11.1 Respiratory Tract Infections: Otitis, Sinusitis, Pharyngitis, CAP & HAP

Respiratory Tract Infections: Otitis, Sinusitis, Pharyngitis, CAP & HAP

Respiratory tract infections represent the most frequent indication for outpatient antimicrobial prescriptions in Canada. However, because viral etiologies predominate in upper respiratory presentations, inappropriate antimicrobial utilization drives significant community-wide selective pressure and adverse drug events. Pharmacists play a pivotal clinical role in evaluating symptom severity, distinguishing viral illnesses from invasive bacterial infections, calculating precise weight-based posology, and applying validated risk-stratification scoring systems to optimize patient outcomes and antimicrobial stewardship.

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Acute Otitis Media (AOM): Triage, Dosing & Watchful Waiting

Acute otitis media (AOM) is defined by rapid onset of middle ear inflammation accompanied by middle ear effusion (MEE), evidenced by tympanic membrane (TM) bulging, erythema, cloudiness, or otorrhea. The primary bacterial pathogens responsible for AOM in Canadian pediatric populations include:

• Streptococcus pneumoniae (35%–40%): Associated with severe pain, high fever, and suppurative complications.
• Haemophilus influenzae (non-typeable) (30%–35%): Frequently isolated in otitis-conjunctivitis syndrome; approximately 30%–45% of Canadian strains produce beta-lactamase.
• Moraxella catarrhalis (10%–15%): Greater than 90% produce beta-lactamase.
• Respiratory viruses (RSV, rhinovirus, influenza): Often co-present, causing mucosal swelling and Eustachian tube dysfunction.

Watchful Waiting Protocol vs. Immediate Antimicrobial Therapy

Because up to 80% of uncomplicated AOM cases resolve spontaneously within 2 to 7 days without antibacterial therapy, clinical practice guidelines recommend a structured watchful waiting (delayed prescription) approach for select low-risk pediatric patients.

First-Line Antimicrobial Posology: High-Dose Amoxicillin

When antibiotic therapy is indicated, high-dose amoxicillin remains the undisputed first-line agent:

• Pediatric Posology (CPS): 75 to 90 mg/kg/day orally divided BID, or 45 to 60 mg/kg/day divided TID. Amoxicillin's half-life is about 1 hour, so twice-daily dosing needs the higher total dose to keep middle-ear levels adequate for over 50% of the day.
• Adult Posology: 1,000 mg PO TID (or 875 mg PO BID for mild presentations).
• Mechanistic Rationale: Intermediate and high-level penicillin-resistant S. pneumoniae (PRSP) possess altered penicillin-binding proteins (PBP2x, PBP2b). Conventional amoxicillin dosing (40–45 mg/kg/day) yields middle ear fluid concentrations below the minimum inhibitory concentration (MIC), whereas high-dose amoxicillin achieves middle ear concentrations exceeding 2–4 mg/L, maintaining free drug levels above the MIC (T > MIC) for greater than 50% of the dosing interval.

Treatment Duration

• Children < 2 years of age, perforated eardrum, recurrent AOM, or failure of initial therapy: 10 full days.
• Children ≥ 2 years of age with uncomplicated AOM: 5 days (shorter course decreases selective pressure and diarrhea without compromising clinical cure).

Managing Treatment Failure and High-Risk Features (7:1 Amoxicillin-Clavulanate)

If a child experiences treatment failure (persistent fever, otalgia, or worsening TM bulging after 48–72 hours of high-dose amoxicillin), has received a beta-lactam within the preceding 30 days, or presents with concurrent purulent conjunctivitis (otitis-conjunctivitis syndrome), the likely pathogen is a beta-lactamase-producing strain of H. influenzae or M. catarrhalis.

• Second-Line Choice: Amoxicillin-clavulanate, 7:1 formulation (400 mg amoxicillin / 57 mg clavulanate per 5 mL). The CPS notes that a 14:1 formulation is not available in Canada.
• Posology (CPS): For a child weighing 35 kg or less, 45 to 60 mg/kg/day of the amoxicillin component divided TID for 10 days. For a child over 35 kg, 500 mg tablets TID for 10 days.
• Formulation Science: Clavulanate causes dose-related diarrhea, and more than about 10 mg/kg/day is linked with a higher risk. The 7:1 suspension gives the most amoxicillin with the least clavulanate available in Canada. At 60 mg/kg/day of amoxicillin it delivers about 8.5 mg/kg/day of clavulanate, which stays under that limit. Do not reach a higher amoxicillin dose by increasing a 4:1 or 7:1 product, because this overdoses clavulanate.
• If oral therapy fails or cannot be taken: ceftriaxone 50 mg/kg IM (or IV) once daily for 3 days, or referral for tympanocentesis.

