Chapter 13. Rash

Dermatologists define several broad categories of skin disease based on clinical features and pathophysiology. The categories that are of primary interest from an infectious disease perspective include bacterial and mycobacterial infections, fungal infections, protozoal infections, viral infections, reactive erythemas, drug- and viral-induced hypersensitivity syndromes, and vasculitic diseases and purpura (Table 13–1).

Table 13–1. Diagnostic Categories of Selected Infectious and Non-Infections Etiologies of Rashes

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Table 13–1. Diagnostic Categories of Selected Infectious and Non-Infections Etiologies of Rashes

Diagnostic Category	Examples
Bacterial and mycobacterial infections	Cellulitis Impetigo Staphylococcal-scalded skin syndrome Toxic shock syndrome Ecthyma Ecthyma gangrenosum Meningococcemia Rocky mountain spotted fever Rickettsialpox Cutaneous tuberculosis Cutaneous anthrax Nontuberculous (atypical) mycobacterial infections Leprosy Secondary syphilis Ulceroglandular tularemia Actinomycosis
Viral infections	Erythema infectiosum (parvovirus B19 infection) Papular purpuric socks and gloves syndrome (parvovirus B19 infection) Infectious mononucleosis (Epstein–Barr virus (EBV) infection) Roseola infantum (human herpesvirus-6 and human herpesvirus-7 infection) Rubeola (measles) Hand-foot-and-mouth disease (coxsackie virus infection) Herpangina (coxsackie virus infection) Varicella Herpes zoster Cutaneous herpes and herpetic whitlow Herpes labialis Herpetic gingivostomatitis Eczema herpeticum Molluscum contagiosum Warts (verruca vulgaris, verruca plana, verruca plantaris, condylomata acuminata) Smallpox
Fungal infections	Dermatophyte infections (tinea capitis, tinea corporis, tinea pedis, onychomycosis) Tinea versicolor Cutaneous candidiasis Subcutaneous mycoses (sporotrichosis) Systemic mycoses (blastomycosis, histoplasmosis, coccidiomycosis) Opportunistic mycoses (aspergillosis, mucormycosis, cryptococcosis)
Protozoal infections	Cutaneous leishmaniasis Muocutaneous leishmaniasis
Reactive erythemas	Erythema multiforme Urticaria Serum sickness and serum sickness-like reactions Erythema marginatum (acute rheumatic fever) Erythema chronicum migrans (Lyme disease)
Hypersensitivity syndromes	Morbilliform drug eruption Stevens–Johnson syndrome Toxic epidermal necrolysis Drug reaction with eosinophilia and systemic symptoms
Vasculitic diseases and purpura	Rocky mountain spotted fever Erlichiosis Purpura fulminans Kawasaki disease Hypersensitivity (leukocytoclastic) vasculitis

Establishing the differential diagnosis of a rash relies both on an appropriately focused history and on correctly describing the morphology and distribution of the rash. A brief overview of common terms used in dermatology is presented in Table 13–2. Common patterns of distribution are presented in Table 13–3.

Table 13–2. Common Morphologic Patterns of Dermatologic Disease

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Table 13–2. Common Morphologic Patterns of Dermatologic Disease

Morphology	Description	Examples
*Primary lesions*
Macule	A flat lesion <1 cm in diameter	Leukocytoclastic vasculitis
Papule	A raised lesion <1 cm in diameter	Molluscum contagiosum
Patch	A flat lesion >1 cm in diameter	Erythema chronicum migrans (Lyme disease)
Plaque	A raised lesion with a flat top >1 cm in diameter	Verucca vulgaris
Nodule	A raised lesion >1 cm in diameter	Erythema nodosum
Vesicle	A clear fluid-filled lesion <1 cm in diameter	Herpes zoster
Bullae	A fluid-filled lesion >1 cm in diameter	Impetigo
Pustule	A cloudy fluid-filled lesion <1 cm in diameter	Neonatal staphylococcal pustulosis
Erosion	A loss of the epidermis (superficial)	Streptococcal intertrigo
Wheal	A transient edematous lesion, often with blanching or pallor centrally with surrounding erythema	Urticaria
Ulcer	A loss of the epidermis and part of the dermis and sometimes the subcutis (deep)	Echthyma gangrenosum
Fissure	A linear cleft or ulcer	Angular chelistis (Candida ...

Morphology

Description

Examples

Primary lesions

Macule

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