Chapter 19

Chapter 19: Infectious Diseases Manifesting in the Skin and Eyes

Section 19.1: The Skin, Its Defenses, and Normal Biota

Learning Outcomes

• Describe the important anatomical features of the skin.
• List the natural defenses present in the skin.
• List the types of normal biota presently known to occupy the skin.

The Skin and Its Defenses: Integument

• Integument components:
    - Skin:
      - Total surface area: $1.5$ to $2$ m²
      - Thickness varies from $1.5$ to $4$ mm
      - Composed of several layers at each thickness
    - Accessories (associated structures):
      - Hair
      - Nails
      - Sweat and oil glands

Important Layers of the Skin

Epidermis

• Stratum corneum:
    - Thick layer of epithelial cells packed with keratin, providing strength against damage and abrasion.
    - Contains lipids that provide water-repellent properties.
    - The entire epidermis is replaced every $25$ to $45$ days.
• Stratum basale:
    - The basal layer, serving as the source for all epidermal cells.

Dermis

• Composition:
    - Made of connective tissue, rich in fibroblast cells and fibers (like collagen).
    - Contains macrophages, mast cells, blood vessels, nerves, and lymphatic vessels.
• Effects of Damage:
    - Damage to the dermis can result in bleeding and blister formation.
    - Blisters arise from friction, trauma, or burns causing a separation between epidermis and dermis.

Skin Appendages: Follicles and Glands

• Follicles:
    - Hair roots are located in the dermis along with associated sebaceous (oil) and scent glands.
• Sweat Glands:
    - Separate from hair follicles, also located in the dermis.
    - All glands have openings on the surface, passing through the epidermis.

Natural Antimicrobial Defenses

• Antimicrobial Peptides:
    - Positively charged chemicals disrupting negatively charged bacterial membranes.
    - Diverse types responsible for maintaining low microbial counts on skin.
• Sebum:
    - Secreted by sebaceous glands, has a low pH making skin inhospitable to most microbes.
    - Contains high lipid concentrations, which serve as nutrients for normal microbiota.
    - Breakdown of fatty acids produces toxic byproducts inhibiting non-adapted microbes.
• Sweat:
    - Exhibits low pH and high salt concentration inhibiting microbial growth.
• Lysozyme:
    - An enzyme found in sweat, tears, and saliva that specifically breaks down peptidoglycan in bacterial cell walls.

Normal Biota of the Skin

• The skin's environment is generally inhospitable to microbes due to its vast, dry, salty surfaces.
• Denser microbial populations are found in moist areas, skin folds, hair follicles, and glandular ducts.
• Microbiome Observations:
    - Hundreds of species distributed across various body areas.
    - Five major taxa present, with their predominance varying in different regions sampled.
    - Significant variability exists among individuals regarding skin microbiota composition.
    - Stable skin microbiota observed over time for individuals.
    - Key Species: Staphylococcus epidermidis and Propionibacterium acnes are prevalent due to their salt-tolerance.
    - About 4 ext{%} of the population carries Staphylococcus aureus, which is a potential pathogen.

Defenses and Normal Biota Summary

• Defenses:
    - Keratinized surface, sloughing off, low pH, high salt concentration, presence of lysozyme.
• Normal Biota:
    - Predominant bacteria: Staphylococcus epidermidis, Propionibacterium, Corynebacterium, Lactobacillus, Bacteroides, Prevotella, Haemophilus.
    - Yeasts: Malassezia, Candida.

Concept Check—Section 19.1

1. Cells in the epidermis contain keratin, giving them the ability to withstand damage.
2. Damage to the dermis causes bleeding.
3. The low pH of sebum along with toxic by-products of its metabolism inhibits the growth of most microbes.
4. Lysozyme is an enzyme that targets peptidoglycan.
5. There are denser populations of bacteria on the skin in moist areas and skin folds.
6. True/False: Everyone carries about the same population of skin biota. False.

Section 19.2: Infectious Diseases Manifesting on the Skin

Learning Outcomes

• List possible causative agents for skin conditions: MRSA, impetigo, cellulitis, staphylococcal scalded skin syndrome, gas gangrene, vesicular/pustular rash diseases, maculopapular rash diseases, wart-like eruptions, large pustular lesions, and cutaneous/superficial mycosis.
• Identify conditions transmitted to the respiratory tract through droplet contact.
• List skin conditions for which vaccination is recommended.

Staphylococcus aureus

• Characteristics:
    - Gram-positive coccus forming grape-like clusters.
    - Nonmotile and sturdiest non-endospore-forming pathogen.
    - Capable of withstanding high salt, extremes in pH, and high temperatures.
    - Viable after months of air drying.

MRSA and Soft Tissue Infections

• Common cause of skin lesions, particularly in non-hospitalized individuals.
• Known for resistance to multiple antibiotics.
• Infections manifest as raised, red, tender lesions, often with pus and heat.
• Symptoms may include fever.
• Often originate from skin breaks due to injury or shaving.

