Skin and Eye Infections and Respiratory System Infections

CHAPTER 21: SKIN AND EYE INFECTIONS

  • Major Anatomical Features of the Skin and Eyes

    • The skin serves as the body's largest organ, comprising the epidermis, dermis, and subcutaneous tissue. The epidermis is the outermost protective layer, primarily composed of keratinocytes, and provides a physical barrier. The dermis lies beneath, containing connective tissue, hair follicles, sweat glands, blood vessels, and nerves, providing strength and elasticity. The subcutaneous tissue (hypodermis) consists of fat and loose connective tissue, insulating the body and cushioning underlying organs.

    • Eyes are protected by structures such as eyelids (which spread tears and block foreign objects), eyelashes (to trap particles), and tears. Tears contain antimicrobial substances like lysozyme (which breaks down bacterial cell walls) and lactoferrin (which sequesters iron, a vital nutrient for bacteria), as well as antibodies. The conjunctiva (a mucous membrane lining the eyelids and covering the sclera) and the transparent cornea also serve as protective barriers.

  • Microbiomes of Various Body Sites

    • Skin microbiome varies by location, influenced by factors like moisture, pH, temperature, and sebum production:

    • Hands: High bacterial diversity due to frequent contact with various surfaces. Common genera include Staphylococcus (coagulase-negative species like S. epidermidis), Corynebacterium, and Micrococcus.

    • Back: Moderate bacterial density, often drier than other areas. Habitat for gram-positive bacteria such as Propionibacterium species (e.g., P. acnes, now Cutibacterium acnes, which thrives in sebaceous follicles) and Staphylococcus species.

    • Feet: Typically has higher moisture levels and is prone to occlusive environments, supporting fungal growth (e.g., Candida species causing candidiasis, and dermatophytes like Trichophyton causing athlete's foot). The interdigital spaces are particularly moist.

    • Eyes: Generally low microbial load due to constant flushing by tears and presence of antimicrobial factors. Staphylococcus epidermidis is predominantly found, often in the conjunctiva, along with some Corynebacterium and Propionibacterium species.

  • Microorganisms Overcoming Defenses

    • Pathogens employ various mechanisms to breach and evade host defenses, including:

    • Enzymes: Some bacteria produce enzymes to break down skin barriers or host tissues. Examples include hyaluronidase (degrades hyaluronic acid in connective tissue), collagenase (breaks down collagen), coagulase (clots plasma, used by S. aureus to form fibrin barrier), and proteases (degrade host proteins).

    • Adhesins: Surface molecules that help bacteria adhere to epithelial cells on the skin and eyes, preventing their removal. Examples include fimbriae/pili, capsules (which also provide immune evasion), and biofilm-forming capabilities.

    • Immunomodulatory factors: Mechanisms to inhibit or evade the host immune response. These include capsules (prevent phagocytosis), Protein A (binds to antibodies in an inverted orientation, preventing opsonization), leukocidins (toxins that kill phagocytic cells), and various toxins that interfere with immune signaling.

  • General Signs and Symptoms of Skin and Eye Infections

    • Common symptoms, resulting from inflammation and tissue damage, include:

    • Redness (erythema): Caused by localized vasodilation and increased blood flow to the infection site.

    • Swelling (edema): Accumulation of fluid in the tissues due to increased vascular permeability.

    • Heat and pain at the infection site: Result from increased metabolic activity, blood flow, and release of inflammatory mediators that stimulate nociceptors.

    • Discharge (purulent or clear): Purulent discharge (pus) indicates a bacterial infection and consists of dead leukocytes, bacteria, and tissue debris. Clear discharge may indicate viral or allergic etiologies.

    • Itching and irritation: Often associated with fungal or parasitic infections, or allergic reactions.

  • Common Bacterial Pathogens

    • Skin Infections:

      • Staphylococcus aureus: A gram-positive, coagulase-positive bacterium, a frequent cause of pyogenic (pus-forming) skin infections such as folliculitis, furuncles, carbuncles, impetigo, and cellulitis. Can also cause more severe invasive infections. Methicillin-resistant S. aureus (MRSA) is a significant concern.

      • Streptococcus pyogenes: A Group A Streptococcus (GAS), gram-positive coccus, responsible for superficial infections like impetigo and erysipelas, as well as more severe conditions like necrotizing fasciitis.

    • Eye Infections:

      • Neisseria gonorrhoeae: Can cause severe conjunctivitis (ophthalmia neonatorum) in newborns exposed during birth, potentially leading to blindness. Also causes adult conjunctivitis.

      • Chlamydia trachomatis: Causes trachoma, a leading infectious cause of blindness worldwide, and inclusion conjunctivitis, particularly in newborns and sexually active adults.

