Respiratory Infections: TB and Pneumonia Overview

Respiratory Infections

Tuberculosis (TB):
- Caused by Mycobacterium tuberculosis, an acid-fast bacillus.
- Pathophysiology: Inhaled droplets containing the bacteria are engulfed by alveolar macrophages. Bacteria resist degradation, leading to granuloma (tubercle) formation. The center of the granuloma develops caseous necrosis (cheese-like). Latent TB infection (LTBI) occurs if the immune system contains the infection. If the immune system fails, active pulmonary TB occurs, with lung tissue destruction and lesions.
- Transmission: Airborne transmission via respiratory droplets (coughs, sneezes, talking).
- Risk factors: Close contact with infected individuals, crowded living conditions (prisons, shelters, etc.), weakened immune systems (HIV/AIDS, malnutrition), and lack of healthcare access.
Pneumonia:
- Caused by bacteria, viruses, fungi, and protozoa.
- Common pathogens include: community-acquired pneumonia (CAP): Streptococcus pneumoniae, Mycoplasma pneumoniae, Legionella pneumophila, Haemophilus influenzae, and influenza viruses; hospital-acquired pneumonia (HAP): Pseudomonas aeruginosa, Staphylococcus aureus (including MRSA), Klebsiella pneumoniae, Enterobacter species, and E. coli.
- Pathophysiology: Pathogens enter the lower respiratory tract, evading airway defenses. Alveolar macrophages release TNF-α, IL-1, and other cytokines, triggering neutrophil recruitment. Neutrophils release proteolytic enzymes, reactive oxygen species. Alveoli fill with exudate, infectious debris, and fluid leading to impaired gas exchange and V/Q mismatch, resulting in hypoxemia and dyspnea.

Bronchiolitis

Cause: Primarily caused by Respiratory Syncytial Virus (RSV). Other causes include Adenovirus, Human metapneumovirus, influenza virus, and parainfluenza virus.
Pathophysiology: Viral infection causes inflammation of the bronchioles. Bronchiole wall swelling causes airway narrowing, increasing airway resistance. Mucus and cellular debris buildup further obstructs airflow, leading to atelectasis and impaired ventilation. Hypoxemia results from ventilation-perfusion mismatch.
Transmission: Contact with respiratory secretions of infected individuals.
Risk Factors: Young children (especially infants under 6 months), premature birth, chronic lung disease, exposure to daycare settings, and crowded environments.

Croup (Laryngotracheobronchitis)

Cause: Most commonly caused by Parainfluenza virus. Other causes include RSV, influenza, adenovirus, coronavirus, and metapneumovirus.
Pathophysiology: Viral infection causes inflammation of the larynx, trachea, and bronchi. Swelling in the subglottic region narrows the airways, resulting in stridor and a barking cough. Increased mucus production and airway obstruction worsen breathing difficulty. Airflow limitation leads to increased respiratory effort and hypoxemia.
Transmission: Respiratory droplets and direct contact with contaminated surfaces.
Risk Factors: Children aged 6 months to 5 years, exposure to daycare settings, and crowded environments.

Acute Epiglottitis

Cause: Most commonly caused by Haemophilus influenzae type b (Hib). Other bacterial causes: Streptococcus pneumoniae and Streptococcus pyogenes.
Pathophysiology: Bacterial infection causes inflammation of the epiglottis, leading to rapid swelling. The swollen epiglottis obstructs the airway, causing severe respiratory distress.
Transmission: Respiratory droplets and close contact with infected people.
Risk Factors: Children aged 2 to 7 years (since Hib vaccination has reduced cases). Unvaccinated children are at higher risk.

Tonsillitis

Cause: Viral tonsillitis usually caused by Rhinovirus, Influenza virus, Herpes simplex virus (HSV), Epstein-Barr virus (EBV), Coronavirus, and Adenovirus. Bacterial tonsillitis is most often caused by Group A Streptococcus (GAS) or Streptococcus pyogenes.
Pathophysiology: Viral or bacterial pathogens invade the tonsillar crypts, triggering an immune response. This causes tonsil inflammation, swelling, pain, difficulty swallowing, and fever. In severe cases, abscess formation may occur.
Transmission: Respiratory droplets, saliva, or direct contact with contaminated surfaces.
Risk Factors: Young children and adolescents, exposure to crowded places like schools and daycare centers.