrounding the central opening |
| Yellowish structure with black barb-like spines | Visible within or protruding from the opening; represents the larval body and its spiracular apparatus |
| Larval movement | Movements of the larva can be directly observed under dermoscopy |
Check Your Understanding
What is the dermoscopic 'jet-with-contrail' sign, and which infection is it pathognomonic for?
The jet-with-contrail sign is pathognomonic for scabies. It consists of a dark triangular structure at the leading edge (the mite's anterior body) followed by a trailing whitish to brown wavy line (the burrow created by the mite tunneling through the stratum corneum). Dermoscopy can directly visualize the Sarcoptes scabiei mite.
Key Takeaways
- The delta-wing jet sign (triangular dark structure representing the mite head and front legs) at the end of a burrow is pathognomonic for scabies under dermoscopy.
- Dermoscopy can rapidly confirm scabies diagnosis in the clinic, reducing the need for skin scraping, particularly useful in pediatric patients.
- The burrow appears as a wavy line with a small dark triangle at the leading edge; this finding has near-100% specificity for scabies when properly identified.
Clinical Scenario
A 35-year-old woman presents with intense pruritus on her hands and wrists that is worse at night. Her partner has similar symptoms. Clinical examination reveals excoriated papules in the finger web spaces and on the wrists. Dermoscopy of an intact burrow on the lateral aspect of her index finger reveals a small dark-brown triangular structure at the leading end of a thin wavy whitish trail, with tiny brown dots along the trail.
What is the pathognomonic dermoscopic sign, and what are its anatomic correlates?
Scabies -- "jet with contrail" sign
The pathognomonic dermoscopic sign of scabies is the "jet with contrail" (also called the delta-wing jet sign). The dark-brown triangular structure represents the anterior body of the female Sarcoptes scabiei mite (her mouth and two front legs), while the whitish wavy trail is the burrow (contrail) she has excavated through the stratum corneum. The small brown dots along the trail are mite feces (scybala). This dermoscopic finding has near-100% specificity for scabies and allows rapid diagnosis without the need for skin scraping. Dermoscopy is particularly useful for diagnosing scabies in pediatric patients and in institutional settings where rapid confirmation is needed to initiate appropriate treatment and contact prophylaxis.
4.3 Fungal Diseases
4.3.1 Tinea Nigra
Etiology and clinical context: Tinea nigra (TN) is a superficial fungal infection caused by Hortaea (Phaeoannellomyces) werneckii (formerly Cladosporium werneckii or Exophiala werneckii). It characteristically presents as asymptomatic, irregular, brown-black macules, often mottled in coloration and darker at the border. Infection occurs on the stratum corneum of the palms and/or soles, predominantly in children living in or visiting tropical and subtropical regions (South and Central America, Asia, Africa).
Critical differential diagnosis: TN may mimic junctional nevi or acral lentiginous melanoma, potentially leading to unnecessary skin biopsies.
Dermoscopic features:
| Feature | Description |
|---|---|
| Pigmented spicules / wispy brown strands | Fine, superficial, wispy-like gray to brownish lines |
| Non-anatomic pigment distribution | The pigmented strands do not follow the anatomic structures of the furrows or ridges on glabrous skin |
| Absence of acral melanocytic patterns | Does NOT show parallel ridge pattern, fibrillar pattern, or lattice-like pattern characteristic of melanocytic lesions of palms/soles |
| Superficial removability | The superficial pigmentation can be easily removed by scraping the stratum corneum with a scalpel (confirmatory clue) |
KOH examination: Reveals brown to olive-colored, thick, branching hyphae as well as budding yeast cells.
Key diagnostic distinction: Melanocytic lesions of the palms and soles follow and respect the dermatoglyphic furrows and ridges (e.g., parallel ridge pattern in melanoma, parallel furrow pattern in nevi). Tinea nigra does NOT respect these anatomic landmarks -- the pigmented spicules are randomly oriented, crossing both ridges and furrows. This single dermoscopic observation can prevent an unnecessary biopsy.
4.3.2 Tinea Corporis and Vellus Hair Involvement
Etiology and clinical context: Tinea corporis (TC) is a superficial fungal infection caused by dermatophytes, most commonly Trichophyton rubrum or Microsporum canis. It presents clinically with annular erythematous lesions spreading centrifugally with a scaly, elevated border.