Penicillin Allergy Alternatives

• Non-Severe (Delayed cutaneous maculopapular rash): Second-generation oral cephalosporins: cefuroxime axetil (30 mg/kg/day divided BID or TID) or cefprozil (30 mg/kg/day divided BID); or ceftriaxone 50 mg/kg IM daily for 3 days.
• Severe / IgE-Mediated (Anaphylaxis, bronchospasm, angioedema): Azithromycin (10 mg/kg on Day 1, followed by 5 mg/kg daily on Days 2–5) or Clarithromycin (15 mg/kg/day divided BID). Note that pneumococcal macrolide resistance in Canada exceeds 25%, carrying an increased risk of microbiological failure.

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Acute Bacterial Rhinosinusitis (ABRS): Differentiating Viral vs. Bacterial

Acute rhinosinusitis involves inflammation of the mucosal lining of the paranasal sinuses and nasal cavity. Over 98% of episodes originate from viral upper respiratory infections (AVRS). Antimicrobial treatment of viral rhinosinusitis provides zero clinical benefit while exposing patients to adverse reactions.

                  [ Patient with Acute Rhinosinusitis Symptoms ]
                                        │
      Does the clinical presentation meet ANY of the 3 bacterial criteria?
       1. Persistent symptoms >= 10 days without clinical improvement
       2. Severe onset: Fever >= 39°C + purulent nasal discharge >= 3-4 consecutive days
       3. "Double sickening": Acute worsening after initial 5-6 day viral recovery
                                        │
                       ┌────────────────┴────────────────┐
                      YES                                NO
                       │                                 │
             [ Acute Bacterial ]               [ Acute Viral ]
             [ Rhinosinusitis  ]               [ Rhinosinusitis ]
                       │                                 │
           Initiate Amoxicillin or              Symptomatic Relief:
          Amoxicillin-Clavulanate            Saline rinse + INCS spray
            (Adults: 5-7 days)                  (No Antibiotics)

The Three Validated Clinical Presentations of ABRS

Under Canadian clinical practice guidelines, acute bacterial rhinosinusitis should be diagnosed only when symptoms satisfy at least one of the following patterns:

1. Persistent Symptoms: Signs and symptoms (purulent anterior/posterior rhinorrhea, nasal obstruction, facial pain/pressure) lasting ≥ 10 calendar days without evidence of clinical improvement.
2. Severe Onset: Presentation with high fever (≥ 39°C) accompanied by purulent (thick, discolored) nasal discharge and facial pain lasting for at least 3 to 4 consecutive days at illness onset.
3. "Double Sickening": Classic biphasic pattern where a typical viral upper respiratory infection begins to improve over 5 to 6 days, followed by sudden, acute deterioration marked by new-onset fever, increased headache, facial pressure, or intensified purulent rhinorrhea.

Pharmacotherapy for ABRS

• First-Line Uncomplicated Adults: Amoxicillin 500 mg PO TID or 1,000 mg PO TID for 5–7 days.
• First-Line with Risk Factors for Resistance: Amoxicillin-clavulanate 875/125 mg PO BID (or 2,000/125 mg extended-release PO BID) for 5–7 days. Risk factors include: age ≥ 65 years, antibiotic exposure within past 30 days, hospitalization within past 5 days, or immunocompromise.
• Penicillin Allergy: Doxycycline 100 mg PO BID for 5–7 days OR Respiratory Fluoroquinolones (Levofloxacin 500 mg PO daily or Moxifloxacin 400 mg PO daily).
• Adjunctive Therapies: Intranasal saline irrigation and intranasal corticosteroid (INCS) sprays (e.g., mometasone, fluticasone) reduce mucosal inflammation. Topical alpha-agonist decongestants (e.g., xylometazoline 0.1%) provide rapid temporary relief but must be strictly limited to ≤ 3 to 5 consecutive days to prevent rhinitis medicamentosa (rebound mucosal hyperemia and intractable congestion).