Pathogenic Characteristics of S. aureus

• Typical enzymes expressed:
    - Coagulase: Coagulates plasma.
    - Hyaluronidase: Digests hyaluronic acid binding connective tissue.
    - Staphylokinase: Digests blood clots.
    - DNase: Degrades DNA.
    - Lipase: Colonizes oily skin.
    - Catalase: Breaks down hydrogen peroxide.

The Coagulase Test

• Used for identifying S. aureus:
    - Confirmatory for virulence.

MRSA Skin and Soft Tissue Infections Disease Table

• Causative Organism: Methicillin-resistant Staphylococcus aureus
• Modes of Transmission: Direct and indirect contact.
• Virulence Factors: Coagulase and other enzymes, superantigens.
• Diagnosis: Utilizing culture and Gram stain, coagulase and catalase tests, multitest systems.
• Prevention: Promoted through hygiene practices.
• Treatment: Vancomycin; noted as a Serious Threat in CDC reports.
• Epidemiology: Commonly found among children and young to middle-aged adults.

Impetigo

• Definition: A superficial bacterial infection resulting in skin flaking or peeling.
• Causative Agents: Either S. aureus or S. pyogenes.
• Primary Victims: Typically children.
• Symptoms: Similar regardless of the causative agent.
    - Mechanism: S. pyogenes may initiate the infection; S. aureus can take over due to its production of bacteriocin that targets S. pyogenes.

Impetigo Disease Table

• Causative Organisms: S. aureus and S. pyogenes.
• Modes of Transmission: Direct contact and indirect contact.
• Virulence Factors: Enzymes such as streptokinase, plasminogen-binding ability, hyaluronidase, M protein (for S. pyogenes), and exfoliative toxins (for S. aureus).
• Diagnosis: Based on clinical signs or cultured when necessary.
• Prevention: Hygiene practices.
• Treatment: Topical treatments like mupirocin, and oral antibiotics for more severe cases.

Cellulitis

• Definition: A fast-spreading infection in the dermis and subcutaneous tissues.
• Symptoms: Pain, tenderness, swelling, warmth, fever, and swollen lymph nodes.
• Progression: Red lines may lead away from an affected area, often following bacteria or fungi introduction through trauma, though no obvious break is necessary.
• At-Risk Factors: Immunocompromised individuals or those with cardiovascular insufficiency.

Cellulitis Disease Table

• Causative Organisms: MRSA, S. pyogenes, and other bacteria or fungi.
• Modes of Transmission: Parenteral implantation.
• Treatment: May require aggressive antibiotics or surgical interventions.

Staphylococcal Scalded Skin Syndrome (SSSS)

• Definition: A dermolytic infection chiefly affecting newborns.
• Causative Organism: S. aureus producing specific exfoliative toxins leading to blistering and desquamation
• Symptoms: Bullous lesions resembling wrinkled tissues. Vulnerability to secondary bacterial infections.

Gas Gangrene

• Definition: Caused by Clostridium perfringens, an endospore-forming bacterium needing anaerobic conditions for its exotoxins.
• Two Forms: Anaerobic cellulitis (localized) and true myonecrosis (spreading).

Disease Tables for Central Conditions

1. Impetigo
      - Causes: S. aureus, S. pyogenes.
      - Transmission: Direct/indirect contact.
      - Treatment/Diagnosis: Clinical signs or cultures.
2. Cellulitis
      - Causes: MRSA, S. pyogenes.
3. SSSS
      - Causes: S. aureus, treatment with systemic antibiotics recommended.
4. Gas Gangrene
      - Causes: C. perfringens, treatment includes surgical intervention and antibiotics.
      - Mortality rates vary; untreated can approach 100 ext{%}.

Terms Used in Describing Skin Conditions and Infections

• Bulla: Large vesicle, e.g., blisters in gangrene.
• Cyst: Raised encapsulated lesion, e.g., severe acne.
• Macule: Flat, well-demarcated lesion, e.g., freckles.
• Papule: Elevated solid bump, e.g., warts.
• Pustule: Small elevated lesion filled with pus.

Vesicular or Pustular Rash Diseases

• Common Viral Diseases:
    1. Chickenpox
    2. Smallpox
    3. Monkeypox
    4. Hand, foot, and mouth disease (HFMD).

Chickenpox

• Typically mild but can be serious for immunocompromised individuals.
• Incubation Period: $10$ to $20$ days; rash forms in a centrifugal pattern.
• Vaccine is available and highly effective.

Smallpox

• Contained two forms: Variola major (highly virulent) and Variola minor.

Maculopapular Rash Diseases

• Key infectious conditions include measles, rubella, fifth disease, and roseola.
• Measles (Rubeola): Significant past mortality; requires a 95 ext{%} vaccination rate for herd immunity.
• Rubella: Minor rash but serious fetal complications.
• Fifth Disease: Characterized by a “slapped-cheek” appearance.
• Roseola: High fever in infants, often with a rash that appears later.

Wartlike Eruptions

• Caused by human papillomaviruses (HPVs), with over $130$ types.
• Molluscum contagiosum also causes wart-like bumps.
  

Leishmaniasis and Cutaneous Anthrax

• Leishmaniasis: Zoonosis from sand flies, presents with skin lesions.
• Cutaneous Anthrax: Infection from Bacillus anthracis leading to black eschar.