Dermoscopic features:
| Feature | Description |
|---|---|
| Erythematous background | Diffuse redness corresponding to the inflammatory infiltrate |
| Yellowish scales and crusts | Due to vesicle rupture; overlying the erythematous base |
| No specific vascular pattern | Unlike many inflammatory dermatoses, TC lacks a diagnostic vascular pattern |
| Coarse scale with chaotic directionality | The orientation of scaling is disordered and mixed (from) |
| Mixed vascular pattern | Non-specific vascular pattern (from) |
The critical role of dermoscopy -- detecting vellus hair involvement:
A pivotal clinical application of dermoscopy in TC is the detection of fungal invasion of vellus hairs on glabrous skin. This finding has direct therapeutic implications:
| Dermoscopic Finding | Clinical Significance |
|---|---|
| Multiple small brown spots surrounded by a white-yellowish halo | Suggestive of fungal invasion of the hair follicle (vellus hair involvement) |
| Implications for treatment | Tinea of vellus hair on glabrous skin that is resistant to topical antifungal therapy requires systemic antifungal treatment |
Clinical Pearl: Dermoscopy can guide the treatment of tinea corporis by revealing vellus hair involvement that is invisible to the naked eye. When the dermatoscope shows multiple small brown spots with white-yellowish halos, this indicates dermatophyte invasion of hair follicles, which cannot be eradicated by topical agents alone and requires systemic therapy. This explains many cases of "treatment-resistant" tinea corporis.
4.4 Bacterial Diseases
While bacterial infections are less extensively covered compared to viral and parasitic conditions, the following dermoscopic patterns have been described for three bacterial entities:
4.4.1 Folliculitis
| Feature | Description |
|---|---|
| White to yellow dot | Filling the hair follicle opening |
| Dotted vessels | Sometimes surrounding the follicular dot |
4.4.2 Lupus Vulgaris (Cutaneous Tuberculosis)
| Feature | Description |
|---|---|
| Fine focused telangiectasias | Over a typically yellow to golden background |
| Yellow to golden background | The "apple jelly" color corresponding to dermal granulomas |
| Milia-like cysts (occasional) | Small white globular structures |
| Whitish lines (occasional) | Linear white structures |
Differential note: The golden-yellow background with focused telangiectasias distinguishes lupus vulgaris from other granulomatous conditions. However, lupus vulgaris must be differentiated from lupus pernio (sarcoidosis) and discoid lupus erythematosus, which display different dermoscopic patterns (discussed in Chapter 12b).
4.4.3 Trichomycosis Palmellina
| Feature | Description |
|---|---|
| Yellowish casts | Surrounding/encasing the hair shafts |
Note: Despite the "mycosis" in its name, trichomycosis palmellina is a bacterial infection caused by Corynebacterium species. The yellowish concretions visible on dermoscopy correspond to bacterial colonies adhering to the hair cuticle.
Check Your Understanding
How does dermoscopy help in the diagnosis of viral warts?
Dermoscopy of viral warts reveals a characteristic pattern of thrombosed capillaries appearing as black, red, or purple dots (sometimes called 'frog spawn' pattern) surrounded by a whitish halo. This pattern corresponds to the dilated and thrombosed dermal capillary loops within the papillomatous projections. The pattern helps distinguish warts from calluses, which show no vascular dots.
Key Takeaways
- Viral warts show the frog-spawn or tadpole pattern of clustered dilated vessels surrounded by a white halo, corresponding to thrombosed capillaries in elongated dermal papillae.
- Pediculosis (lice) shows torpedo-shaped nits firmly attached to the hair shaft, distinguishable from pseudonits (hair casts) that slide freely along the shaft.
- Molluscum contagiosum under dermoscopy shows a central white-to-yellow polylobulated amorphous structure (molluscum body) surrounded by crown vessels.
4.5 Comparison of Dermoscopic Features Across Infectious Conditions
The following comparative analysis helps organize the diagnostic approach by morphologic pattern:
Conditions with identifiable organisms:
| Condition | What You See | Key Identifying Feature |
|---|---|---|
| Scabies | Mite body + burrow | Delta-wing triangle at end of S-shaped white trail |
| Pediculosis | Nits on hair shaft | Ovoid structures cemented to hair; color indicates viability |
| Tick bite | Tick body parts | Anterior legs protruding + chitinous "shield" |
| Tungiasis | Embedded flea | Central pigmented ring with pore + eccentric blotch |
| Cutaneous larva migrans | Larval body in burrow | Translucent brownish segmental structure + red dots in empty trail |
| Myiasis | Larva in central opening | Yellowish body with black barb-like spines; visible movement |
Conditions with pattern-based diagnosis:
| Condition | Pattern | Key Feature |
|---|---|---|
| Molluscum contagiosum | Central structureless + peripheral vessels | Polylobular white-yellow center + crown vessels |
| Common warts | Papillomatous + vascular dots | Frog spawn / mosaic pattern |
| Plantar warts | Structureless + hemorrhagic dots | Yellowish background + scattered black dots/streaks |
| Tinea nigra | Pigmented linear structures | Wispy brown spicules NOT following dermatoglyphics |
| Tinea corporis | Inflammatory + follicular | Erythema + yellow scale; brown spots with halos if vellus hair involved |
Key Takeaways
- Dermoscopy of infectious diseases has high clinical utility because it provides rapid, noninvasive confirmation that can eliminate the need for more invasive diagnostic procedures.
