21: Skin and Eye Infections
The human body is covered in skin , and like most coverings, skin is designed to protect what is underneath. One of its primary purposes is to prevent microbes in the surrounding environment from invading underlying tissues and organs. But in spite of its role as a protective covering, skin is not itself immune from infection. Certain pathogens and toxins can cause severe infections or reactions when they come in contact with the skin. Other pathogens are opportunistic, breaching the skin’s natural defenses through cuts, wounds, or a disruption of normal microbiota resulting in an infection in the surrounding skin and tissue. Still other pathogens enter the body via different routes—through the respiratory or digestive systems, for example—but cause reactions that manifest as skin rashes or lesions.
Nearly all humans experience skin infections to some degree. Many of these conditions are, as the name suggests, “skin deep,” with symptoms that are local and non-life-threatening. At some point, almost everyone must endure conditions like acne, athlete’s foot, and minor infections of cuts and abrasions, all of which result from infections of the skin. But not all skin infections are quite so innocuous. Some can become invasive, leading to systemic infection or spreading over large areas of skin, potentially becoming life-threatening.
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- 21.1: Anatomy and Normal Microbiota of the Skin and Eyes
- Human skin consists of two main layers, the epidermis and dermis, which are situated on top of the hypodermis, a layer of connective tissue. The skin is an effective physical barrier against microbial invasion. The skin’s relatively dry environment and normal microbiota discourage colonization by transient microbes. The skin’s normal microbiota varies from one region of the body to another. The conjunctiva of the eye is a frequent site for microbial infection; deeper infections are less common.
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- 21.2: Bacterial Infections of the Skin and Eyes
- Staphylococcus and Streptococcus cause many different types of skin infections, many of which occur when bacteria breach the skin barrier through a cut or wound. S. aureus are frequently associated with purulent skin infections that manifest as folliculitis, furuncles, or carbuncles. S. aureus is also a leading cause of staphylococcal scalded skin syndrome (SSSS). S. aureus is generally drug resistant and current MRSA strains are resistant to a wide range of antibiotics.
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- 21.3: Viral Infections of the Skin and Eyes
- Papillomas (warts) are caused by human papillomaviruses. Herpes simplex virus (especially HSV-1) mainly causes oral herpes, but lesions can appear on other areas of the skin and mucous membranes. Roseola and fifth disease are common viral illnesses that cause skin rashes; roseola is caused by HHV-6 and HHV-7 while fifth disease is caused by parvovirus 19. Viral conjunctivitis is often caused by adenoviruses and may be associated with the common cold. Herpes keratitis is caused by herpesviruses.
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- 21.4: Mycoses of the Skin and Eyes
- Mycoses can be cutaneous, subcutaneous, or systemic. Common cutaneous mycoses include tineas caused by dermatophytes of the genera Trichophyton, Epidermophyton, and Microsporum. Tinea corporis is called ringworm. Tineas on other parts of the body have names associated with the affected body part. Aspergillosis is a fungal disease caused by molds of the genus Aspergillus. Primary cutaneous aspergillosis enters through a break in the skin, such as the site of an injury or a surgical wound.
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- 21.5: Protozoan and Helminthic Infections of the Eyes
- The protozoan Acanthamoeba and the helminth Loa loa are two parasites that can breach the skin barrier, causing infections of the skin and eyes. Acanthamoeba keratitis is a parasitic infection of the eye that often results from improper disinfection of contact lenses or swimming while wearing contact lenses. Loiasis, or eye worm, is a disease endemic to Africa that is caused by parasitic worms that infect the subcutaneous tissue of the skin and eyes. It is transmitted by deerfly vectors.
Thumbnail: Multiple plantar warts have grown on this toe.