Respiratory Tract Infections

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Introduction

  • Respiratory Tract Infections (RTIs) are infections affecting any part of the respiratory system.

  • The respiratory tract is divided into:

    • Upper respiratory tract (nose, sinuses, pharynx, larynx)

    • Lower respiratory tract (trachea, bronchi, lungs)

  • RTIs are among the most common infectious diseases worldwide.

  • They can range from mild self-limiting illnesses (common cold) to severe life-threatening conditions (pneumonia).

  • The causative agents include viruses (most common), bacteria, and occasionally fungi.

  • Transmission mainly occurs through respiratory droplets, direct contact, and contaminated surfaces.

  • Children, elderly individuals, smokers, and immunocompromised patients are at higher risk.

  • Early diagnosis, appropriate treatment, and preventive measures such as vaccination and hygiene are essential to reduce complications and mortality.

 

Classification

1. Upper Respiratory Tract Infections (URTI)

These involve structures above the vocal cords:

  • Rhinitis (common cold)

  • Sinusitis

  • Pharyngitis

  • Tonsillitis

  • Laryngitis

  • Otitis media

2. Lower Respiratory Tract Infections (LRTI)

These involve structures below the vocal cords:

  • Bronchitis

  • Bronchiolitis

  • Pneumonia

  • Tuberculosis

  • Lung abscess

 

Etiological Agents of RTIs

A. Viral Causes (Most Common)

  1. Rhinovirus

  2. Influenza virus

  3. Respiratory syncytial virus

  4. Adenovirus

  5. SARS-CoV-2

B. Bacterial Causes

  1. Streptococcus pneumoniae

  2. Haemophilus influenzae

  3. Mycoplasma pneumoniae

  4. Staphylococcus aureus

  5. Klebsiella pneumoniae

C. Fungal Causes (Opportunistic)

  • Aspergillus fumigatus

  • Candida albicans

 

Mode of Transmission

  • Droplet infection (coughing, sneezing)

  • Airborne transmission

  • Direct contact

  • Contaminated surfaces

  • Aspiration (especially in elderly)

 

Pathogenesis

  1. Entry of pathogen into respiratory tract

  2. Adherence to epithelial cells

  3. Colonization and multiplication

  4. Local inflammation

  5. Tissue damage and systemic response

In pneumonia, alveoli fill with inflammatory exudate leading to impaired gas exchange.

Clinical Features

Upper RTI Symptoms

  • Sneezing

  • Nasal congestion

  • Sore throat

  • Hoarseness

  • Mild fever

Lower RTI Symptoms

  • Cough (productive or dry)

  • High-grade fever

  • Dyspnea

  • Chest pain

  • Wheezing

  • Cyanosis (severe cases)

 

Pneumonia: A Major LRTI

Pneumonia is inflammation of lung parenchyma and may be:

  • Lobar pneumonia

  • Bronchopneumonia

  • Atypical pneumonia

  • Aspiration pneumonia

Sample Collection

The choice of specimen is crucial for accurate diagnosis and varies depending on the infection site:

Sputum

  • Collection Method: Patients are instructed to take deep breaths and cough to produce sputum. The sample should be collected in a sterile container, ideally early in the morning when secretions are most concentrated.
  • Quality Assessment: A Gram stain is performed to evaluate the quality of the sputum. A good-quality sample shows numerous neutrophils and few epithelial cells.

Nasopharyngeal Swabs

  • Collection Method: A sterile swab is inserted into the nasopharynx, ideally reaching the posterior nasopharynx to collect secretions.
  • Use: Commonly used for viral testing, including PCR for influenza and RSV.

Throat Swabs

  • Collection Method: A sterile swab is rubbed over the tonsils and posterior pharynx to collect secretions.
  • Use: Primarily for detecting Streptococcus pyogenes in cases of suspected strep throat.

Bronchoalveolar Lavage (BAL)

  • Collection Method: Conducting via bronchoscopy, sterile saline is instilled into a lung segment and then aspirated back.
  • Use: Useful for diagnosing pneumonia, particularly in immunocompromised patients or when standard cultures are inconclusive.

Pleural Fluid

  • Collection Method: Obtained through thoracentesis in cases of suspected pleural effusion.
  • Use: Cultured to identify pathogens in cases of empyema or complicated pneumonia.

Blood Cultures

  • Collection Method: Blood samples are taken in sterile conditions, typically in pairs from different sites (e.g., peripheral veins) to enhance yield.
  • Use: Important in severe cases of pneumonia or suspected sepsis.

 

Laboratory Techniques for Diagnosis

Culture Methods

A. Bacterial Culture

  • Media Selection: Appropriate media include:
    • Blood Agar: Supports the growth of most respiratory pathogens and helps identify hemolytic activity.
    • Chocolate Agar: Used for fastidious organisms such as Haemophilus influenzae.
    • MacConkey Agar: Selective for Gram-negative bacteria, useful for isolating enteric pathogens.
  • Incubation Conditions: Typically at 35-37°C in aerobic conditions, some pathogens require CO₂-enriched environments.

