L-21 Pathology of infectious diseases III

Overview

• Lecture 21 discusses the pathology of infectious diseases, focusing on gram-positive and gram-negative bacterial infections along with vaccine-preventable childhood diseases.

Gram-Positive Pyogenic Infections

• Staphylococcal infections include Furuncles, Carbuncles, Phlegmon, Abscesses, Impetigo, Scalded Skin Syndrome, Toxic Shock Syndrome, Pneumonia, Acute Endocarditis, and Food Poisoning.
• Streptococcal infections feature Pharyngitis, Scarlet Fever, Erysipelas, Post-Abortum Endometritis, Neonatal Sepsis, and Necrotizing Fasciitis.

Staphylococcal Infections

• Furuncle: Localized skin abscess with a central core of pus.
• Carbuncle: Larger, deeper infection involving multiple hair follicles.
• Phlegmon: Diffuse, purulent inflammation in the soft tissues.
• Abscess: Localized collection of pus within tissues, typically enclosed by a fibrous capsule.
• Impetigo: Superficial skin infection with honey-colored crusts.
• Scalded Skin Syndrome: Exfoliative toxin-induced skin sloughing, often in children.
• Toxic Shock Syndrome: Systemic inflammatory response with fever, rash, and organ dysfunction.
• Pneumonia: Inflammation of the lungs, leading to consolidation and pus formation.
• Acute Endocarditis: Infection of the heart valves resulting valve destruction.
• Food Poisoning: Gastroenteritis due to ingestion of enterotoxins.

Streptococcal Infections

• Pharyngitis: Throat inflammation, commonly with fever and exudates.
• Scarlet Fever: Characterized by a red, sandpaper-like rash, often following pharyngitis.
• Erysipelas: Superficial, raised skin infection with a definitive border.
• Post-Abortum Endometritis: Uterine lining infection following miscarriage or abortion.
• Neonatal Sepsis: Systemic infection in newborns, often caused by group B streptococcus.
• Necrotizing Fasciitis: Rapidly progressing infection of the deep soft tissues, often with gas formation and tissue death.

Pneumonia

• Defined as an infection, inflammation, and consolidation of lung parenchyma.
• Inflammation: Bacterial invasion of the lung parenchyma causes the alveoli to fill with an inflammatory exudate.
• Consolidation: "Solidification" of the pulmonary tissue.

Stages of Inflammatory Response in Pneumonia

• Congestion: Lungs are heavy, edematous, and red.
• Congestion Microscopy: Vascular engorgement, intra-alveolar edema fluid with neutrophils and bacteria are present.
• Red Hepatization: Lobe is red, firm, and airless with a liver-like consistency.
• Red Hepatization Microscopy: Massive confluent exudation with neutrophils, red cells, and fibrin fills the alveolar spaces.
• Gray Hepatization: Progressive disintegration of red cells and persistence of fibrinosuppurative exudate, leading to a grayish-brown color.
• Resolution: Enzymatic digestion breaks down exudate within the alveolar spaces; debris gets resorbed, ingested by macrophages, expectorated, or organized by fibroblasts.
• Pleura: Pleuritis can result in fibrous thickening or permanent adhesions.

Staphylococcal Pneumonia

• Chest X-rays show multiple, irregularly marginated patches of airspace disease in both lungs.

Scalded Skin Syndrome (SSSS)

• SSSS in infants causes very superficial erosions that begin as small blisters particularly in the axillae and neck region; gentle pressure causes skin to peel off.

Infective Endocarditis

• Endocarditis Definition: Inflammation of the endocardium and typically refers to valve infections.
• Staphylococcus Aureus: Can infect healthy or deformed valves; responsible for 20% to 30% of cases and is a prevalent offender in IE among intravenous drug abusers.
• Prosthetic Valve Endocarditis: Coagulase-negative staphylococci (e.g., S. Epidermidis) are commonly involved.

Streptococcus Pyogenes

• This bacteria causes suppurative, toxin-related, and immunologic reactions.
• Clinical Case: A 20-year-old male with streptococcal pharyngitis, treatable but important to prevent rheumatic fever.
• Skin infections manifest as diffuse, edematous, acute inflammatory reaction in the epidermis/dermis, extending into subcutaneous tissues, with a neutrophil infiltrate.

Impetigo

• Impetigo is a localized, intraepidermal infection caused by S. pyogenes or S. aureus.
• The infection causes erythematous papules with central ulceration and crusts.

Streptococcus Pneumoniae

• Streptococcus Pneumoniae (or pneumococcus) causes pneumonia, otitis media, sinusitis, and meningitis.
• Most children worldwide experience at least one episode of pneumococcal disease by age 5, commonly otitis media.
• It is the most common etiologic agent of lobar pneumonia.