      • Pseudomonas aeruginosa: A common cause of bacterial keratitis, particularly associated with contact lens wear, and can lead to rapid corneal damage.

  • Bacterial Diseases – Characteristics

    • Impetigo: A highly contagious superficial skin infection, primarily affecting children. It is characterized by reddish sores that quickly rupture, ooze, and form yellowish-brown crusts. Can be caused by S. aureus (bullous and non-bullous forms) or S. pyogenes (non-bullous).

    • Folliculitis: Inflammation of hair follicles, often presenting as small red bumps or pus-filled pimples around hair shafts. Commonly caused by S. aureus. A specific form, 'hot tub folliculitis,' is often caused by Pseudomonas aeruginosa.

    • Cellulitis and Erysipelas: Deeper skin infections. Cellulitis affects the dermis and subcutaneous tissue, presenting as a spreading area of redness, swelling, warmth, and pain. It's often caused by S. pyogenes or S. aureus. Erysipelas is a more superficial form of cellulitis, characterized by a raised, sharply demarcated lesion, typically caused by S. pyogenes.

  • Common Viral Pathogens

    • Skin Infections:

      • Herpes simplex virus (HSV): Primarily HSV-1 (oral herpes, cold sores) and HSV-2 (genital herpes), but both can infect either site, as well as the eyes. They cause painful vesicular lesions.

      • Varicella-zoster virus (VZV): Causes chickenpox (varicella) as a primary infection and shingles (herpes zoster) upon reactivation.

      • Molluscum contagiosum virus: A poxvirus causing pearly, flesh-colored, umbilicated papules, common in children and immunocompromised individuals.

    • Eye Infections:

      • Adenovirus: A common cause of viral conjunctivitis ('pink eye') and keratoconjunctivitis, often highly contagious.

      • Herpes simplex virus (HSV): Can cause ocular herpes (herpes keratitis), a serious infection of the cornea that can lead to scarring and vision loss.

  • Viral Diseases – Characteristics

    • Herpes Simplex Virus (HSV): Causes painful, fluid-filled lesions that recur periodically. The virus can remain dormant (latent) in nerve cells (e.g., trigeminal ganglia for oral herpes, sacral ganglia for genital herpes) and reactivate under stress, fever, or immunosuppression.

    • Varicella-Zoster Virus (VZV): Chickenpox is a generalized vesicular rash. After primary infection, VZV remains latent in dorsal root ganglia. Reactivation causes shingles, characterized by painful blistering rashes typically confined to a dermatome (area supplied by a single nerve).

  • Common Fungal Pathogens

    • Cutaneous Mycoses: Infections of the skin, hair, and nails caused by dermatophytes, a group of fungi that thrive on keratin. Genera include Trichophyton, Microsporum, and Epidermophyton. These cause various forms of tinea (ringworm).

    • Subcutaneous Mycoses: Infections that extend into the dermis, subcutaneous tissue, muscle, and fascia. Sporothrix schenckii is a dimorphic fungus from rose thorns or soil that causes sporotrichosis, characterized by nodular lesions along lymphatic channels.

    • Candida species: Yeasts that are part of the normal microbiota but can cause opportunistic infections (candidiasis) of the skin and mucous membranes, especially in moist areas or immunocompromised individuals.

  • Fungal Diseases – Characteristics

    • Tinea (ringworm): General term for dermatophyte infections, named for the body part affected (e.g., tinea pedis - athlete's foot, tinea capitis - scalp ringworm, tinea cruris - jock itch, tinea corporis - body ringworm). They typically cause a circular rash with raised, scaly borders and central clearing.

    • Candidiasis: Caused by Candida species, most commonly Candida albicans. It can manifest as oral thrush (white patches on mucous membranes), vulvovaginitis (vaginal yeast infection), diaper rash (cutaneous candidiasis in moist folds), or intertrigo (infection in skin folds).

  • Parasitic Pathogens

    • Common Parasites:

      • Sarcoptes scabiei: The scabies mite, which burrows into the stratum corneum of the skin.

      • Lice: Wingless insects that infest hair-bearing parts of the body. Species include head lice (Pediculus humanus capitis), body lice (P. h. humanus), and pubic lice (Pthirus pubis or 'crabs').

  • Parasitic Diseases – Characteristics

    • Scabies: An infestation leading to intense itching, especially at night, and characteristic skin inflammation with small, raised red bumps and visible burrows. The symptoms are largely due to an allergic reaction to the mites, their eggs, and their feces.

    • Lice: Infestation causes severe itching (pruritus) due to saliva injected during feeding. Persistent scratching can lead to excoriations and secondary bacterial infections. Lice lay eggs called nits which are firmly attached to hair shafts. Transmission is primarily through direct contact.