B. Identification of Isolates

  • Colony Morphology: Initial observations include colony size, shape, color, and hemolytic patterns on blood agar.
  • Gram Staining: A fundamental technique to differentiate between Gram-positive and Gram-negative bacteria.

Biochemical Tests

  • Catalase Test: Differentiates Staphylococcus (catalase-positive) from Streptococcus (catalase-negative).
  • Coagulase Test: Identifies Staphylococcus aureus (coagulase-positive).
  • Oxidase Test: Useful for identifying Pseudomonas aeruginosa and differentiating it from Enterobacteriaceae.
  • Lactose Fermentation: Evaluated on MacConkey agar to identify lactose fermenters like E. coli and Klebsiella pneumoniae.

Molecular Methods

A. Polymerase Chain Reaction (PCR)

  • Nucleic Acid Amplification: PCR allows for rapid identification of specific pathogens, especially in cases where cultures may be negative or slow.
  • Real-Time PCR: Provides quantitative results and is particularly useful for detecting viral pathogens and determining bacterial loads.

B. Nucleic Acid Hybridization

  • Fluorescent In Situ Hybridization (FISH): Utilizes fluorescently labeled probes to identify specific bacteria in clinical samples, particularly useful in polymicrobial infections.

Serological Tests

  • Antigen Detection: Rapid antigen tests (e.g., for influenza) and enzyme-linked immunosorbent assays (ELISA) can detect specific pathogens in respiratory specimens.
  • Antibody Detection: Serological tests may indicate previous exposure but are less reliable for diagnosing acute infections.

Imaging Studies

Imaging can aid in diagnosing respiratory infections:

  • Chest X-ray assesses pneumonia, pleural effusions, and other lung pathologies.
  • CT Scan: Provides detailed imaging for complicated infections and helps identify abscesses or consolidations.

 

Interpretation of Results

Culture Results

  • Positive Culture: The presence of a pathogen confirms infection; identification allows for targeted treatment.
  • Negative Culture: May indicate:
    • Fastidious organisms not captured in standard cultures.
    • Contamination during sampling.
    • Recent antibiotic therapy affecting culture results.

Sensitivity Testing

Antibiotic susceptibility testing (AST) is critical for guiding treatment:

  • Disk Diffusion Method (Kirby-Bauer): Measures the diameter of inhibition zones around antibiotic disks to classify susceptibility.
  • Minimum Inhibitory Concentration (MIC): Determines the lowest concentration of an antibiotic that inhibits bacterial growth.

Interpretation of Sensitivity Results

  • Susceptible: Indicates that the antibiotic is likely effective for treating the infection.
  • Intermediate: Indicates uncertain efficacy; higher doses may be necessary.
  • Resistant: Indicates that the pathogen is unlikely to respond to the antibiotic, necessitating alternative therapies.

 

Specific Respiratory Tract Infections and Their Laboratory Diagnosis

Viral Upper Respiratory Infections

Common Pathogens: Rhinoviruses, influenza viruses, and RSV.

Diagnostic Methods:

    • Nasopharyngeal Swabs: Collected for PCR testing to detect viral RNA.
    • Rapid Antigen Tests: Useful for detecting influenza and RSV quickly.

Streptococcal Pharyngitis

Common Pathogen: Streptococcus pyogenes.

Diagnostic Methods:

    • Throat Swab: Cultured on blood agar for confirmation.
    • Rapid Antigen Detection Tests: Provide quick results, with negative results sometimes requiring follow-up culture.

Bacterial Pneumonia

Common Pathogens: Streptococcus pneumoniae, Staphylococcus aureus, Klebsiella pneumoniae.

Diagnostic Methods:

    • Sputum Culture: Analyzing for bacterial growth; quality assessed via Gram stain.
    • Blood Cultures: Particularly in severe cases or when bacteremia is suspected.
    • Chest X-ray: Used to visualize pneumonia and assess severity.

Chronic Respiratory Infections

Common Pathogens: Pseudomonas aeruginosa in cystic fibrosis patients.

Diagnostic Methods:

    • Sputum Samples: Cultured for routine bacterial culture and sensitivity testing.
    • Bronchoalveolar Lavage (BAL): This may be performed for detailed microbiological analysis.

Pleural Effusion or Empyema

Common Pathogens: Mixed flora, including Staphylococcus aureus and Streptococcus pneumoniae.

Diagnostic Methods:

    • Pleural Fluid Culture: Analysis of collected fluid for bacterial growth.
    • Cytology: Assessing for malignant cells if cancer is suspected.