Ringworm (Cutaneous Mycoses)

• Dermatophytes: Cause infections strictly confined to skin.
• Various tinea conditions include tinea capitis (scalp), tinea cruris (groin), tinea pedis (foot) etc.

Superficial Mycoses

• Example: Tinea versicolor, which affects superficial skin layers without inflammation.

Concept Check—Section 19.2 Performance

1. True/False: You can “catch” shingles from someone with an outbreak. False.
2. The most serious form of rubella is congenital rubella.
3. True/False: Most warts disappear after 2 to 3 years. True.
4. Leishmania is transmitted by the female sand fly.
5. True/False: Cutaneous mycoses invade the dermal tissue. False.

Section 19.3: The Surface of the Eye, Its Defenses, and Normal Biota

Learning Outcomes

• Describe anatomical features of the eye.
• List natural defenses present in the eye.
• List types of normal biota known to occupy the eye.

Anatomical Features of the Eye

• Conjunctiva: Thin membrane covering the eye and eyelids, secreting protective fluids.
• Cornea: Dome-shaped, can regenerate quickly after superficial damage, acts as the “windshield” of the eye.
• Tears: Composed of aqueous fluid that protects and lubricates the surface of the eye.
• Immune Privilege: Reduced innate immunity to protect vision by limiting B- and T-cell responses and isolating the anterior chamber from blood supply.

Normal Biota of the Eye

• Dominant genera include Corynebacterium, Staphylococcus epidermidis, Micrococcus, and Streptococcus.

Concept Check—Section 19.3

1. The cornea can regenerate quickly if superficially damaged.
2. Tears are formed in the lacrimal gland.
3. Lysozyme and lactoferrin in tears have antimicrobial properties.
4. The reduced innate immunity of the eye to protect vision is known as immune privilege.
5. Staphylococcus is the dominant genus in the eye microbiome.

Section 19.4: Infectious Diseases Manifesting in the Eye

Learning Outcomes

• List possible causative agents for diseases such as conjunctivitis, trachoma, keratitis, and river blindness.
• Discuss differential diagnoses for neonatal vs nonneonatal conjunctivitis.

Conjunctivitis

• Commonly known as pink eye: infection of the conjunctiva.
• Bacterial: usually presents milky discharge.
• Viral: presents with clear exudate.
• Allergic: causes copious amounts of clear fluid, with redness and swelling.

Neonatal vs Nonneonatal Conjunctivitis Disease Table

• Neonatal conjunctivitis
    - Causes: C. trachomatis, N. gonorrhoeae.
    - Modes of Transmission: Vertical.

• Bacterial conjunctivitis
    - Causes: Various (S. aureus, S. pneumoniae).
    - Modes of Transmission: Direct and indirect contact.

• Viral conjunctivitis
    - Causes: Adenoviruses and others.
    - Modes of Transmission: Direct and indirect contact.

Ocular Trachoma

• Chronic infection of epithelial cells caused by C. trachomatis; major global blindness cause.
• Transmission: Indirect and mechanical vector.

Keratitis

• More severe than conjunctivitis, can lead to corneal destruction.
• May arise from trauma or pathogens like HSV-1 and Acanthamoeba.

River Blindness

• Chronic parasitic infection by Onchocerca volvulus, transmitted by black flies.
• Causes significant inflammatory damage in human tissues.

Concept Check—Section 19.4

1. Bacterial conjunctivitis has a milky exudate; viral has a clear exudate.
2. True/False: Trachoma is caused by the same strain of C. trachomatis that causes conjunctivitis. True.
3. Which of the following eye diseases does not lead to blindness?
      - a. Trachoma
      - b. Keratitis
      - c. Conjunctivitis (d) River blindness.

Taxonomic Organization: Microorganisms Causing Disease of the Skin and Eyes

Gram-Positive Bacteria

• MRSA: Cause of various conditions such as skin abscesses and conjunctivitis.
• Streptococcus pyogenes: Cause of impetigo, cellulitis, and other infections.
• C. perfringens: Associated with gas gangrene.
• Bacillus anthracis: Causes cutaneous anthrax.
• S. pneumoniae: Linked to conjunctivitis.

Gram-Negative Bacteria

• C. trachomatis: Causes neonatal and bacterial conjunctivitis.
• Neisseria gonorrhoeae: Associated with conjunctivitis and trachoma.
• Wolbachia: Known for synergy with Onchocerca in river blindness.

DNA Viruses

• Human herpesvirus 3: Causes chickenpox.
• Variola virus: Causes smallpox.
• Parvovirus B19: Caused fifth disease.
• HPV: Linked with warts.

RNA Viruses

• Coxsackie: Responsible for HFMD.
• Measles virus: Causes measles.
• Rubella virus: Causes rubella.

Fungi

• Trichophyton, Microsporum, and Epidermophyton: Linked to ringworm.
• Malassezia species: Known for superficial mycoses.

Protozoa

• Leishmania spp.: Causes leishmaniasis.
• Acanthamoeba: Associated with keratitis.

Helminths

• Onchocerca volvulus (with Wolbachia): Causes river blindness.