What is Pneumocystis (Pneumocystis jirovecii) pneumonia?

From the Guidelines

Pneumocystis carinii, now renamed Pneumocystis jirovecii, is a fungal organism that causes Pneumocystis pneumonia (PCP), a serious lung infection primarily affecting individuals with weakened immune systems, and the most recent guidelines recommend prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMX) for high-risk individuals, particularly those with HIV/AIDS, cancer, or taking immunosuppressive therapy, as stated in the 2024 guidelines 1. The infection presents with fever, dry cough, shortness of breath, and progressive respiratory difficulty.

• Key populations at risk include:
  • HIV/AIDS patients with CD4 counts below 200 cells/mm³
  • Organ transplant recipients
  • Cancer patients undergoing chemotherapy
  • Those on long-term corticosteroid therapy Diagnosis typically involves identifying the organism in respiratory specimens through specialized staining techniques or PCR testing.
• The first-line treatment is trimethoprim-sulfamethoxazole (TMP-SMX) for 21 days, typically at a dose of 15-20 mg/kg/day of the trimethoprim component, divided into three or four doses, as recommended in the 2015 guidelines 1. Alternative treatments for those who cannot tolerate TMP-SMX include pentamidine, dapsone plus trimethoprim, atovaquone, or clindamycin plus primaquine.
• Prophylaxis with TMP-SMX is recommended for high-risk individuals, and the most recent guidelines suggest that prophylaxis should be considered in patients with autoimmune inflammatory rheumatic diseases (AIIRD) in whom high doses of glucocorticoids are used, especially in combination with immunosuppressants, as stated in the 2023 guidelines 1. The organism was originally classified as a protozoan but is now recognized as a fungus, though it has unique characteristics that distinguish it from typical fungi, including its inability to be cultured on standard fungal media.
• The most commonly used prophylaxis scheme is trimethoprim/sulfamethoxazole (TMP-SMX) 480 mg/day (single-strength) or 960 mg three times a week, with alternative prophylactic medications including atovaquone, dapsone, or nebulised pentamidine, as recommended in the 2023 guidelines 1.

From the Research

Definition and Classification of Pneumocystis

• Pneumocystis jirovecii is an opportunistic fungal pathogen that can cause severe pneumonia in immunocompromised hosts 2.
• The organism was initially mistaken as a trypanosome and later as a protozoan, but genetic analysis identified it as a unicellular fungus 3.
• Pneumocystis carinii is an older term used to describe the organism, but it is now recognized that Pneumocystis jiroveci is the species responsible for human infections 3, 4.

Risk Factors and Clinical Presentation

• Risk factors for Pneumocystis jirovecii pneumonia (PJP) include HIV, organ transplant, malignancy, certain inflammatory or rheumatologic conditions, and associated therapies and conditions that result in cell-mediated immune deficiency 2.
• Clinical signs of PJP are nonspecific, and definitive diagnosis requires direct detection of the organism in lower respiratory secretions or tissue 2.
• The clinical presentation of PJP can vary, with a slow indolent time course in HIV-infected patients and fulminant respiratory failure in non-HIV-infected patients 3.

Diagnosis and Treatment

• Definitive diagnosis of PJP relies on histopathological testing of sputum, induced or sampled by fiberoptic bronchoscopy with bronchoalveolar lavage 3.
• The first-line drug for treatment and prevention of PJP is trimethoprim-sulfamethoxazole 2, 3, 5.
• Second-line therapies, such as dapsone, pentamidine, atovaquone, clindamycin, and primaquine, may be used in patients who are intolerant or allergic to trimethoprim-sulfamethoxazole 2, 6.