CHAPTER 22: RESPIRATORY SYSTEM INFECTIONS

  • Major Anatomical Features of the Upper and Lower Respiratory Tract

    • The upper respiratory tract includes: the nasal cavity (which warms, humidifies, and filters air, containing turbinates), pharynx (throat, subdivided into nasopharynx, oropharynx, and laryngopharynx), and larynx (voice box, containing vocal cords and protected by the epiglottis). These structures primarily prepare and direct air to the lungs.

    • The lower respiratory tract includes: the trachea (windpipe, lined with ciliated pseudostratified columnar epithelium and supported by C-shaped cartilage rings), bronchi (primary, secondary, and tertiary branches leading into the lungs), bronchioles (smaller airways without cartilage), and alveoli (tiny air sacs where gas exchange occurs, surrounded by capillaries and containing alveolar macrophages for immune defense).

  • Normal Microbiota of Upper and Lower Respiratory Tracts

    • Upper tract: Heavily colonized by a diverse microbiota, as it is exposed to the external environment. Common bacteria include Staphylococcus epidermidis, viridans streptococci (e.g., Streptococcus mitis, Streptococcus oralis), Corynebacterium spp., Neisseria spp. (non-pathogenic species), and Haemophilus spp. Some anaerobes also inhabit the oral cavity and nasopharynx.

    • Lower tract: Traditionally considered largely sterile due to protective mechanisms like the mucociliary escalator (ciliated cells sweeping mucus and trapped particles upward), alveolar macrophages, and lysozyme in mucus. However, modern techniques suggest a scant, transient microbial population may exist, mainly Streptococcus and Pseudomonas species, which are typically cleared rapidly.

  • Microorganisms Overcoming Defenses

    • Respiratory pathogens employ various virulence factors to evade and overcome host defenses:

    • Pathogens utilize toxins (e.g., pertussis toxin from Bordetella pertussis), and adhesins (e.g., hemagglutinin on influenza virus, fimbriae on bacteria like Bordetella pertussis) to attach to respiratory epithelial cells and prevent removal by the mucociliary escalator.

    • Other evasion mechanisms include capsules (e.g., Streptococcus pneumoniae) that prevent phagocytosis, IgA proteases (e.g., Neisseria and Haemophilus) that cleave secretory IgA antibodies, and the ability to survive intracellularly (e.g., Mycobacterium tuberculosis).

  • Microbial Interactions with the Respiratory System

    • The healthy microbiota in the upper respiratory tract provide competition against pathogens (competitive exclusion), preventing their colonization. They occupy ecological niches and produce antimicrobial substances.

    • When infections occur, they can disrupt the normal flora, leading to an imbalance (dysbiosis), and can overwhelm or impair the immune response, causing inflammation, tissue damage (e.g., to cilia), and reduced host defenses.

  • Common Bacteria Causing Respiratory Infections

    • Streptococcus pneumoniae: The most common bacterial cause of community-acquired pneumonia, otitis media, meningitis, and sinusitis. Known for its polysaccharide capsule.

    • Haemophilus influenzae: Can cause pneumonia, otitis media, sinusitis, and epiglottitis. The Hib vaccine targets type b, but non-typable strains remain a concern.

    • Mycoplasma pneumoniae: Causes 'atypical' or 'walking pneumonia'. Lacks a cell wall and is resistant to many common antibiotics.

    • Bordetella pertussis: Causes pertussis (whooping cough).

    • Mycobacterium tuberculosis: The causative agent of tuberculosis.

  • Characteristics of Specific Bacterial Diseases

    • Pneumonia: Inflammation of the lung parenchyma, particularly the alveoli, which fill with fluid and inflammatory cells (consolidation). Symptoms may present with cough (productive or non-productive), fever, chest pain, and shortness of breath (dyspnea). Can be lobar (affecting a whole lobe) or bronchopneumonia (patchy distribution).

    • Bronchitis: Inflammation of the bronchial tubes, often causing a persistent cough that may be productive (with sputum). Can be acute (often viral, sometimes secondary bacterial) or chronic (associated with smoking or irritants).

  • Common Viruses Causing Respiratory Infections

    • Influenza virus: Types A, B, and C. Known for causing seasonal epidemics and periodic pandemics (Influenza A).

    • Rhinoviruses: Over 100 serotypes, the most frequent cause of the common cold.

    • Respiratory Syncytial Virus (RSV): A major cause of lower respiratory tract infections (bronchiolitis and pneumonia) in infants and young children.

    • Adenovirus and Coronaviruses (e.g., Human CoV-229E, -OC43, and SARS-CoV-2) also cause a range of respiratory illnesses.

