• Previously known as Pneumocystis carinii, Pneumocystis jiroveci is the causal agent of P. carinii pneumonia (PCP).
• PCP is the most prevalent opportunistic infection among HIV-positive individuals.
• Pneumocystis is a unicellular fungus that inhabits the respiratory passages of numerous mammals and humans.
• Initially, the genus Pneumocystis was misidentified as a trypanosome, and then as a protozoan.
• In the 1980s, biochemical studies of the rRNA and mitochondrial DNA of Pneumocystis proved that it is a fungus.
• The cyst wall is remarkably similar to that of fungus. However, unlike fungus, it lacks ergosterol in its membrane and instead has cholesterol.

Properties 

There are three unique morphological phases in this organism:

• The trophozoite or tropic form, in which it frequently occurs in clusters.
• The sporozoite is a precystic stage.
• The cyst, which contains many spore-like intracystic entities.

Pathogenesis and Immunity 

• Infection is transmitted through inhalation.
• The tropic form of the bacterium adheres to the alveoli during inhalation.
• Defects in both cellular immunity and humoral immunity permit uncontrolled proliferation of the organism and disease progression.
• Without CD4 cells, activated alveolar macrophages are incapable of eradicating the pathogen.

Clinical Syndromes 

• PCP is caused by P. jiroveci in HIV patients with a CD4 cell count below 200 cells/mL.
• It also develops PCP in individuals with primary immunological weaknesses, such as hypogammaglobulinemia and severe combined immunodeficiency, in organ-transplant recipients’ long-term immunosuppressive regimens, and in patients with hematologic and nonhematologic cancers.
• The majority of PCP cases are asymptomatic.
• Symptomatic cases are characterised by abrupt onset of fever, nonproductive cough, dyspnea, and tachypnea. There are bilateral rales and ronchi present.
• Rarely do extrapulmonary symptoms occur. It occurs in the later stages of HIV infection.

Epidemiology 

• Pneumocystis was discovered to cause interstitial pneumonia in very malnourished and preterm infants in Central and Eastern Europe during World War II.
• Prior to 1981, there were fewer than 100 reports from the United States. However, in 1981, the CDC in the United States reported the prevalence of PCP in five healthy Los Angeles-based homosexual males.
• P. jiroveci is now recognised as one of the most common causes of life-threatening opportunistic infections in HIV-positive individuals around the world. The fungus resides within the lungs of healthy persons.
• By ages 3 or 4, the majority of youngsters are estimated to have been exposed to the organism. PCP causes 10–20% mortality in HIV-positive people, depending on the disease’s stage.
• It also causes substantial death and morbidity among non-HIV patients. Due to a lack of access to modern healthcare, many cases in developing nations go unreported.
• In Africa, Pneumocystis is responsible for up to 80% of HIV-infected newborns with pneumonia.

Laboratory Diagnosis

• The majority of patients had diffuse bilateral infiltrates spreading from the perihilar region on chest radiography. Pneumatoceles and asymmetric infiltrates are uncommon findings.
• The sputum produced by inhaling a hypertonic saline solution is a valuable specimen for microscopic examination of the organism. Pneumocystis is observed less commonly in expectorated sputum. Bronchoalveolar lavage is the most often utilised invasive method for diagnosing PCP. When PCP is strongly suspected but induced sputum samples are frequently negative, this is indicated. It has a sensitivity of greater than 90 percent.
• Wright, Cresyl violet, and Giemsa stains identify the trophozoite and cyst forms, but not the cyst wall. Methenamine silver, Gram-Weigert, and toluidine blue stain the Pneumocystis cyst wall preferentially. Papanicolaou smear is useful for demonstrating eosinophilic material with a foamy appearance surrounding Pneumocystis. Histologic approaches are less sensitive than direct immunofluorescence using monoclonal antibodies for detecting Pneumocystis in histopathologic specimens.
• The open lung biopsy has a sensitivity and specificity of one hundred percent because it gives the most diagnostic tissue. This is an invasive operation, hence it is reserved for instances in which bronchoscopy is inconclusive.

Treatment

• Although P. jiroveci is classified as a fungus, it does not react to antifungal medications.
• The medicine of choice for treating PCP is a combination of trimethoprim and sulfamethoxazole. Pentamidine and atovaquone are alternative medicines.

Prevention and Control 

• For the prevention of infection in individuals with CD4 counts < 200/L, chemoprophylaxis with trimethoprim and sulfamethoxazole or aerosolized pentamidine is helpful.