 

MCQs

  • Respiratory tract infections primarily affect the:
    A. Digestive system
    B. Respiratory system
    C. Nervous system
    D. Urinary system
    Answer: B

  • Upper respiratory tract includes:
    A. Lungs
    B. Bronchi
    C. Nose and pharynx
    D. Alveoli
    Answer: C

  • Lower respiratory tract includes:
    A. Nose
    B. Sinuses
    C. Larynx
    D. Lungs
    Answer: D

  • The most common cause of RTIs is:
    A. Fungi
    B. Bacteria
    C. Viruses
    D. Parasites
    Answer: C

  • Common cold is most commonly caused by:
    A. Rhinovirus
    B. Influenza virus
    C. Streptococcus pneumoniae
    D. SARS-CoV-2
    Answer: A

  • The most common bacterial cause of pneumonia is:
    A. Staphylococcus aureus
    B. Streptococcus pneumoniae
    C. Escherichia coli
    D. Klebsiella pneumoniae
    Answer: B

  • Infection of bronchi is called:
    A. Pneumonia
    B. Bronchitis
    C. Sinusitis
    D. Pharyngitis
    Answer: B

  • Infection of sinuses is known as:
    A. Tonsillitis
    B. Sinusitis
    C. Laryngitis
    D. Bronchiolitis
    Answer: B

  • Atypical pneumonia is commonly caused by:
    A. Mycoplasma pneumoniae
    B. Staphylococcus aureus
    C. Clostridium tetani
    D. Vibrio cholerae
    Answer: A

  • RTIs are mainly transmitted by:
    A. Vector bite
    B. Droplet infection
    C. Sexual contact
    D. Blood transfusion
    Answer: B

  • High fever with productive cough suggests:
    A. URTI
    B. Pneumonia
    C. Rhinitis
    D. Otitis
    Answer: B

  • The most common viral cause of bronchiolitis in infants:
    A. Respiratory syncytial virus
    B. Adenovirus
    C. Influenza virus
    D. Rhinovirus
    Answer: A

  • Lobar consolidation is seen in:
    A. Asthma
    B. Bronchitis
    C. Pneumonia
    D. Rhinitis
    Answer: C

  • Gram-positive diplococci in sputum suggest:
    A. Staphylococcus
    B. Streptococcus pneumoniae
    C. E. coli
    D. Klebsiella
    Answer: B

  • Pink colonies on MacConkey agar indicate:
    A. Non-lactose fermenter
    B. Lactose fermenter
    C. Anaerobe
    D. Fungus
    Answer: B

  • Influenza virus belongs to family:
    A. Retroviridae
    B. Orthomyxoviridae
    C. Herpesviridae
    D. Adenoviridae
    Answer: B

  • Cyanosis indicates:
    A. Hyperglycemia
    B. Hypoxia
    C. Dehydration
    D. Anemia
    Answer: B

  • Chest X-ray is useful in diagnosing:
    A. Sinusitis
    B. Rhinitis
    C. Pneumonia
    D. Otitis
    Answer: C

  • Most common complication of pneumonia:
    A. Pleural effusion
    B. Gastritis
    C. Arthritis
    D. Nephritis
    Answer: A

  • Smoking increases risk of:
    A. RTIs
    B. Skin infection
    C. UTI
    D. Hepatitis

  • Hospital-acquired pneumonia develops after:
    A. 12 hours of admission
    B. 24 hours of admission
    C. 48 hours of admission
    D. 72 hours of discharge
    Answer: C

  • Most common cause of bronchiolitis in infants:
    A. Respiratory syncytial virus
    B. Rhinovirus
    C. Influenza virus
    D. Streptococcus pneumoniae
    Answer: A

  • Atypical pneumonia is characterized by:
    A. Sudden onset with high fever
    B. Gradual onset with dry cough
    C. Productive cough with pus
    D. Severe chest pain
    Answer: B

  • Most common cause of community-acquired pneumonia:
    A. Staphylococcus aureus
    B. Streptococcus pneumoniae
    C. Klebsiella pneumoniae
    D. Pseudomonas aeruginosa
    Answer: B

  • Klebsiella pneumonia commonly affects:
    A. Children
    B. Alcoholics
    C. Athletes
    D. Pregnant women
    Answer: B

  • Rusty sputum is classically seen in:
    A. Asthma
    B. Tuberculosis
    C. Pneumococcal pneumonia
    D. Viral infection
    Answer: C

  • Acid-fast bacilli in sputum suggest infection with:
    A. Mycobacterium tuberculosis
    B. Haemophilus influenzae
    C. Staphylococcus aureus
    D. Bordetella pertussis
    Answer: A

  • Whooping cough is caused by:
    A. Bordetella pertussis
    B. Rhinovirus
    C. Respiratory syncytial virus
    D. Klebsiella pneumoniae
    Answer: A

  • Pleural effusion is accumulation of fluid in:
    A. Alveoli
    B. Bronchi
    C. Pleural cavity
    D. Pericardium
    Answer: C

  • Most common viral cause of pneumonia in adults:
    A. Influenza virus
    B. Rhinovirus
    C. Adenovirus
    D. SARS-CoV-2
    Answer: A

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