Skin diseases can be caused by viruses, bacteria, fungi, or parasites. The most common bacterial skin pathogens are Staphylococcus aureus and group A b-hemolytic streptococci. Herpes simplex is the most common viral skin disease. Of the dermatophytic fungi, Trichophyton rubrum is the most prevalent cause of skin and nail infections.
Primary Infections: Primary skin infections have a characteristic clinical picture and disease course, are caused by a single pathogen, and usually affect normal skin. Impetigo, folliculitis, and boils are common types. The most common primary skin pathogens are S aureus, b-hemolytic streptococci, and coryneform bacteria. These organisms usually enter through a break in the skin such as an insect bite. Many systemic infections involve skin symptoms caused either by the pathogen or by toxins; examples are measles, varicella, gonococcemia, and staphylococcal scalded skin syndrome. Dermatophytic fungi have a strong affinity for keratin and therefore invade keratinized tissue of the nails, hair, and skin.
Secondary Infections: Secondary infections occur in skin that is already diseased. Because of the underlying disease, the clinical picture and course of these infections vary. Intertrigo and toe web infection are examples.
Most skin infections cause erythema, edema, and other signs of inflammation. Focal accumulations of pus (furuncles) or fluid (vesicles, bullae) may form. Alternatively, lesions may be scaling with no obvious inflammation. Nail infections cause discoloration of the nail and thickening of the nail plate.
Clinical examination and staining and/or culturing of a specimen of pus or exudate are often adequate for diagnosis. Ultraviolet light (Wood's lamp) is helpful in diagnosing erythrasma and some toe web and fungal infections. Microscopic examination of a KOH preparation of skin scales, nail scrapings, or loose hair is useful for fungal infections. For viral infections, stained smears of vesicle fluid are examined under the microscope for typical cytopathology.
Cleansing and degerming the skin with a soap or detergent containing an antimicrobial agent may be useful. Drying agents, such as aluminum chloride, and keratinolytic agents, such as topical salicylate, are also helpful. Topical antimicrobial agents can be used for some infections, but systemic therapy may be necessary for patients with extensive disease.
Skin diseases are caused by viruses, rickettsiae, bacteria, fungi, and parasites. This chapter focuses on the common bacterial diseases of skin. Viral infections are also described, but of the cutaneous fungal diseases, only nail infections are included. The other fungal diseases are described in the Mycology section.
Skin infections may be either primary or secondary (Fig. 98-1). Primary infections have characteristic morphologies and courses, are initiated by single organisms, and usually occur in normal skin. They are most frequently caused by Staphylococcus aureus, Streptococcus pyogenes, and coryneform bacteria. Impetigo, folliculitis, boils, and erythrasma are common examples. Systemic infections may also have skin manifestations. Secondary infections originate in diseased skin as a superimposed condition. Intertrigo and toe web infections are examples of secondary infections.