Chapter 16: Infectious Diseases Affecting the Skin and Eyes

Learning Outcomes Section 16.1

  1. Anatomy of the Skin

    • Epidermis: Outer layer of skin, consisting of several layers, key features include:

      • Stratum corneum: 25 cells thick; composed of dead cells that have migrated from deeper layers.

      • Millions of these cells slough off daily, carrying microorganisms with them.

      • Entire epidermis is replaced every 25 to 45 days.

    • Natural Defenses

      • Importance of skin as a barrier against pathogens.

Layers of the Skin

Epidermis

  • Structure: Consists of several layers, notably the stratum corneum.

  • Function: Protects against pathogens and regulates water loss.

Defenses of the Skin

  1. Antimicrobial Peptides

    • Positively charged chemicals that aid in keeping microbial counts low on the skin.

    • Various types exist, critical for skin's innate immunity.

  2. Sebum

    • Oily substance, gives skin low pH, inhospitable for many microorganisms.

    • Contains lipids that provide nutrients for normal skin microbiota.

    • Toxic by-products from fatty acid metabolism inhibit growth of non-adapted microorganisms.

  3. Sweat

    • Environmental factor that inhibits microorganisms due to its low pH and high salt concentration.

  4. Lysozyme

    • Enzyme found in sweat, tears, and saliva; it breaks down peptidoglycan in bacterial cell walls.

Concept Check 1

  • Question: What substance produced by the skin is metabolized by normal microbiota, producing toxic byproducts that inhibit the growth of potential pathogens?

    • Answer: C. Sebum

Learning Outcomes Section 16.2

  1. Characteristics of Skin's Normal Microbiota

    • Must thrive in dry and salty conditions.

    • Sparse in dry areas (e.g., back) but dense in moist areas (e.g., armpits, groin).

    • Can inhabit hair follicles and gland ducts.

    • Key organisms: Streptococcus, Staphylococcus, Corynebacterium, Propionibacterium, Pseudomonas, Lactobacillus; yeasts such as Candida.

Normal Biota of the Skin

  1. Microbial Diversity

    • Recent microbiome studies reveal hundreds of species; individual differences exist in microbiota composition.

    • Stability over time for an individual.

Concept Check 2

  • Statement: Normal biota grow into dense populations over dry, flat areas of the human body.

    • Answer: B. False

Learning Outcomes Section 16.3

  1. MRSA Skin and Soft-Tissue Infection

    • Signs and Symptoms:

      • Raised, red, tender lesions filled with pus; heat upon touch.

      • Fever common.

    • Transmission:

      • Common on gym equipment, electronic devices, razors, and surfaces.

      • Active lesions should remain covered to prevent spread.

    • Pathogenesis and Virulence Factors:

      • Key diagnostic marker: Coagulase.

      • Produces hyaluronidase, staphylokinase, DNase, lipase.

    • Culture/Diagnosis:

      • Techniques include PCR, blood agar or mannitol salt agar culture.

      • Catalase test and Coagulase test.

  2. Highlight Disease: Maculopapular Rash Diseases

    • Caused by various microorganisms.

    • Characterized by flat or slightly raised colored bumps.

    • Examples include Measles, Rubella, Fifth Disease, Roseola.

  3. Measles

    • Signs and Symptoms:

      • Sore throat, dry cough, headache, conjunctivitis, fever, oral lesions (Koplik’s spots), maculopapular rash starting from the head.

    • Complications include laryngitis, bronchopneumonia, pneumonias, encephalitis.

    • Pathogenesis: virus travels via respiratory droplets, infects lymphatic system, and enters bloodstream; leads to cell fusion causing syncytia.

    • Transmission:

      • Highly contagious; human-only reservoir; infectious period prior to rash emergence.

    • Diagnosis: Clinical presentation, ELISA for IgM to measles antigen.

    • Prevention:

      • MMR vaccine provides long-term protection.

    • Treatment:

      • Focuses on symptom relief (fever reduction, cough suppression).

