4.3 Severe Community-Acquired & Viral Pneumonia

Defining Severe Community-Acquired Pneumonia

The 2019 ATS/IDSA guideline defines severe CAP by one major or three or more minor criteria.

Leukopenia, hypothermia, and thrombocytopenia are ominous markers of overwhelming infection.

Microbiology and Testing

The leading organisms in severe CAP are:

• Streptococcus pneumoniae.
• Legionella.
• Staphylococcus aureus (including post-influenza MRSA).
• Haemophilus influenzae.
• Enteric gram-negative bacilli.
• Pseudomonas aeruginosa in patients with structural lung disease or recent antibiotics.
• Respiratory viruses.

For severe CAP, obtain:

• Blood cultures, and sputum or tracheal aspirate Gram stain and culture.
• Pneumococcal and Legionella urinary antigens.
• Influenza and SARS-CoV-2 testing, ideally a multiplex respiratory panel.
• A MRSA nasal PCR.

A negative MRSA nasal PCR has a high negative predictive value for MRSA pneumonia and supports early de-escalation. Procalcitonin should not be used to withhold antibiotics at presentation.

Empiric Antibiotics for Severe CAP (ATS/IDSA 2019)

• No MRSA or Pseudomonas risk factors: a beta-lactam (ceftriaxone, cefotaxime, ampicillin-sulbactam, or ceftaroline) plus a macrolide (azithromycin), or a beta-lactam plus a respiratory fluoroquinolone (levofloxacin or moxifloxacin). Observational data favor including a macrolide in severe disease.
• Prior respiratory isolation of MRSA or Pseudomonas: add coverage empirically (vancomycin or linezolid for MRSA; piperacillin-tazobactam, cefepime, ceftazidime, meropenem, or imipenem for Pseudomonas) and de-escalate with cultures.
• Recent hospitalization with IV antibiotics in the past 90 days: add MRSA or Pseudomonas coverage pending cultures and rapid tests in severe CAP.
• Aspiration: routine anaerobic coverage is not recommended unless there is lung abscess or empyema.
• Duration: at least 5 days, continuing until the patient is clinically stable.

Corticosteroids in Severe CAP

In CAPE COD (NEJM 2023), ICU patients with severe CAP were randomized to hydrocortisone 200 mg/day for 4 or 8 days with a taper, or placebo. Hydrocortisone lowered 28-day mortality from 11.9% to 6.2% and reduced the need for intubation and vasopressors. Patients with septic shock or influenza were excluded. The 2024 SCCM corticosteroid guideline update suggests corticosteroids for hospitalized adults with severe bacterial CAP. Observational data link steroids to worse outcomes in influenza pneumonia, so avoid them there unless another indication, such as septic shock, exists.

Legionnaires' Disease

Suspect Legionella when severe pneumonia comes with:

• High fever with relative bradycardia.
• Diarrhea and confusion.
• Hyponatremia.
• Elevated liver enzymes and creatine kinase.
• Failure to respond to beta-lactams.

Exposures include cooling towers, plumbing, and hot tubs. The urinary antigen detects only L. pneumophila serogroup 1, so a negative test does not exclude other strains; use respiratory PCR or culture on buffered charcoal yeast extract. Treat with azithromycin or levofloxacin.

Influenza

• Oseltamivir (75 mg twice daily) is recommended for every hospitalized patient with influenza as early as possible, regardless of symptom duration (IDSA 2018). Longer courses are used for critically ill or immunocompromised patients, and IV peramivir is an alternative when enteral absorption is unreliable.

• Secondary bacterial pneumonia is caused by S. aureus (including MRSA), S. pneumoniae, and Streptococcus pyogenes. Suspect it when the patient worsens after initial improvement, or with severe necrotizing disease.

• Necrotizing staphylococcal pneumonia, often from Panton-Valentine leukocidin-producing strains, presents with:

  • Leukopenia.
  • Hemoptysis.
  • Rapid cavitation.
  • Septic shock.

  Use linezolid, or vancomycin plus clindamycin, to suppress toxin production.

• Influenza also causes myocarditis, rhabdomyolysis, and encephalopathy. Avian influenza (H5N1, H7N9) requires airborne precautions and early oseltamivir.

Critical COVID-19

• Dexamethasone 6 mg daily for up to 10 days reduced mortality in patients requiring oxygen or ventilation (RECOVERY).
• Add an IL-6 receptor antagonist (tocilizumab) or a JAK inhibitor (baricitinib) for patients with rising oxygen needs and systemic inflammation.
• Remdesivir benefits are greatest in patients on low-flow oxygen who are not yet ventilated.
• Anticoagulation: critically ill patients receive standard prophylactic doses, because therapeutic heparin helped only non-critically ill hospitalized patients in the multiplatform trials.
• Awake prone positioning in patients on high-flow oxygen reduced intubation in a pooled meta-trial.

Other Viral and Atypical Causes

Complications of severe pneumonia include parapneumonic effusion and empyema, lung abscess, ARDS, myocardial infarction, and arrhythmias. Reassess any patient who is not improving by 48 to 72 hours for resistant organisms, an undrained collection, or a noninfectious mimic.