Pneumonia

Aetiology and Risk Factors

• Streptococcus pneumoniae is the most common cause of CAP in adults and remains the primary pathogen in severe and fatal cases.

• Atypical bacteria such as Mycoplasma pneumoniae, Chlamydophila (Chlamydia) pneumoniae, and Legionellaspecies also play important roles.

• Haemophilus influenzae is more frequently observed in individuals with chronic obstructive pulmonary disease (COPD).

• Staphylococcus aureus (including methicillin-resistant strains) is less common but can cause severe or cavitating pneumonia (especially post-influenza).

• Gram-negative bacilli, including Enterobacteriaceae (e.g. Klebsiella pneumoniae) and Pseudomonas aeruginosa, are more frequent in patients with comorbidities (e.g. bronchiectasis, chronic kidney disease) or in severe CAP.

• Respiratory viruses such as influenza, respiratory syncytial virus (RSV), metapneumovirus, and SARS-CoV-2 can act as sole pathogens or lead to secondary bacterial infections.

• In tropical regions of Australia (north of ~20°S latitude), Burkholderia pseudomallei (melioidosis) and Acinetobacter baumannii must be considered, especially in those with diabetes, excessive alcohol intake, or chronic lung/kidney disease.

Clinical Presentation

Typical symptoms include:

• Cough (with or without sputum)

• Dyspnoea

• Fever

• Chills

• Pleuritic chest pain

Some patients, particularly older individuals, may present with confusion. Gastrointestinal symptoms, such as diarrhoea, can also occur. On examination, there may be crackles, dullness on percussion if consolidation is present, and reduced breath sounds.

Severity Assessment

Careful assessment helps determine whether outpatient or inpatient care is required and guides antibiotic selection.

Red flags for hospital admission include:

• Tachypnoea (respiratory rate ≥22 breaths/min)

• Hypotension (systolic blood pressure <90 mmHg)

• Heart rate >100 beats/min

• Acute confusion

• Oxygen saturation <92% on room air (or < baseline in chronic respiratory conditions)

• Multilobar involvement on chest radiograph

• Lactate >2 mmol/L

High-severity features (possible need for ICU) include:

• Respiratory rate ≥30 breaths/min

• Oxygen saturation <90% on room air, or PaO₂/FiO₂ <250

• Rapid progression on chest X-ray or sepsis (elevated lactate, hypotension)

• Marked acute kidney injury or altered mental state

Investigations

• Chest X-ray: Confirms an infiltrate and assesses for consolidation, cavitation, or effusions.

• Sputum Gram stain and culture: Ideally from a high-quality specimen, to detect bacterial causes.

• Blood cultures: Two sets in moderate to high-severity CAP, ideally before starting antibiotics.

• Pneumococcal and Legionella urinary antigen tests: Useful in moderate to high-severity CAP to guide targeted therapy.

• Nucleic acid amplification tests (PCR): Can identify influenza, SARS-CoV-2, and other viruses; also used for Legionella, Mycoplasma, and Chlamydophila if indicated.

• Biomarkers (CRP, procalcitonin): May assist with clinical decision-making regarding antibiotics but do not definitively distinguish viral from bacterial infection.

Antibiotic Therapy for Community-Acquired Pneumonia

Therapy depends on severity, local epidemiology (e.g. tropical vs non-tropical areas), and likelihood of atypical pathogens.

Low-Severity CAP

Patients without red flags can usually be managed in the community.

• First-line:

  • Amoxicillin 1 g orally, three times daily for up to 5 days (extend to 7 days if response is slower).

• If atypical organisms are suspected or there is penicillin hypersensitivity:

  • Doxycycline 100 mg orally, twice daily, or

  • Clarithromycin 500 mg orally, twice daily.

• Combination therapy (amoxicillin plus doxycycline) may be needed if follow-up is uncertain or if minimal improvement is seen after 48 hours of monotherapy.

Moderate-Severity CAP

Hospital admission is generally indicated.

• Typical regimen:

  • Benzylpenicillin IV plus doxycycline (or clarithromycin) orally.

  • IV azithromycin is an alternative if the patient cannot tolerate oral doxycycline or clarithromycin.

• Duration: About 5–7 days in total (including IV-to-oral switch).

• Penicillin allergy:

  • Consider ceftriaxone IV (or cefuroxime orally once stable) plus doxycycline or clarithromycin.

  • In severe penicillin allergy, use moxifloxacin alone.

High-Severity CAP

Often requires ICU-level care.

• Empirical coverage:

  • Ceftriaxone (2 g IV once daily, or 1 g twice daily if septic shock) plus IV azithromycin.

  • In tropical regions during the wet season, meropenem plus azithromycin is recommended due to Burkholderia pseudomallei.

• Duration: At least 7 days in most cases; azithromycin is typically given for 3–5 days.

• Additional considerations:

  • Suspected Staphylococcus aureus: add vancomycin.

  • Risk of Pseudomonas aeruginosa: use an antipseudomonal beta-lactam (e.g. piperacillin+tazobactam or cefepime) plus azithromycin.

Aspiration Pneumonia

Many CAP regimens already cover typical oropharyngeal bacteria. Antibiotics specifically targeting anaerobes (e.g. metronidazole) may be considered if there is putrid sputum or severe periodontal disease. Differentiate aspiration pneumonitis (chemical injury) from aspiration pneumonia (bacterial infection); if in doubt, start standard CAP treatment and review early.

Paediatric and Special Groups

Children, residents of aged-care facilities, and immunocompromised individuals may require different regimens or specialist input. In older patients, comorbidities must be considered, and early follow-up is essential if managed in the community.

Supportive Management

• Ensure adequate hydration and provide supplemental oxygen if the patient is hypoxic.

• Offer appropriate analgesia for pleuritic pain.

• Encourage early mobilisation.

• Monitor closely for sepsis, septic shock, or acute respiratory decompensation.

• Consider antiviral therapy (e.g. for influenza) during relevant seasons.