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Group A Streptococcal (GAS) Pharyngitis & Centor/McIsaac Scoring

Acute pharyngitis is overwhelmingly viral (rhinovirus, adenovirus, coronavirus, Epstein-Barr virus). Streptococcus pyogenes (Group A beta-hemolytic Streptococcus [GAS]) accounts for only 15%–30% of pediatric and 5%–15% of adult sore throats. The primary rationale for antibiotic treatment of GAS pharyngitis is preventing acute rheumatic fever (ARF), mitigating suppurative complications (e.g., peritonsillar abscess), shortening symptom duration by approximately 24 hours, and preventing transmission.

Modified Centor (McIsaac) Clinical Decision Score

The McIsaac score stratifies the pre-test probability of GAS pharyngitis and guides diagnostic swab testing:

Clinical Management Based on Score

• Score 0 to 1 (Risk < 10%): No throat swab, no rapid antigen testing, no antibiotic therapy. Provide symptomatic analgesics.
• Score 2 to 3 (Risk 15%–32%): Perform throat swab for Rapid Antigen Detection Testing (RADT) or throat culture. Treat with antibiotics only if microbiologically confirmed positive.
• Score ≥ 4 (Risk 50%–60%): In Canadian clinical practice, microbiologic confirmation is still recommended prior to initiating antibacterial therapy to curb inappropriate prescribing, though empiric therapy may be initiated pending rapid test results in severely symptomatic individuals.

Regimens for Confirmed GAS Pharyngitis

• First-Line Drug of Choice: Penicillin V oral:
  • Children: 250 mg PO BID or TID for 10 days (or 40 mg/kg/day divided BID–TID).
  • Adolescents/Adults: 300 mg PO TID–QID or 600 mg PO BID for 10 full days.
  • Alternative First-Line: Amoxicillin 50 mg/kg once daily (max 1,000 mg/day) or 25 mg/kg BID for 10 days. Amoxicillin suspension is widely favored in pediatric practice due to superior palatability.
• Penicillin Allergy:
  • Non-severe allergy (non-IgE): Cephalexin 20 mg/kg/dose BID (max 500 mg BID) for 10 days.
  • Severe allergy (IgE-mediated anaphylaxis): Clindamycin 20–30 mg/kg/day divided TID (max 300 mg TID) for 10 days, OR Azithromycin 12 mg/kg once daily (max 500 mg) for 5 days.

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Community-Acquired Pneumonia (CAP): Triage & Empiric Regimens

Community-Acquired Pneumonia (CAP) involves infection of the pulmonary parenchyma acquired outside of a healthcare facility. Common pathogens include:

• Typical Bacteria: Streptococcus pneumoniae (most common bacterial pathogen), Haemophilus influenzae, Moraxella catarrhalis.
• Atypical Pathogens: Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella pneumophila (intracellular organisms that lack peptidoglycan cell walls, rendering beta-lactams completely ineffective).
• Viruses: Influenza A/B, Respiratory Syncytial Virus (RSV), SARS-CoV-2.

Inpatient Triage: The CURB-65 Criteria

Clinical triage determines outpatient versus hospital admission based on the validated CURB-65 score (1 point for each):

• C — Confusion: New disorientation to person, place, or time.
• U — Urea: Blood urea nitrogen > 7 mmol/L (> 19 mg/dL).
• R — Respiratory Rate: ≥ 30 breaths/minute.
• B — Blood Pressure: Systolic < 90 mmHg OR Diastolic ≤ 60 mmHg.
• 65 — Age: ≥ 65 years.

Outpatient Empiric Therapy (CTS / ATS-IDSA Guidelines)

Canadian practice guidelines stratify outpatient CAP based on underlying comorbidity status:

                      [ Outpatient Diagnosis of CAP ]
                                     │
        Does the patient have chronic comorbidities or risk factors?
      (COPD, diabetes, CHF, CKD, liver disease, malignancy, asplenia,
               or antibiotic exposure within past 3 months)
                                     │
                     ┌───────────────┴───────────────┐
                    NO                              YES
                     │                               │
       [ Outpatient WITHOUT Comorbidities ]  [ Outpatient WITH Comorbidities ]
                     │                               │
           Option 1: Amoxicillin           Option 1: Beta-lactam Combo
             1 g PO TID x 5 days             Amoxicillin-Clavulanate 875/125 mg BID
                     OR                      PLUS Azithromycin 500 mg Day 1, then
           Option 2: Doxycycline             250 mg daily (or Doxycycline 100 mg BID)
             100 mg PO BID x 5 days                  OR
                                           Option 2: Respiratory Fluoroquinolone
                                             Levofloxacin 750 mg PO daily OR
                                             Moxifloxacin 400 mg PO daily x 5 days

