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Pneumocystis

Authors: Anusmita Tripathy, M.D., Aliya N. Husain, M.D.

Editorial Board Member: Matthew J. Cecchini, M.D., Ph.D.

Last author update: 26 October 2023

Last staff update: 26 March 2024

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PubMed Search: Pneumocystis pneumonia

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Table of Contents

Cite this page: Tripathy A, Husain AN. Pneumocystis. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/lungnontumorPCP.html. Accessed April 24th, 2024.

Definition / general

Pneumocystis pneumonia (PCP) is a serious infection caused by the fungus Pneumocystis jirovecii in immunocompromised patients

Essential features

One of the most common AIDS defining diseases in HIV infected patients
Substantial mortality and morbidity
Direct microscopic examination of bronchoalveolar lavage, sputum and lung biopsy are the gold standard methods of diagnosis
Cup shaped organisms with a central dot-like density

Terminology

Pneumocystis jirovecii pneumonia (PJP)
Formerly known as Pneumocystis carinii

ICD coding

ICD-10: B59 - pneumocystosis

Epidemiology

Incidence of Pneumocystis pneumonia has declined substantially with widespread use of Pneumocystis pneumonia prophylaxis and antiretroviral therapy
Recent incidence among patients with AIDS in Western Europe and the United States is < 1 case per 100 person years
Commonly seen in HIV and immunocompromised patients (hematologic malignancies followed by autoimmune diseases) (Int J Infect Dis 2022;121:172)

Sites

Lung and rarely extrapulmonary sites, which include lymph nodes, spleen, liver, bone marrow, adrenal glands, gastrointestinal tract, kidneys, thyroid gland, heart, pancreas, eyes, ears, lumina of blood vessels, perivascular involvement and lymphatic invasion (Arch Intern Med 1991;151:1205)

Pathophysiology

Lung injury during infection is mostly due to host inflammatory response mediated by CD4+ T cells, alveolar macrophages, neutrophils and the soluble mediators
- Inflammatory response is primarily responsible for the damage of the alveolar capillary interface activity (N Engl J Med 2004;350:2487)

Etiology

Immunocompromised conditions (e.g., cancer, HIV, transplant recipients on immunosuppressant therapies) (N Engl J Med 2004;350:2487)

Diagrams / tables

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Life cycle

Clinical features

Symptoms include fever, nonproductive cough, difficulty breathing, chest pain, fatigue and chills (Respiration 2018;96:52)
These symptoms develop over several days to weeks

Diagnosis

Sputum, bronchoalveolar lavage or lung biopsy
Direct microscopic examination, polymerase chain reaction (PCR, highly sensitive but less specific), blood test to detect beta D glucan (Med Mycol 2020;58:1015)
In direct microscopy, Grocott-Gomori methenamine silver (GMS) stain should be performed on tissue biopsy and lavage specimens; immunofluorescence may be performed
Papanicolaou (Pap) and Romanowsky (Giemsa, Diff-Quik) barely stain the cyst wall of the organisms and infection can easily be missed

Laboratory

Hypoxemia is the most characteristic laboratory abnormality
Elevation of lactate dehydrogenase levels to > 500 mg/dL is common but also nonspecific

Radiology description

Chest radiograph typically demonstrates diffuse, bilateral, symmetrical ground glass interstitial infiltrates emanating from the hila in a butterfly pattern (AIDS 1998;12:885)
In early disease, a chest radiograph may be normal
Atypical radiographic presentations also occur, e.g., nodules, blebs / cysts, asymmetric disease, upper lobe localization, intrathoracic adenopathy and pneumothorax
Spontaneous pneumothorax in a patient with HIV infection should raise the suspicion of PCP
Cavitation and pleural effusion are uncommon in the absence of other pulmonary pathogens or malignancy and their presence may indicate an alternative diagnosis or an additional pathology such as tuberculosis (TB), Kaposi sarcoma or bacterial pneumonia

Radiology images

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Chest imaging

Bilateral well defined cystic spaces in lung

Prognostic factors

Treatment changes course of disease

Case reports

45 year old woman presented with dyspnea on exertion and significant weight loss within the preceding 4 months (BMC Infect Dis 2023;23:185)
50 year old man presented with hypoxia, 1 week history of pleuritic chest pain and fever (Turk Patoloji Derg 2020;1:246)
59 year old man presented with Pneumocystis jirovecii associated pneumonia and hypercalcemia due to ectopic production of 1,25 dihydroxyvitamin D, 6 years after renal transplantation (Acta Clin Belg 2021;76:75)

Treatment

Trimethoprim and sulfamethoxazole

Gross description

Lungs are heavy, firm and gray-brown
- There is fine porosity due to trapped pockets of air
- There may be small yellow indurated areas in the lung parenchyma and occasional emphysematous subpleural blebs (Can Med Assoc J 1969;100:846)
Lung nodules with or without cavitation have been demonstrated in response to granulomatous inflammation (Thorax 2002;57:435)

Gross images

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Confluent consolidation and fine porosity

Frozen section description

Touch preparations with Giemsa stain of fresh frozen lung infected with Pneumocystis is preferred over histologic sections as morphologic features are more apparent in the former (Can Med Assoc J 1969;100:846)

