Microbial Diseases of the Skin and Eyes: Structure, Pathogenesis, and Clinical Manifestations

Diseases of the Skin and Eyes

Introduction

The skin and eyes serve as important barriers against microbial invasion. However, various microorganisms can overcome these defenses, leading to a range of infectious diseases. This chapter explores the structure of the skin, its normal microbiota, and the major bacterial, viral, fungal, and arthropod-associated diseases affecting the skin and eyes.

Skin: Structure and Defense Mechanisms

Properties of the Skin

• Salty, dry, and low pH environment: These properties inhibit microbial growth.

• Lysozymes: Enzymes that hydrolyze peptidoglycan, breaking down bacterial cell walls.

• Normal microbiota: Includes Gram-positive, salt-tolerant bacteria such as Staphylococci, Micrococci, and diphtheroids.

Mucous Membranes

• Mucus secretion: Traps foreign invaders.

• Ciliary escalator: Some cells have cilia that move mucus and trapped particles out of the body.

Common Signs and Symptoms of Skin Diseases

Exanthem and Enanthem

• Exanthem: Skin rash arising from disease conditions.

• Enanthem: Rash on mucous membranes (e.g., inside the mouth).

Lesions and Pus Formation

• Lesions: Formed from fluid within skin layers; differences depend on the layer affected.

• Pus: Protein-rich fluid containing dead leukocytes and cellular debris.

Types of Skin Lesions

• Lesions can be classified based on appearance and underlying pathology (e.g., macule, papule, vesicle, pustule, ulcer, etc.).

Bacterial Infections of the Skin

Staphylococcus Species

• Staphylococcus epidermidis: Gram-positive, normal flora, opportunistic.

• Staphylococcus aureus: Gram-positive, often found in nasal passages, most pathogenic of staphylococci, causes nosocomial infections, several antibiotic-resistant strains (e.g., MRSA).

• Virulence factors: Coagulase (clots fibrin), toxins (leukocidins, hemolysins, exfoliative toxin, enterotoxins).

Common Staphylococcal Infections

• Folliculitis: Infection of hair follicles.

• Sty (stye): Folliculitis of an eyelash.

• Furuncle: Abscess/boil; localized pus surrounded by inflamed tissue.

• Carbuncle: Inflammation of tissue under the skin, can lead to fever and generalized illness.

• Impetigo: Highly contagious skin infection, primarily in children.

• Scalded Skin Syndrome (SSS): Toxin-mediated loss of skin adhesion.

• Toxic Shock Syndrome (TSS): TSST-1 superantigen triggers massive cytokine release, leading to shock and potentially death.

MRSA (Methicillin-resistant Staphylococcus aureus)

• HA-MRSA: Hospital-acquired, serious for immunocompromised patients.

• CA-MRSA: Community-acquired, more deadly, secretes high levels of modulins (peptides that destroy neutrophils).

Streptococcus Species

• Streptococcus pyogenes: Gram-positive, grows in chains, catalase negative, Group A Streptococci (GAS), produces beta-hemolysins, M proteins on cell walls.

Virulence Factors

• Hemolysins: Destroy red blood cells (alpha, beta, gamma types).

• Leukocidins: Destroy leukocytes.

• M-protein: Antiphagocytic, aids in adherence, antigenic variation, degrades complement protein C3b.

• Streptokinases: Dissolve blood clots.

• Streptococcal pyrogenic exotoxins (SPEs): Superantigens causing fever and hypersensitivity (e.g., scarlet fever, STSS).

Common Streptococcal Infections

• Erysipelas: Erythema affecting the dermal layer, sometimes includes lymphatics.

• Impetigo: Also caused by Streptococcus.

Necrotizing Fasciitis

• Caused by GAS producing streptokinases, hyaluronidase, and SPE superantigens.

• Results in rapid death of soft tissue, can be fatal if untreated.

Pseudomonads

• Pseudomonas aeruginosa: Gram-negative, aerobic rod, produces blue-green pus (pyocyanin).

• Pseudomonas dermatitis: Associated with swimming pools and hot tubs.

• Otitis externa: Swimmer’s ear infection.

• Major concern for nosocomial infections, especially in burn patients.