Inpatient Non-Severe CAP Regimens

For patients admitted to the hospital ward (CURB-65 score ≥ 2):

• Preferred Regimen: IV/Oral Beta-Lactam (Ceftriaxone 1 to 2 g IV daily OR Cefotaxime 1 to 2 g IV Q8H OR Ampicillin-sulbactam 1.5 to 3 g IV Q6H) PLUS Macrolide (Azithromycin 500 mg IV/PO daily) or Doxycycline (100 mg IV/PO BID).
• Alternative Monotherapy: Respiratory Fluoroquinolone (Levofloxacin 750 mg IV/PO daily OR Moxifloxacin 400 mg IV/PO daily).
• Duration: Standard course is 5 days, provided the patient achieves clinical stability (afebrile for ≥ 48 hours, heart rate < 100 bpm, respiratory rate < 24 bpm, SBP ≥ 90 mmHg, room air SpO2 ≥ 92%, and tolerating oral intake).

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Hospital-Acquired (HAP) & Ventilator-Associated Pneumonia (VAP)

• Hospital-Acquired Pneumonia (HAP): Pneumonia occurring ≥ 48 hours after hospital admission that was not incubating upon entry.
• Ventilator-Associated Pneumonia (VAP): Pneumonia developing ≥ 48 hours after endotracheal intubation and mechanical ventilation.

Microbiology and Resistant Pathogens

Unlike CAP, nosocomial pneumonia involves multidrug-resistant (MDR) pathogens:

• Gram-Negative Bacilli: Pseudomonas aeruginosa, Klebsiella pneumoniae, Enterobacter cloacae, Escherichia coli, Acinetobacter baumannii.
• Gram-Positive Cocci: Methicillin-Resistant Staphylococcus aureus (MRSA), Methicillin-Susceptible S. aureus (MSSA).

Empiric Regimen Architecture

Empiric therapy must provide high-dose, bactericidal coverage against Pseudomonas aeruginosa and MRSA:

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Clinical Case Vignette: Evaluating Pediatric Otalgia & Treatment Failure

A mother brings her 14-month-old son (weight: 10 kg) to the community pharmacy. The child was diagnosed with acute otitis media 3 days ago at an after-hours walk-in clinic and prescribed amoxicillin 125 mg/5 mL suspension at a dose of 250 mg (10 mL) PO twice daily (50 mg/kg/day). The mother reports the child is still pulling at his right ear, has had little sleep, and developed a fever of 39.2°C this morning.

Pharmacist Clinical Assessment:

1. Evaluate Current Posology: The prescribed dose of 50 mg/kg/day represents conventional standard dosing, which is inadequate in Canadian practice to overcome intermediate and penicillin-resistant S. pneumoniae. The child received subtherapeutic therapy.
2. Identify Treatment Failure: The persistence of moderate-to-severe symptoms and fever beyond 48 to 72 hours of amoxicillin confirms clinical treatment failure.
3. Formulate Recommendation: Because the child failed amoxicillin therapy at 72 hours, the likelihood of beta-lactamase-producing H. influenzae or M. catarrhalis (or resistant S. pneumoniae) is high. The pharmacist contacts the clinic prescriber to recommend switching to amoxicillin-clavulanate (7:1 formulation), as the CPS advises after amoxicillin failure.
4. Calculate Posology:
   • Target amoxicillin dose: 60 mg/kg/day × 10 kg = 600 mg/day = 200 mg PO TID.
   • Product selection: Amoxicillin-clavulanate 7:1 suspension (400 mg amoxicillin / 57 mg clavulanate per 5 mL = 80 mg/mL amoxicillin).
   • Dose volume: 200 mg ÷ 80 mg/mL = 2.5 mL PO TID for 10 days (treatment failure, and the child is under 2 years).
   • Clavulanate check: 600 mg × 57/400 = about 85 mg/day = about 8.5 mg/kg/day, below the 10 mg/kg/day diarrhea threshold.
5. Patient Counseling: Counsel the mother to administer the medication at the start of a meal to enhance absorption and minimize GI intolerance, schedule acetaminophen or ibuprofen for acute pain control, and follow up if no improvement is noted within 48 hours.