Pneumocystis pneumonia (PCP)

Definition

Pneumocystis pneumonia (PCP, also PJP) is a pneumonia caused by the atypical fungus Pneumocystis jirovecii (formerly P. carinii). It occurs almost exclusively with impaired cell-mediated immunity and is one of the most important opportunistic infections in advanced HIV infection; in people with HIV it is an AIDS-defining illness.

Occurrence & epidemiology

• P. jirovecii occurs worldwide. Primary infection usually occurs in early childhood; most immunocompetent children have developed specific antibodies by the age of 4 years.
• Since effective HIV medicines became available, the frequency among people with HIV has fallen sharply in well-resourced countries. A high risk persists in undiagnosed HIV infection, especially with CD4 counts below 200/µL or below 14 % (HIV stage 3).

Aetiopathogenesis

• Pathogen: Pneumocystis jirovecii, originally classified as a protozoon, has been considered a fungus since the 1990s based on molecular analyses. Its life cycle consists of trophic forms and cysts.
• Transmission: airborne; causes no disease in immunocompetent people.
• Risk groups: HIV infection with a low CD4 count; organ transplant recipients; hematologic cancers; use of immunosuppressive drugs.

Clinical features

• Leading symptoms: fever, dyspnea (initially on exertion) and dry, nonproductive cough; occasionally chest discomfort.
• Course: insidious over several weeks in HIV infection, faster over a few days in other forms of immunodeficiency.
• Findings: tachypnea and tachycardia, especially on exertion; on auscultation sometimes diffuse fine crackles, but often normal.
• Hypoxemia as the hallmark finding, often with oxygen desaturation on exertion.
• Complications: spontaneous pneumothorax, respiratory failure.

Histology

In tissue, bronchoalveolar lavage or induced sputum, the organizms are found in the alveolar spaces. Giemsa, Diff-Quik and Wright stains show trophic forms and cysts without staining the cyst wall; Grocott-Gomori methenamine silver (GMS), Gram-Weigert, cresyl violet and toluidine blue stains specifically mark the cyst wall. Most stains have low sensitivity; some laboratories additionally use direct immunofluorescence with monoclonal antibodies.

Diagnosis

• Chest X-ray: typically bilateral, diffuse perihilar or fine granular opacities; in 20–30 % the radiograph is normal.
• Chest CT: ground-glass opacities, even with a normal radiograph; a normal CT makes PCP very unlikely.
• Pulse oximetry and arterial blood gas analysis: hypoxemia and increased alveolar-arterial oxygen gradient, sometimes before infiltrates are visible.
• Laboratory tests: elevated serum 1,3-beta-D-glucan and LDH support the suspicion but are nonspecific.
• Pulmonary function testing: reduced diffusing capacity (rarely used for diagnosis).
• Pathogen detection: from induced sputum, bronchoalveolar lavage or tracheal aspirate; bronchoalveolar lavage is considerably more sensitive than induced sputum. Nucleic acid amplification (PCR) is considered the gold standard with the highest yield; in addition immunofluorescence and microscopic stains.
• HIV test in every case of PCP without a known cause of immunodeficiency.

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Further reading (open access)

1. MSD Manual Professional: Pneumocystis jirovecii Pneumonia
2. RKI-Ratgeber: HIV-Infektion/AIDS

Cross-references