Propionibacterium

• Propionibacterium acnes: Gram-positive, anaerobic rod, causes inflammatory acne by metabolizing sebum.

• Treatment: Antibiotics, benzoyl peroxide, visible (blue) light therapy.

Viral Infections of the Skin

Papillomaviruses (HPV)

• Over 50 types; cause warts, transmitted by contact.

• HPV-16 associated with cancers; vaccine reduces cervical cancer risk by 85–90%.

• Epidermodysplasia verruciformis ("tree man" disease) is a rare susceptibility to HPV.

Poxviruses

• Cause ulcers, macular rash, fever; transmitted via respiratory route.

• Smallpox (variola virus): Variola major (~20% mortality), variola minor (<1% mortality).

• First disease with a vaccine (Edward Jenner, 1796).

• Monkeypox: Zoonotic, milder than smallpox, recent outbreaks.

Herpesviruses

• Varicella-zoster virus (HHV-3): Causes chickenpox (varicella) and shingles (zoster); latent in dorsal root ganglion.

• HSV-1: Oral herpes (cold sores, blisters).

• HSV-2: Genital herpes.

• Other forms: Herpes gladiatorum (skin), herpes whitlow (fingers), herpes encephalitis (brain, high fatality).

• Treatment: Antivirals such as acyclovir.

Measles Virus (Rubeola)

• Highly contagious; transmitted via respiratory route.

• Symptoms: Koplik’s spots (enanthem in mouth), macular rash, fever.

• Complications: Immune amnesia, encephalitis (1/1,000 cases).

• Vaccine: MMR (measles, mumps, rubella).

German Measles Virus (Rubella)

• Milder than measles; macular rash, fever.

• Congenital rubella syndrome: Severe fetal damage (deafness, cataracts, learning disabilities).

• Vaccine: MMR.

Parvoviruses and Roseola

• Human parvovirus B19: Erythema infectiosum (fifth disease), flu-like symptoms, "slapped cheek" rash.

• HHV-6 and HHV-7: Roseola (sixth disease), high fever, rash lasting 1–2 days.

Fungal Infections of the Skin

Cutaneous Mycoses

• Fungi that metabolize keratin (dermatomycoses, tineas/ringworm).

• Trichophyton: Infects hair, skin, nails.

• Epidermophyton: Infects skin, nails.

• Microsporum: Infects hair, skin.

• Treatment: Oral griseofulvin, topical miconazole.

Subcutaneous Mycoses

• Sporotrichosis (Sporothrix schenckii): Enters via puncture wound, causes nodular lesions, can spread if untreated. Treated with potassium iodide (KI).

Candidiasis

• Candida albicans: Causes oral thrush (white patches, soreness), urogenital yeast infections (itching, discharge).

• Predisposing factors: Antibiotic therapy, immunosuppression.

• Treatment: Miconazole, nystatin, diflucan.

Arthropod Infections of the Skin

Scabies

• Sarcoptes scabiei (mite): Burrows in skin to lay eggs, causes itching and rash.

• Transmitted via contact and fomites (clothes, bedding).

• Treatment: Topical insecticides.

Pediculosis (Lice)

• Pediculus humanus capitis (head louse): Causes itching, mild reaction to bites, swelling of lymph nodes.

• Transmitted via contact; females lay eggs (nits) on hair.

• Treatment: Topical insecticides, removal of eggs.

Diseases of the Eyes

Bacterial Infections

• Conjunctivitis (pinkeye): Most commonly caused by Haemophilus influenzae; symptoms include swelling of conjunctiva and red eyes. Transmitted via contact, associated with unsanitary conditions. Treated with topical antibiotics.

• Ophthalmia neonatorum: Caused by Neisseria gonorrhoeae, transmitted to newborns during birth, can lead to blindness. Prevented with antibiotics or silver nitrate.

• Inclusion conjunctivitis and trachoma: Caused by Chlamydia trachomatis, can lead to blindness, transmitted via birth canal, fomites, or arthropods. Treated with tetracycline.

Protozoan Infections

• Acanthamoeba keratitis: Caused by Acanthamoeba spp., slowly destroys the cornea, leading to blindness. Transmitted via contaminated water or unsanitary contact lenses.