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Chapter 18
Infectious Diseases Affecting the Skin and Eyes
Chapter Outline
18.1. The Skin and Its Defenses: Constantly exposed to the environment; a vulnerable site for infection
A. Integument
1. Hair, nails, sweat, skin, and oil glands make up the integument
B. The skin is organized in layers
1. The epidermis
a. The stratum corneum (epithelial cells) contains the protein keratin
i. Keratin protects the cells from damage, abrasion, and water
penetration
b. Three or more layers of epithelial cells lie below the stratum corneum
c. Lowest layer is stratum basale (basal layer), which is attached to the
underlying dermis
2. The dermis is composed of connective tissue instead of epithelium
3. Also present in the dermis are:
a. Hair follicles
b. Sebaceous glands
C. Defenses of the skin
1. Keratinized surface
2. Low pH of sebum
3. High salt in sweat
4. Lysozyme in sweat
5. Antimicrobial peptides
18.2. Normal Biota of the Skin
A. The Diphtheroids
1. Corynebacterium
2. Propionibacterium acnes
B. Micrococci
1. Staphylococci
a. Coagulase-negative Staphylococci (Staphylococcus epidermidis)
b. Coagulase-positive Staphylococci (Staphylococcus aureus)
2. Streptococci
3. Micrococcus
C. Yeasts
18.3. Skin Diseases Caused by Microorganisms
A. Acne
1. Follicle-associated lesions
2. Microbial digestion of excess sebum
3. Causative agent
a. Propionibacterium acnes
4. Transmission and epidemiology
a. Normal biota
b. Adolescents and young adults
c. Male hormones
5. Prevention and treatment
a. Topical sloughing agents
1-1
b. Antibiotics
c. Oral contraceptives for females
d. Accutane
B. Impetigo
1. A highly contagious superficial bacterial infection
2. Causative organisms
a. Staphylococcus aureus
b. Streptococcus pyogenes.
3. Signs and symptoms
4. Impetigo caused by Staphylococcus aureus
a. Staphylococcus aureus
i. Facultative anaerobe
ii. High salt and extreme pH tolerance
b. Pathogenesis and virulence factors
i. Coagulase
ii. Exfoliative exotoxins A and B
iii. Hyaluronidase
iv. Lipases
v. DNase
c. Culture and diagnosis
i. Blood agar
ii. Mannitol salt agar
iii. Catalase
iv. Coagulase
5. Impetigo caused by Streptococcus pyogenes
a. Streptococcus pyogenes
i. Lancefield group A
ii. ß-hemolytic on blood agar.
b. Pathogenesis and virulence factors
i. M protein
ii. Hyaluronidase
iii. Streptokinase
iv. Associated with acute post streptococcal glomerulonephritis
6. Transmission and epidemiology of impetigo
a. Highly contagious
b. Affects mostly preschool children
c. Prevention
i. Good hygiene
d. Treatment
i. Topical mupirocin
ii. Treats both causative organisms
C. Cellulitis
1. A fast spreading infection of the dermis and subcutaneous tissue
2. Causative organisms
a. Staphylococcus aureus
b. Streptococcus pyogenes
3. Immunocompromised and those with cardiac insufficiency are at high risk
4. Treatment is through oral or IV antibiotics
5. Surgical debridement in severe cases
D. Staphylococcal Scalded Skin Syndrome (SSSS)
1. Dermolytic condition caused by Staphylococcus aureus
1-2
2. Affects mostly newborns and babies
3. Systemic form of impetigo
4. Desquamation of the skin
5. Exfoliative toxin A and B
E. Gas gangrene
1. Most often caused by Clostridium perfringens
2. Also called clostridial myonecrosis
3. Signs and symptoms
a. Anaerobic cellulitis
i. Damaged necrotic muscle tissue or myonecrosis
ii. Toxins and gas
4. Pathogenesis and virulence factors
a. Alpha toxin
b. Collagenase, hyaluronidase, DNAse
5. Transmission and epidemiology
6. Prevention and treatment
a. Cleaning and surgical repair of wounds
b. Broad spectrum antibiotics
c. Hyperbaric oxygen therapy
F. Vesicular or pustular rash diseases
1. Chickenpox
a. Signs and symptoms
i. Skin lesions
ii. Macules and papules to itchy fluid-filled vesicles
iii. Encephalopathy in 0.1% of cases
b. Shingles
i. Herpes zoster
ii. Reactivation of virus in ganglia
c. Causative agent
i. Human Herpesvirus 3 (HHV-3)
ii. Varicella-Zoster virus
iii. Enveloped DNA virus
d. Pathogenesis and virulence factors
i. HHV-3 remains latent in ganglia
ii. Protects virus from attack by the immune system
e. Transmission and epidemiology
f. Prevention
i. Attenuated vaccine
ii. Oka strain
iii. Zostavax for prevention of shingles in adults 60 years and older
g. Treatment
f. Special note about Reye’s syndrome
2. Smallpox
a. Signs and symptoms
