Respiratory Tract Infections and Natural Defenses

• Lower respiratory tract infections include pneumonia.

Normal Flora of the Respiratory Tract

• The nares (nostrils) contain Staphylococcus epidermidis, corynebacteria, and Staphylococcus aureus.
• The pharynx (throat) contains Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae, and Neisseria meningitidis.
• The oral cavity contains streptococci, lactobacilli, staphylococci, and corynebacteria, as well as anaerobes like bacteroides.
• The lower respiratory tract (trachea, bronchi, pulmonary tissues) is typically free of microorganisms due to the ciliated epithelium's cleansing action.

Natural Defenses of the Respiratory Tract

• Defenses include: Physical, Physiological, and Humoral/Cellular Mechanisms.
• Physical mechanisms are: humidification, particle removal, and particle expulsion (coughing, sneezing, gagging).

Physiological Mechanisms in the Respiratory Tract

• A gel layer secreted by goblet cells and mucus glands contains: mucus secretion (mucins), antimicrobial molecules (lysozymes, defensins), specific antibodies (IgA), and cytokines.
• Cilia push the mucus blanket upwards to the pharynx for swallowing or coughing up.

Humoral And Cellular Mechanisms

• These mechanisms act by opsonization to help macrophages ingest foreign material.
• Neutrophils migrate into air spaces, phagocytose, and kill microbes using antimicrobial proteins (lactoferrin), degradative enzymes (elastase), and oxidant radicals.
• Neutrophils generate mediators like tumor necrosis factor alpha (TNF-α), interleukin 1 (IL-1), and chemokines to activate dendritic and B cells.
• Dendritic cells present antigens and are key to adaptive immune response.

Non-Specific Soluble Factors

• Lysozyme, defensins, and lactoferrin are bactericidal enzymes found in granulocytes.
• Interferons are produced by most cells in response to viral infection and enhance lymphocyte function.
• Complement enhances cytotoxicity with antibodies.
• Surfactant protein A (SPA) is one of four surfactant proteins that opsonize bacteria/particles, enhancing phagocytosis by macrophages.
• Dimeric secretory IgA targets specific antigens.

Infections of the Upper Respiratory Tract

• The common cold (acute coryza) results from respiratory viruses like rhinoviruses, coronaviruses, and adenoviruses.
• Sinusitis is a bacterial infection of the paranasal sinuses, mainly Streptococcus pneumoniae and Haemophilus influenzae, or occasionally fungal.
• Pharyngitis is commonly caused by adenoviruses, of which there are about 32 serotypes.
• Acute laryngotracheobronchitis is commonly caused by parainfluenza viruses and measles.
• Acute epiglottitis is caused by H. influenzae type b (Hib), and can be life-threatening in children under 5.
• Influenza viruses, belonging to the orthomyxovirus group, exist in forms A and B. Influenza B causes localized outbreaks, while influenza A causes worldwide pandemics.

Pneumonia

• Pneumonia is defined as a general term denoting inflammation of the gas exchanging region of the lung, usually due to infection.
• Pneumonia is an acute inflammation/infection of lung parenchyma, and inflammation due to other causes is called 'pneumonitis'.
• The common feature of pneumonias is a cellular exudate in the alveolar spaces.

Classifications of Pneumonia

• Anatomical: lobar, interstitial, or bronchopneumonia.
• By infecting organism: bacterial vs. atypical.
• By source of infection: community-acquired, hospital-acquired (HCAP), aspiration, or in the immunocompromised patient.

Community-Acquired Pneumonia

• The commonest causative organism of community acquired pneumonia is Streptococcus Pneumoniae.
• It is less commonly caused by Haemophilus influenzae, Klebsiella pneumoniae, Staphylococcus aureus and Streptococcus pyogenes, viral also is CAP
• Atypical organisms responsible are Mycoplasma pneumoniae (the commonest of the atypical organisms), Chlamydia pneumoniae, and Legionella pneumophila.

Nosocomial Pneumonia

• Nosocomial or hospital-acquired pneumonia is defined as an infection of the lower respiratory tract in hospitalized patients, not incubating at the time of hospital admission, and occurs 2-3 days after admission.
• Organisms causing it are gram-negative bacteria (Pseudomonas, E Klebsiella, Coli), Staphylococcus Aureus, Streptococcus Pneumoniae,.

Aspiration Pneumonia

• Aspiration of food, drink, saliva, or vomitus can lead to pneumonia.
• It is more likely in people with altered consciousness (due to anesthesia, alcohol, or drug abuse), swallowing problems (neuromuscular issues or oesophageal disease).
• Causative organisms consist of oral flora & anaerobes.

Pneumonia in the Immunocompromised Patient

• Patients with poor immune response are susceptible to a range of organisms, such as Pneumocystis jiroveci, Aspergillus spp., Cytomegalovirus, and others.

Clinical Presentation of Pneumonia

• Can be variable.
• Systemic features: malaise, fever
• Local features: cough productive of sputum
• Sputum can be purulent or rusty colored (due to blood) or frankly blood stained.
• Common symptoms are pleuritic chest pain and breathlessness (dyspnoea).
• Pneumonias can be of rapid onset, especially pneumococcal or staphylococcal infections, with potentially fatal outcomes.
• Atypical pneumonias (e.g., Mycoplasma pneumonia) may have a prolonged prodromal period lasting weeks.

Assessing the Severity of Pneumonia

• CURB-65 score is used.
• The presence of 2 or more features indicates need for hospital treatment and potentially ICU.
  • C: New mental confusion
  • U: Urea > 7 mmol/L
  • R: Respiratory rate > 30 per minute
  • B: Blood pressure (systolic BP < 90 or diastolic BP <60 mmHg)
  • 65: Age > 65 years

Diagnosis of Pneumonia

• Chest X-ray: Usually reveals shadowing in at least one section of the lung field.
• Microbiology: Identifying the infecting organism via Gram stain and sputum culture is helpful; blood culture is important in severely ill patients.

Etiological Clues for Respiratory Pathogens

• Herpes labialis may indicate streptococcal infection, as can rusty sputum.
• Poor dental hygiene may suggest Klebsiella or Actinomyces israelii.
• Mycoplasma pneumoniae is more common in young people and rare in the elderly.
• Haemophilus influenzae is more common in the elderly, especially with underlying lung disease.
• Legionella pneumophila occurs in local outbreaks centered on contaminated cooling towers.
• Staph. aureus is more common following an episode of influenza.
• Klebsiella pneumonia has a specific association with alcohol abuse and often presents with a severe bacteraemic illness.

Prognosis of Pneumonia

• Pneumococcal pneumonia has a mortality rate of approximately 5% in previously well individuals.
• Poorer prognosis is associated with: extremities of age, high CURB 65 score, very high or low white cell count, absence of fever, extensive x-ray shadowing, significant hypoxia, and rise in blood urea.

Management of Pneumonia

• General measures: encourage good oral fluid intake to avoid dehydration.
• Anti-pyretic drugs (e.g., paracetamol) are used to reduce fever and malaise.
• Use stronger analgesics for pleural pain.
• Severe illness may require intravenous fluids and oxygen if there is cyanosis with good respiratory drive.
• Specific therapy includes antibiotics, which vary by type of pneumonia.
• Community-acquired pneumonia typically targets Pneumococcus, usually sensitive to penicillin or related antibiotics.
• For hospital-acquired pneumonia, antibiotics should cover gram-negative organisms.

Complications of Pneumonia

• Local complications: Pleural effusion, empyema, lung abscess.
• Systemic complications: Septicemia, AKI (acute kidney injury), septic emboli, VTE (venous thromboembolism).