  • Viral Diseases – Characteristics

    • Influenza: Characterized by sudden onset of systemic symptoms including high fever, body aches (myalgia), headache, fatigue, and chills, in addition to respiratory symptoms like cough and sore throat. Antigenic drift and shift lead to new strains and recurrent epidemics.

    • Common Cold: Caused predominately by rhinoviruses, presents with milder and more localized symptoms such as sneezing, runny nose (rhinitis), nasal congestion, and sore throat, typically self-resolving within a week.

  • Common Fungal Pathogens

    • Histoplasma capsulatum: A dimorphic fungus common in areas with moist soil, especially in the Ohio and Mississippi River valleys, associated with bat and bird droppings. Causes histoplasmosis.

    • Coccidioides immitis: A dimorphic fungus found in the desert soils of the Southwestern US, causing coccidioidomycosis (Valley fever). Forms spherules in host tissue.

    • Cryptococcus neoformans: An encapsulated yeast often associated with pigeon droppings, causing opportunistic infections (cryptococcosis), especially in immunocompromised individuals, potentially leading to pneumonia and meningitis.

  • Fungal Diseases – Characteristics

    • Histoplasmosis: A pulmonary infection that can range from asymptomatic to severe. Primarily affects the lungs but can disseminate to other organs, particularly in immunocompromised individuals, forming granulomatous lesions.

    • Coccidioidomycosis: Often asymptomatic or mild, flu-like. In some cases, it can cause severe pneumonia, chronic lung disease, or disseminate to other body parts like skin, bones, and meninges.

  • Signs and Symptoms of Respiratory Infection

    • Rhinitis: Inflammation of the nasal lining, leading to a runny nose (rhinorrhea), sneezing, and nasal congestion.

    • Sinusitis: Inflammation of the paranasal sinuses, causing facial pain or pressure, headache, and nasal discharge.

    • Pharyngitis: Inflammation of the pharynx (sore throat), often accompanied by pain and difficulty swallowing.

    • Laryngitis: Inflammation of the larynx, resulting in hoarseness or complete voice loss.

    • Bronchitis: Inflammation of the bronchial tubes, characterized by a persistent cough, which may produce sputum.

    • Pneumonia: Infection of the lung tissue, leading to cough, fever, shortness of breath, chest pain (pleuritic pain), and fatigue. Can be potentially severe and lead to respiratory failure.

  • Streptococcal Infections

    • Pharyngitis (strep throat): Most commonly caused by Streptococcus pyogenes (Group A Strep). Symptoms include a severe sore throat, sudden onset, fever ( > 38^ ext{o}C ), swollen lymph nodes in the neck, and sometimes a rash (scarlet fever). The presence of M protein is a key virulence factor.

    • Scarlet Fever: A complication of strep throat, caused by S. pyogenes strains that produce erythrogenic toxins (pyrogenic exotoxins). Characterized by a distinctive red, sandpaper-like rash, especially on the trunk and neck, and a strawberry tongue.

  • Diagnosis and Treatment of Streptococcal Infections

    • Diagnosis involves rapid antigen detection tests (RADT) for quick screening, with throat culture being the gold standard for confirmation. Early and accurate diagnosis is crucial to prevent complications.

    • Treatment typically involves antibiotics such as amoxicillin or penicillin, administered for 10 days to eradicate the bacteria, reduce symptom duration, and, most importantly, prevent complications.

  • Complications of Untreated Infections

    • Possible serious sequelae of untreated S. pyogenes infections include:

      • Rheumatic fever: An autoimmune response triggered by molecular mimicry between bacterial M proteins and host tissues. It primarily affects the heart (leading to rheumatic carditis and lifelong heart valve damage), joints (migratory arthritis), brain (neuropsychiatric symptoms like Sydenham's chorea), and skin.

      • Glomerulonephritis: A post-streptococcal kidney inflammation (Type III hypersensitivity reaction) caused by immune complex deposition, leading to blood and protein in the urine (dark urine), edema, and hypertension. This is often an acute, self-limiting condition, but can rarely lead to chronic kidney disease.

  • Acute Otitis Media (AOM)

    • A common infection in children, characterized by inflammation and pus accumulation behind the eardrum, causing ear pain, fever, and sometimes temporary hearing loss. It is primarily caused by Streptococcus pneumoniae (most common), Haemophilus influenzae, and Moraxella catarrhalis.

    • Treatment includes antibiotics, often after an observation period (watchful waiting), and pain relief measures. In recurrent cases, myringotomy (insertion of ear tubes) may be performed to drain fluid and equalize pressure.

  • Bacterial Rhinosinusitis

    • Often occurs as a secondary bacterial infection following a viral upper respiratory infection (URI). It is characterized by prolonged symptoms (e.g., nasal congestion, facial pain/pressure, purulent discharge) lasting more than 10 days, or by worsening symptoms after initial improvement (