i. Fever
ii. Rash in the mouth
iii. Macular, papular, vesicular, and pustular
b. Causative agent
i. Variola virus is an Orthopoxvirus
ii. Enveloped, brick-shaped DNA virus
c. Pathogenesis and virulence factors
1-3
d. Transmission and epidemiology
e. Prevention
i. Jenner
ii. Vaccinia virus vaccine
f. Treatment
G. Maculopapular rash diseases
1. Measles
a. Rubeola
b. Signs and symptoms
i. Koplik’s spots
ii. Maculopapular exanthemum
iii. Subacute sclerosing panencephalitis (SSPE)
c. Causative agent
i. Morbillivirus genus
ii. Single-stranded enveloped RNA virus
d. Pathogenesis and virulence factors
i. Causes viremia
ii. Induces formation of large syncytia (giant cells with many nuclei)
iii. Disables many aspects of the host immune response
e. Transmission and epidemiology
i. Transmitted through respiratory droplets
f. Culture and diagnosis
i. Clinical presentation
ii. ELISA
g. Prevention
i. The MMR vaccine (for measles, mumps, and rubella)
h. Treatment
2. Rubella
a. German measles
b. Signs and symptoms
i. Postnatal
ii. Congenital (prenatal) infection of the fetus
c. Causative agent
i. Rubivirus found in the family Togavirus
ii. Single-stranded RNA virus with a loose lipid envelope
d. Pathogenesis and virulence factors
e. Transmission and epidemiology
f. Culture and diagnosis
g. Prevention
i. The MMR vaccination
h. Treatment
3. Fifth Disease
a. Erythema infectiosum
b. Characteristic “slapped-cheek” appearance
c. The causative agent is parvovirus B19
d. Very contagious; can cause stillbirth in pregnant women
4. Roseola
a. Maculopapular rash
b. HHV-6 and HHV-7
5. Scarlet fever
1-4
a. Usually results of a respiratory infection with Streptococcus pyogenes
b. Most often pharyngitis
c. S. pyogenes strain must carry a bacteriophage with a gene for an
exotoxin called erythrogenic toxin
H. Wart-like eruptions
1. Viruses cause all warts
2. Warts (papillomas)
a. Benign, squamous epithelial growths
b. Human Papilloma Virus (HPV)
c. Can become malignant, when caused by a particular type of HPV
3. Molluscum contagiosum
a. Poxvirus
I. Larger pustular skin lesions
1. Leishmaniasis
a. Zoonosis transmitted by female sand fly
b. Leishmania tropica (cutaneous leishmaniasis)
2. Cutaneous anthrax
a. Bacillus anthracis
b. Eschar formation
c. Penicillin and ciproflaxin treatment
J. Ringworm (Cutaneous Mycoses)
1. Fungal dermatophytes cause mycoses that are confined to the nonliving epidermal
tissues, hair and nails
a. Ringworm of the scalp (Tinea Capitis)
b. Ringworm of the beard (Tinea Barbae)
c. Ringworm of the body (Tinea Corporis)
d. Ringworm of the groin (Tinea Cruris)
e. Ringworm of the foot (Tinea Pedis)
f. Ringworm of the hand (Tinea Manuum)
g. Ringworm of the nail (Tinea Unguium)
2. Causative agents
a. There are about 39 species in the genera Trichophyton, Microsporum, and
Epidermophyton
3. Pathogenesis and virulence factors
a. Digest keratin
4. Transmission and epidemiology
5. Prevention and treatment
K. Superficial mycosis
1. Outer epidermis.
2. Tinea versicolor
a. Malassezia furfur
18.4. The Surface of the Eye and Its Defenses
A. Conjunctiva
B. Cornea
C. The flushing action of tears, which contain lysozyme and lactoferrin, is a major
protective feature
D. Immune privilege
18.5. Normal Biota of the Eye
A. Generally sparse
18.6. Eye Diseases Caused By Microorganisms
1-5
A. Conjunctivitis
1. Infection of the conjunctiva
2. Signs and symptoms
a. Most bacterial infections produce a milky discharge
b. Viral infections produce a clear exudate
c. Commonly called “pinkeye”
3. Causative agents and their transmission
a. Neonatal eye infection—Neisseria gonorrhoeae or Chlamydia trachomatis
b. Bacterial conjunctivitis in other ages is usually caused by Staphylococcus
epidermidis, Streptococcus pyogenes, Streptococcus pneumoniae, Haemophilus
influenzae, and Moraxella species
c. Viral conjunctivitis: usually caused by adenoviruses
d. Both bacterial and viral conjunctivitis is highly contagious
4. Prevention and treatment
B. Trachoma
1. Ocular trachoma
a. Preventable form of blindness
b. Caused by Chlamydia trachomatis
C. Keratitis
1. Keratitis is a more serious eye infection than conjunctivitis
2. Herpes simplex virus type 1 HSV-1 and HSV-2
D. River blindness
1. Chronic parasitic helminth infection
2. Onchocerca volvulus—transmitted by small biting black flies
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