Pulmonary Pneumocystis jirovecii infection

Last revised by Yoshi Yu on 25 Aug 2023

Pulmonary Pneumocystis jirovecii infection, also known as Pneumocystis jirovecii pneumonia (PJP) or Pneumocystis pneumonia (PCP), is an atypical pulmonary infection and the most common opportunistic infection in patients with acquired immunodeficiency syndrome (AIDS).

Classically, "PCP" was the acronym for Pneumocystis carinii pneumonia, but the causative organism was reclassified as Pneumocystis jirovecii. Pneumocystis carinii refers to a species found in rats, while Pneumocystis jirovecii refers to the human isolate 14. However, there continues to be widespread use of the acronym PCP; a post hoc justification for its use is it stands for Pneumocystis pneumonia 14,15. The acronym "PJP" for Pneumocystis jirovecii pneumonia is also in use. 

Pneumocystis pneumonia is virtually never present in immunocompetent individuals. It is one of the most common causes of life-threatening pulmonary infections in HIV-positive patients. It occurs twice as frequently in homosexual males versus intravenous drug users (IVDU). It typically occurs at CD4 counts <200 cells/mm 3,8.

It is seen particularly in patients with AIDS; therefore, the demographics closely match those of the AIDS population. Typically, non-AIDS patients are severely immunosuppressed due to other causes, such as hematological malignancy or in bone marrow transplant recipients.

Presentation is usually non-specific and insidious, the most common symptoms being dyspnea and/or non-productive cough. In patients who are profoundly immunocompromised, onset may be more dramatic and resemble other pulmonary infections 7,9.

The diagnosis can often be confirmed with bronchoalveolar lavage which has a sensitivity of 85-90% 8. It is one of the most common causes of life-threatening pulmonary infections in HIV-positive patients.

Pneumocystis jirovecii is an atypical yeast-like fungus of the genus Pneumocystis 10 that was previously thought to be a protozoan.

Histology of infected lung demonstrates intra-alveolar eosinophilic masses with a foamy appearance, due to small cysts within which the Pneumocystis jirovecii organism is found 9

Culturing Pneumocystis jirovecii can be very difficult. Diagnostic confirmation requires identification of organisms in sputum or bronchoalveolar lavage fluid. Monoclonal antibodies for detecting Pneumocystis jirovecii are available and have a sensitivity greater than 90% for detecting Pneumocystis jirovecii in induced sputum from HIV-infected patients 10.

Although up to 90% of chest radiographs in patients with Pneumocystis pneumonia are abnormal, appearances are often non-specific. Between 10-15% of patients have normal chest radiographs and close to 30% have non-specific or inconclusive findings 2-4,6,7.

Features which are highly suggestive of pneumocystis pneumonia in patients with CD4 counts below 200/mm3 include 5:

  • small pneumatoceles

  • subpleural blebs

  • fine reticular interstitial changes

  • predominantly perihilar in distribution

Pleural effusions are normally not a feature, seen in less than 5% of cases 9.

High-resolution computed tomography is more sensitive and may be used to exclude PCP in patients with clinical suspicion for PCP but normal or inconclusive chest radiographs 3.

Features include 2,3,7:

  • ground-glass pattern

    • considered a principal finding

    • predominantly involving perihilar or mid zones

      • there may be a mid, upper or lower zone predilection depending on whether the patient is on prophylactic aerosolized medication

      • if they are, then the poorly ventilated upper zones are prone to infection 9, whereas, in those who are not, the lower zones are more frequently involved

      • there may be relative preservation of previously irradiated areas

      • show some peripheral sparing in a considerable number of patients (~40%) 10

  • reticular opacities or septal thickening may also be present; a crazy paving pattern may, therefore, be seen when both ground-glass opacities and septal thickening are present

  • pneumatoceles

    • varying shape, size, and wall thickness

    • are seen in up to 30% of cases 7

  • pleural effusions are rare 2,3

  • lymphadenopathy is uncommon (10%)

Atypical features, found more frequently in patients treated prophylactically, include 7:

  • consolidation: can be more common in patients without HIV infection and tends to develop more rapidly, reflecting pulmonary damage from the host immune response.

  • nodules (granulomas)

  • lymphadenopathy

  • pleural effusions are also more frequently encountered in this group of patients

A cystic form of Pneumocystis pneumonia is also recognized; again, more frequently in patients receiving aerosolized prophylaxis. Features of this pattern include 7:

  • thin walled cysts: in most cases these are pneumatoceles

  • upper lobe predominance

  • may be bilateral

  • increased risk of pneumothorax due to cyst rupture

Gallium-67 lung scintigraphy is highly sensitive for PCP, and a normal gallium scan renders the diagnosis of PCP very unlikely. The gallium scan in patients with PCP demonstrates diffuse pulmonary uptake, which may be heterogeneous or homogeneous.

Despite this, the specificity of the gallium scan is low; hence, it is most useful in patients in whom bronchoalveolar lavage may be less diagnostic (e.g. in suspected relapse).

Most patients with acute infection are treated with trimethoprim-sulfamethoxazole (co-trimoxazole or TMP-SMZ) 17, combined with corticosteroids in patients with moderate to severe infections 8. The same agent may be used as prophylaxis. A number of alternative agents may also be employed, both for acute treatment and prophylaxis, although these are beyond the scope of this article. Some of these include certain naphthoquinones such as atovaquone 17.

Overall, with prompt treatment, survival is good (50-95%), although relapses are common 9.

The differential diagnosis on HRCT is strongly influenced by knowledge of HIV status and CD4 count. In these patients the differential includes:

Other conditions to be considered, which also occur in non-AIDS patients, include:

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Cases and figures

  • Figure 1: gross pathology
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  • Case 1
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  • Case 2
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  • Case 3
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  • Case 4
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  • Case 5
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  • Case 6: with small pneumatocoeles
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  • Case 7: CT chest
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  • Case 8
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  • Case 9
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  • Case 10: with pneumothorax
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  • Case 11
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  • Case 12
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  • Case 13
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  • Case 14
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  • Case 15
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  • Case 16
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  • Case 17
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  • Case 18: probable
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  • Case 19: confirmed
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  • Case 20: Pneumocystis jirovecii pneumonia
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