      • Vitamin A supplementation recommended.

  4. Rubella

    • Postnatal Rubella: Pink rash appearing on the face, mild prevalence.

    • Congenital Rubella: Teratogenic effects if infection occurs during first trimester; can lead to severe birth defects including deafness and heart defects.

    • Causative Agent: Rubivirus, affects mitosis, leading to tissue apoptosis.

    • Transmission: Mostly moderate, requires close contact.

    • Diagnosis: Detecting IgM antibody through ELISA.

    • Prevention: MMR vaccine recommended; treatment is generally symptomatic only.

  5. Impetigo

    • Characteristics: Superficial bacterial infections leading to flaky skin; caused mainly by Staphylococcus aureus or Streptococcus pyogenes.

    • Signs: Peeling skin, crusty scabs, honey-colored crusts found on face and extremities.

    • Pathogenesis: Enzymes and toxins produced by bacteria can complicate or worsen the condition.

  6. Cellulitis

    • Characteristics: Fast-spreading infection in the dermis/subcutaneous tissues.

    • Signs: Pain, tenderness, swelling, fever, lymphadenopathy.

    • Causative Agents: Mainly Streptococcus pyogenes and occasionally Staphylococcus aureus.

    • Treatment: Responsive to antibiotics; severe cases may require IV antibiotics or surgical intervention.

  7. Staphylococcal Scalded Skin Syndrome (SSSS)

    • Caused by exfoliative toxins from Staphylococcus aureus; typically affects newborns.

    • Transmission: Caregivers can transmit via contact; 30% carriers in adults.

    • Symptoms: Bullous lesions, severe damage to epidermis leading to peeling and desquamation.

  8. Vesicular or Pustular Rash Diseases

    • Example: Chickenpox (varicella) and smallpox (variola).

    • Signs of Chickenpox: Fever, rash starts at scalp and face, progresses to trunk; lesions are itchy vesicles.

    • Smallpox: Highly virulent; causes fever and a severe, widespread rash.

  9. Leishmaniasis

    • Cutaneous leishmaniasis affects skin capillaries; mucocutaneous variant is endemic to certain regions.

    • Causative Agents: Various species, e.g., L. tropica.

    • Transmission: By female sand flies; commonly affects travelers.

  10. Cutaneous Anthrax

    • Displays as a necrotic papule leading to a painless black eschar.

    • Causative Agent: Bacillus anthracis.

    • Common in occupations handling animal products.

  11. Superficial Mycoses: Ringworm

    • Caused by dermatophytes; infections are confined to surface skin, hair, and nails.

    • Common types include tinea pedis (athlete's foot), tinea corporis (body), and tinea capitis (scalp).

  12. Keratitis

    • Signs: Deeper tissue invasion leading to corneal damage.

    • Causes include bacteria and herpes simplex virus. Very serious, can cause blindness.

    • Diagnosis: Clinical evaluation, possible culture; treatment depends on causative agent.

  13. Conjunctivitis

    • Signs: Bacterial forms produce milky discharge; viral forms produce clear discharge, often causing morning adherence of eyelids.

    • Causative Agents: Includes Neisseria gonorrhoeae (neonatal) and various bacteria for other age groups.

    • Treatment: Antibiotics for bacterial conjunctivitis; symptomatic for viral.

Concept Check 6

  • Question: What are the most common causes of neonatal conjunctivitis? Select all that apply.

    • Answers: B. Chlamydia trachomatis, E. Neisseria gonorrhoeae.

Summary of Major Skin and Eye Infections

  • Keratitis: caused by Herpes simplex virus and bacteria.

  • Large Pustular Skin Lesions: caused by Leishmania species and Bacillus anthracis.

  • Conjunctivitis: caused by Neisseria gonorrhoeae, Chlamydia trachomatis, various bacteria and viruses.

  • Staphylococcal Scalded Skin Syndrome: caused by Staphylococcus aureus.

  • Vesicular or Pustular Rash Disease: caused by Human herpesvirus 3 and Variola virus.