Frozen section images

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Touch preparation of fresh frozen lung

Microscopic (histologic) description

Alveolar spaces filled with pink, foamy amorphous material composed of proliferating fungi and cell debris
Pneumocystis organisms can be identified in the GMS (Grocott-Gomori methenamine silver) stained tissue sections within airspaces
They are thin walled, oval, round and crescentic forms about the size of red blood cells (7 microns) (Can Med Assoc J 1969;100:846)
Granulomas, although rare, are mostly necrotizing and multiple
More than 1 infection, such as coexisting cytomegalovirus infection, may be present in these patients so it is important to look for other infectious organisms

Microscopic (histologic) images

Contributed by Aliya N. Husain, M.D.

Intra-alveolar Pneumocystis jirovecii organisms

Granulomatous inflammation

Foamy amorphous material

Intra-alveolar Pneumocystis organisms

Cytology description

Characteristic morphological finding is round to oval and cup shaped dark black (GMS stain) cysts with central dot-like density

Cytology images

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Bronchoalveolar lavage washings

Immunofluorescence description

Direct immunofluorescence testing on sputum and bronchoalveolar lavage specimens

Immunofluorescence images

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Direct
immunofluorescence
testing

Positive stains

Grocott-Gomori methenamine silver

Negative stains

Periodic acid-Schiff stain

Electron microscopy description

Trophic forms are 1 - 4 microns and have simple plasma membrane with filopodial projections (Clin Chest Med 2009;30:265)
Cyst forms are 8 - 10 microns with rigid cell walls and up to 8 intracystic bodies (Clin Chest Med 2009;30:265)

Molecular / cytogenetics description

Polymerase chain reaction is sensitive and rapid in detecting the presence of Pneumocystis jirovecii in bronchoalveolar lavage specimens (J Mycol Med 2022;32:101241)

Sample pathology report

Lung, transbronchial biopsy:
- Organisms consistent with Pneumocystis jirovecii infection (see comment)
- Comment: Grocott-Gomori methenamine silver (fungal) stain is positive for organisms morphologically consistent with Pneumocystis jirovecii (control is appropriate).

Differential diagnosis

Pneumocystis jirovecii lack budding and have a central dark dot-like density that is not seen in the following organisms: Candida, Histoplasma and Cryptococcus
Alveolar proteinosis:
- Diffuse pulmonary opacification and intra-alveolar PAS positive material and lipid
- Exudative pulmonary edema, sputum contains gelatinous material
- No inflammation
- Alveolar contents contain type II pneumocytes and necrotic alveolar macrophages
- GMS stain is negative
Anti-GBM disease:
- Necrotizing, hemorrhagic, interstitial pneumonitis; associated with rapidly proliferative glomerulonephritis, linear immunoglobulin deposits on basement membranes of alveolar septal walls
- Also, intra-alveolar hemorrhage, septal thickening and hypertrophy, organization of blood in alveolar spaces
Organizing pneumonia:
- Bronchiolar and alveolar plugs of loose fibrous tissue (Masson bodies)

Board review style question #1

A 50 year old man with hypoxia and bilateral interstitial infiltrates on chest imaging showed intra-alveolar foamy material. Grocott-Gomori methenamine (GMS) stain is shown in the figure above. What is the diagnosis?

Alveolar proteinosis
Candida
Cryptococcus
Pneumocystis pneumonia

Board review style answer #1

D. Pneumocystis pneumonia. Pneumocystis jirovecii are round to oval and cup shaped GMS positive cysts, approximately the size of red blood cells, with a central dot. Answer C is incorrect because Cryptococcus is larger, usually 10 - 20 microns, budding and does not reveal a central dot with GMS stain. Answer A is incorrect because GMS is negative in alveolar proteinosis. Answer B is incorrect because Candida is larger, forms pseudohyphae and budding yeasts and does not reveal a central dot with GMS stain.

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Reference: Pneumocystis

Board review style question #2

A 60 year old woman with dry cough, fever and shortness of breath revealed bilateral pulmonary nodules on imaging. Biopsy revealed features as shown in the figure above. What is the diagnosis?

Blastomyces and respiratory syncytial virus (RSV)
Cryptococcus neoformans and herpes virus
Histoplasma capsulatum and adenovirus
Pneumocystis pneumonia and cytomegalovirus (CMV)

Board review style answer #2

D. Pneumocystis pneumonia and cytomegalovirus (CMV). The presence of intra-alveolar amorphous foamy material is characteristic of pneumonia caused by Pneumocystis jirovecii. Enlarged cells with single intranuclear inclusion and multiple small intracytoplasmic inclusions are diagnostic for cytomegalovirus. Answers A, B and C are incorrect because intra-alveolar amorphous foamy material is rarely seen in Cryptococcus, Histoplasma and Blastomyces pulmonary infection. Adenovirus has smudged intranuclear inclusions. Herpes virus has glassy intranuclear inclusions and respiratory syncytial virus (RSV) causes multinucleation with small intracytoplasmic eosinophilic inclusions.

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Reference: Pneumocystis

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