Gram-Negative Cocci Infections

• Gram-negative cocci infections include Meningococci (meningitis), gonococci (gonorrhea), and arthritis.

Neisseria Meningitides

• Neisseria Meningitides causes Pyogenic Meningitis and Overwhelming Shock.

Gonorrhea

• Gonorrhea is an acute suppurative infection which can cause sterility.

Vaccine-Preventable Childhood Diseases

• Conditions include Diphtheria, Croup, Skin Lesions, Haemophilus Influenzae, epiglottitis, Pneumonia, Meningitis, Arthritis, and Bordetella Pertussis (Whooping Cough).

Diphtheria

• Diphtheria is a necrotizing upper respiratory infection cause by Corynebacterium Diphtheriae (aerobic, pleomorphic and gram-positive rod).
• Infections due to Diphtheria present with cardiac and neurologic disturbances due to toxin production.
• The disease is preventable by vaccination with inactivated C. Diphtheriae toxin (toxoid).
• Pathology will show thick, gray, leathery membranes lining affected respiratory passages.
• Microscopy will show sloughed epithelium, necrotic debris, neutrophils, fibrin, and bacteria.

Pertussis

• Pertussis (gram-negative organism) is characterized by debilitating paroxysmal coughing.
• Bordetella Pertussis causes pertussis.
• The paroxysm is followed by a long, high-pitched inspiration, the "whoop," which gives the disease its name - "whooping cough".
• Pathology: Tracheobronchitis, necrosis of ciliated respiratory epithelium, acute inflammatory response, loss of the protective mucociliary blanket, and increased risk of pneumonia.

Haemophilus Influenzae

• Haemophilus Influenzae causes pyogenic infections in young children.
• Clinical profile/pathology: H. Influenzae elicits strong acute inflammatory responses in the middle ear, sinuses, facial skin, epiglottis, meninges, lungs, and joints.
• This is a major pediatric pathogen and a leading cause of bacterial meningitis worldwide.
• Haemophilus Influenzae is aerobic, pleomorphic and gram-negative coccobacillus
• Nonencapsulated strains (type a) →localized infections
• Encapsulated strains, type b, cause 95% of the invasive bacteremic infections.

Gram-Negative Rods

• Conditions include Legionella (Legionnaires' disease, Pontiac fever) and Pseudomonas (opportunistic nosocomial infections, pneumonia, ecthyma gangrenous, endocarditis, endotoxic shock)

Legionella

• Pathogenesis in Respiratory Tract Diseases: Legionella multiplies in alveolar macrophages, monocytes, and alveolar epithelial cells in infected hosts.
• Bacterial resistance to killing by toxic superoxide, hydrogen peroxide, and hydroxyl radicals leads to phagolysosome fusion inhibition.
• Infected macrophages release chemokines and cytokines, which recruit additional immune cells to the site.
• TH1 cells (sensitized helper T cells) activate parasitized macrophages, enabling bacterial killing.
• IFN-y production is critical for eliminating Legionella organisms.
• Clinical profile: Susceptible individuals inhale infectious aerosols.
• Asymptomatic cases can occur.
• Symptomatic infections primarily affect the lungs.
• Influenza-like illness (referred to as Pontiac fever).
• Historical Data: in 1968, L. Pneumophila caused a self-limited, febrile illness in workers at the Pontiac, Michigan Public Health Department.
• Legionella has slender rods by its nature, or coccobacillary in clinical material.
• Legionella has slender rod in nature; coccobacillary in clinical material.
• Legionella are facultative, intracellular parasites that are unencapsulated and have monotrichous flagella.
• Legionella can be cultured on a special medium.

Pseudomonas Aeruginosa

• Opportunistic infection in the eye (contact lenses), wounds, urinary tract, and burns.
• Colonizes the respiratory tract of persons with cystic fibrosis by formation of a biofilm.
• Can cause - Necrotizing Otitis Externa.
• Clinical Features: A swollen ear with necrotic exudate seen within the external auditory canal and can include progressive ear-related pain.

Ecthyma Gangrenosum

• Caused by Pseudomonas aeruginosa in the setting of sepsis and skin involvement.
• It is seen most common in immunocompromised patients.
• Symptoms include fever, malaise, and diaphoresis.

Pathology of Ecthyma Gangrenosum

• Sequence of tissue manifestations from vascular thromboemboli that cause ischemia:
  • Initial stage is a red, macular lesion.
  • As ischemia continues, vesicles appear that transform into pustules due infiltration by neutrophils.
  • Prolonged tissue then causes ischemia and tissue necrosis that can result in ulceration or black eschar with erythematous borders.

Overview of Pseudomonas

• A recovery of infection of Pseudomonas sp. from the lungs of a child with COPD is diagnostic of CF.