- Tungiasis (sand flea) shows a characteristic ring of dark-brown dots surrounding a central pore, visible under dermoscopy even before clinical features are fully developed.
- Cutaneous larva migrans shows subtle serpiginous burrow-like structures under dermoscopy that can be traced to identify the advancing end for targeted treatment.
5. Infectious Disease Dermoscopy Reference Table
This comprehensive table consolidates all dermoscopic patterns and the narrative text.
| Category | Condition | Pathognomonic / Key Feature | Additional Dermoscopic Features | Key References |
|---|---|---|---|---|
| Viral | Facial-plane warts | Dotted vessels regularly distributed over brown/flesh-colored background | Intermingled whitish lines | Zalaudek et al., 2008 |
| Viral | Common warts (verruca vulgaris) | Frog spawn / mosaic pattern: densely packed papillae with central red or black dots/loops surrounded by whitish halos | Disruption of dermatoglyphics; thrombosed capillaries | Teoli et al., 2006; Vazquez-Lopez et al., 2004 |
| Viral | Palmo-plantar warts | Yellowish structureless pattern with brownish-reddish lines/dots | Black dots/streaks (thrombosed vessels + microhemorrhages from pressure) | Dalmau et al., 2006; Kim et al., 2006 |
| Viral | Genital warts | Whitish reticulation | -- | Zalaudek et al., 2008 |
| Viral | Molluscum contagiosum | Polylobular whitish structureless areas centrally + crown vessels peripherally | Central pore/umbilication; radial and punctiform vessel variants | Morales et al., 2005; Zaballos et al., 2006 |
| Fungal | Tinea corporis | Coarse scale with chaotic orientation of scaling | Mixed vascular pattern; yellowish crusts | Vazquez-Lopez et al., 2004 |
| Fungal | Tinea corporis (vellus hair) | Multiple small brown spots with white-yellowish halo (fungal follicular invasion) | Indicates need for systemic antifungal therapy | Gomez-Moyano et al., 2010, 2016 |
| Fungal | Tinea nigra | Pigmented spicules / wispy brown strands NOT following furrows or ridges | Absence of acral melanocytic patterns; superficially removable by scraping | Smith et al., 2001; Xavier et al., 2008 |
| Bacterial | Folliculitis | White to yellow dot filling the hair follicle | Sometimes surrounded by dotted vessels | Zalaudek et al., 2008 |
| Bacterial | Lupus vulgaris | Fine focused telangiectasias over yellow to golden background | Occasional milia-like cysts or whitish lines | Brasiello et al., 2008 |
| Bacterial | Trichomycosis palmellina | Yellowish casts around hair shafts | -- | Personal observation |
| Parasitic | Scabies | Delta-wing triangle + contrail (jet with contrail): brown triangle at end of curved whitish line | S-shaped burrow; brown dots (feces) within burrow; translucent posterior body | Argenziano et al., 1997; Dupuy et al., 2007 |
| Parasitic | Lice / nits | Vital nits: ovoid, brown structures fixed to hair shaft | Empty nits: translucent with plane/fissured free end; Dead nits: focal brown + air pocket | Di Stefani et al., 2006; Scanni & Bonifazi, 2006 |
| Parasitic | Phthirus pubis | Direct visualization of louse on terminal hair shafts | Can be found on genitalia, eyelashes, scalp, and vellus hair sites | Lacarrubba & Micali, 2013 |
| Parasitic | Tick (Ixodes ricinus) | Double pair of anterior legs protruding from skin surface | Chitinous body as brown-gray translucent "shield" with pigmented streaks | Pagliarello et al., 2006; Zalaudek et al., 2006 |
| Parasitic | Tungiasis | Central pigmented ring with pore + eccentric gray-bluish blotch | Whitish background; "whitish chains" (possible egg chains); visible eggs | Bakos & Bakos, 2008; Cabrera & Daza, 2008 |
| Parasitic | Cutaneous larva migrans | Translucent brownish/yellowish segmental linear structure (larval body) | Empty burrow shows red dotted vessels | Elsner et al., 1997; Zalaudek et al., 2008 |
| Parasitic | Myiasis | Central opening with larva visible | Dilated blood vessels surrounding; yellowish structure with black barb-like spines; visible movement | Bakos & Bakos, 2007 |
6. Clinical Pearls
The "jet with contrail" sign is pathognomonic for scabies. Place the dermatoscope over the suspected burrow endpoint. The dark-brown delta-shaped triangle represents the mite's head and two front legs. This single observation can replace multiple skin scrapings and provide an immediate, confident diagnosis.
