Aetiology of HAP in patients with immune compromise
Patients with immune compromise are at greater risk of pneumonia than people without immune compromise. Patients with immune compromise include those:
- with neutropenia
- with immunodeficiency disorders
- with advanced human immunodeficiency virus (HIV) infection
- with haematological malignancies
- receiving immunosuppressive therapy or chemotherapy.
Consider alternative diagnoses to pneumonia in patients with immune compromise and pulmonary infiltrates on X-ray, given the broad differential. This may include noninfectious pathologies such as:
- acute respiratory distress syndrome
- drug-related pneumonitis
- progression of underlying disease such as malignancy
- connective tissue disease.
Although a broad range of pathogens can cause HAP in patients with immune compromise, the most common pathogens are the same as in patients without immune compromise (eg Streptococcus pneumoniae, gram-negative bacteria).
Note: Even in patients with immune compromise, the usual pneumonia pathogens (eg S. pneumoniae, gram-negative bacteria) are still the most common cause of HAP.
Pathogens to consider in patients with immune compromise who have high-severity HAP or significant immunosuppression include:
- respiratory viruses that can cause pneumonia and pneumonitis – for example, influenza, parainfluenza, human metapneumovirus, coronavirus (eg SARS-CoV-2), cytomegalovirus and RSV
- bacterial pathogens:
- anaerobes not adequately treated with standard empirical therapy – especially in patients with severe periodontal disease or putrid sputum
- Burkholderia pseudomallei – can cause pneumonia in tropical regions of Australia1, particularly in patients with risk factors (such as diabetes, heavy alcohol consumption [including binge drinking], chronic lung or kidney disease, and immunosuppressive therapy [including chronic corticosteroid use])Smith, 2018
- gram-negative bacteria, including Pseudomonas aeruginosa – especially in patients with haematological malignancy or immunoglobulin deficiencies
- Legionella species
- Nocardia species – usually cause focal lung infections
- mycobacterial pathogens
- Mycobacterium tuberculosis – especially in patients from high-prevalence areas
- nontuberculous mycobacteria
- fungal pathogens
- Aspergillus species – can cause invasive aspergillosis in patients with significant immunosuppression (eg neutropenia after intensive chemotherapy)
- Cryptococcus species – usually cause focal lung infections
- Pneumocystis jirovecii
- Strongyloides stercoralis in patients with past or present epidemiological risk of acquiring S. stercoralis.