Crown vessels in molluscum contagiosum are not the only vascular pattern. While the corona-like pattern of fine, blurred telangiectasias is classic, inflamed or excoriated MC lesions may display radial or punctiform vessel patterns instead. Always look for the central polylobular white-yellow amorphous structures as the primary clue.
Thrombosed capillaries in warts = red/black dots within papillae. The "frog spawn" pattern of common warts (densely packed papillae, each with a central vascular dot surrounded by a whitish halo) is the most reliable dermoscopic criterion for differentiating warts from calluses (homogeneous opacities) and corns (translucent central core).
Tinea nigra is the great melanoma mimic on acral skin -- but dermoscopy resolves the dilemma. The wispy brown pigmented spicules of tinea nigra do NOT respect dermatoglyphic furrows or ridges, unlike melanocytic lesions that follow the parallel ridge or parallel furrow patterns. This observation alone can prevent a biopsy; confirmation with KOH preparation showing olive-colored hyphae is definitive.
Nit viability assessment is critical for treatment decisions. Vital (brown, ovoid), dead (brown spot + air pocket), and empty (translucent, flat-ended) nits all have distinct dermoscopic appearances. Only vital nits within 1-2 mm of the scalp indicate active infestation requiring treatment. Do not confuse pseudonits (hair casts, spray debris, seborrheic scales) with true nits.
Dermoscopy after tick removal confirms complete extraction. Following tick removal, use the dermatoscope to verify that no mouthparts remain embedded in the skin. Retained mouthparts may cause a persistent foreign-body granulomatous reaction.
Treatment-resistant tinea corporis? Look for vellus hair involvement. When topical antifungals fail, dermoscopy may reveal multiple small brown spots surrounded by white-yellowish halos, indicating fungal invasion of vellus hair follicles. This finding mandates systemic antifungal therapy because topical agents cannot penetrate to the intrafollicular infection.
Cutaneous larva migrans -- identify the advancing front. The translucent brownish segmental structure visible on dermoscopy represents the larval body at the leading edge of the track. The empty burrow behind it shows only red dotted vessels. Targeted treatment (e.g., cryotherapy, topical ivermectin) should be directed at the advancing front, not the trailing inflammatory track.
Always integrate dermoscopy with clinical context. Entomodermoscopy is a powerful diagnostic adjunct, but clinical history (travel, contacts, exposure), distribution pattern, and laboratory confirmation (KOH, culture, serology) remain essential components of the diagnostic workup for infectious diseases.
Clinical Vignettes
Clinical Scenario A 32-year-old woman presents with intensely pruritic papules and linear tracks on her wrists and interdigital spaces. Her partner has similar symptoms. Dermoscopy of a suspicious linear track on the wrist reveals a dark-brown, delta-shaped (triangular) structure at one end of the track, with two small anterior projections. The track behind it appears as a wavy line.
What is the most likely diagnosis?
Diagnosis: Scabies -- "jet with contrail" sign.
Clinical Pearl 1: the "jet with contrail" sign is pathognomonic for scabies. The dark-brown delta-shaped triangle represents the mite's head and two front legs (Sarcoptes scabiei). This single dermoscopic observation can replace multiple skin scrapings and provide an immediate, confident diagnosis. The interdigital and wrist distribution, intense pruritus, and symptomatic partner support the clinical picture. Clinical Pearl 9: always integrate dermoscopy with clinical context -- treatment of both the patient and close contacts is necessary.
Clinical Scenario A 25-year-old man presents with a brown macule on his right palm that appeared 2 weeks ago after a tropical vacation. He is concerned about melanoma. Dermoscopy reveals wispy brown pigmented spicules that do NOT follow the dermatoglyphic furrows or ridges. The pigmentation has a superficial, "paint-like" quality.
What is the most likely diagnosis?
Diagnosis: Tinea nigra -- the great melanoma mimic on acral skin.
Clinical Pearl 4: tinea nigra's wispy brown spicules do NOT respect dermatoglyphic furrows or ridges, unlike melanocytic lesions that follow the parallel ridge or parallel furrow patterns. This observation alone can prevent an unnecessary biopsy. The superficial quality and travel to a tropical region support the diagnosis. Confirmation with KOH preparation showing olive-colored hyphae of Hortaea werneckii is definitive. This case underscores Clinical Pearl 9: clinical context (travel history) combined with dermoscopy resolves the diagnostic dilemma without biopsy.
Clinical Scenario A 40-year-old woman presents with a persistent, erythematous, scaly patch on the lateral thigh that has not responded to 6 weeks of topical antifungal cream. The KOH preparation is negative from surface scrapings. Dermoscopy reveals multiple small brown spots surrounded by white-yellowish halos, distributed across the patch. Short broken hairs are visible in the affected area.
What is the most likely diagnosis?
Diagnosis: Tinea corporis with vellus hair involvement -- systemic antifungal required.
Clinical Pearl 7: when topical antifungals fail, dermoscopy may reveal multiple small brown spots surrounded by white-yellowish halos, indicating fungal invasion of vellus hair follicles. The negative KOH from surface scrapings does not exclude the diagnosis because the infection is intrafollicular, beyond the reach of surface sampling. This finding mandates systemic antifungal therapy because topical agents cannot penetrate to the intrafollicular infection. The broken vellus hairs confirm follicular involvement. A KOH preparation of a plucked vellus hair, or a fungal culture, would likely yield a positive result.
Clinical Scenario
A 6-year-old girl presents with multiple dome-shaped, umbilicated papules on her trunk and axillae. Dermoscopy reveals polylobular white-to-yellowish structureless areas in the center of each lesion, surrounded by fine, blurred linear telangiectasias arranged in a peripheral "corona"-like pattern. Some lesions show a central punctum.
What is your diagnosis?
Molluscum contagiosum
The polylobular white-yellow amorphous central structures surrounded by a corona of fine blurred telangiectasias are the classic dermoscopic findings of molluscum contagiosum. The central structures correspond to the molluscum body (Henderson-Patterson bodies). While crown vessels are the best-known vascular pattern, inflamed or excoriated lesions may display radial or punctiform vessel patterns instead. The central polylobular structures are the primary diagnostic clue regardless of the vascular pattern. The distribution in a child and the umbilicated morphology clinically support the diagnosis.
8. Cross-References
| Topic | Source |
|---|---|
| Entomodermoscopy overview and | Chapter 12a, pp. 294-295 |
| Molluscum contagiosum dermoscopy | Chapter 12a, p. 294;a,b |
| Cutaneous warts (common, plantar, facial-plane, genital) | Chapter 12a, pp. 294-295;a,b,c |
| Scabies and the jet-with-contrail sign | Chapter 12a, pp. 294-295;a,b,c |
| Lice and nit classification | Chapter 12a, pp. 294-296;a,b,c and 12a.5a,b |
| Tick bites (Ixodes ricinus) | Chapter 12a, pp. 295-296;a,b |
| Tungiasis | Chapter 12a, p. 296 |
| Cutaneous larva migrans | Chapter 12a, p. 297;a,b |
| Tinea nigra | Chapter 12a, p. 297;a,b |
| Tinea corporis and vellus hair involvement | Chapter 12a, p. 297;a,b |
| Inflammatory dermatoses (inflammoscopy) | Chapter 12b (see Module 36) |
| Dermoscopy of acral lesions (parallel patterns) | Chapter 8 (acral lesions module) |
| Differential of unpigmented lesions (wart vs amelanotic melanoma) | Chapter 4b (prediction without pigment); Module 05 |
9. Related Modules
| Module | Title | Relevance to This Module |
|---|---|---|
| Module 01 | Introduction and Principles of Dermoscopy | Foundational equipment and technique knowledge required for performing entomodermoscopy |
| Module 02 | Histopathologic Correlations of Dermoscopic Structures | Understanding histopathologic correlates of vascular patterns (crown vessels, dotted vessels, thrombosed capillaries) and structural features seen in infections |
| Module 05 | Prediction without Pigment | Differential diagnosis of nonpigmented lesions including warts vs amelanotic melanoma; vessel pattern recognition |
| Module 09 | Basal Cell Carcinoma | Differential diagnosis of molluscum contagiosum vs nodular BCC (both may show central structureless area with telangiectasias; arborizing vessels are specific to BCC) |
| Module 13 | Vascular Lesions | Complementary knowledge of vascular patterns and their histopathologic correlates relevant to crown vessels (MC), dotted vessels (folliculitis), and telangiectasias (lupus vulgaris) |
Module 35 completed. For inflammatory dermatoses (inflammoscopy), see Module 36.