Pneumocystis pneumonia

Pneumocystis pneumonia (PCP), also known as Pneumocystis jirovecii pneumonia (PJP), is a form of pneumonia that is caused by the yeast-like fungus Pneumocystis jirovecii. Pneumocystis specimens are commonly found in the lungs of healthy people although it is usually not a cause for disease. However, they are a source of opportunistic infection and can cause lung infections in people with a weak immune system or other predisposing health conditions. PCP is seen in people with HIV/AIDS (who account for 30-40% of PCP cases), those using medications that suppress the immune system, and people with cancer, autoimmune or inflammatory conditions, and chronic lung disease.

Signs and symptoms Signs and symptoms may develop over several days or weeks and may include: shortness of breath and/or difficulty breathing (of gradual onset), fever, dry/non-productive cough, weight loss, night sweats, chills, and fatigue. Uncommonly, the infection may progress to involve other visceral organs (such as the liver, spleen, and kidney).

Cough - typically dry/non-productive because sputum becomes too viscous to be coughed up. The dry cough distinguishes PCP from typical pneumonia.

Complications

Pneumothorax is a well-known complication of PCP. Also, a condition similar to acute respiratory distress syndrome (ARDS) may occur in patients with severe Pneumocystis pneumonia, and such individuals may require intubation.

Pathophysiology The risk of PCP increases when CD4-positive T-cell levels are less than 400 cells/μL. In these immunosuppressed individuals, the manifestations of the infection are highly variable. The disease attacks the interstitial, fibrous tissue of the lungs, with marked thickening of the alveolar septa and alveoli, leading to significant hypoxia, which can be fatal if not treated aggressively. In this situation, lactate dehydrogenase levels increase and gas exchange is compromised. Oxygen is less able to diffuse into the blood, leading to hypoxia, which along with high arterial carbon dioxide (CO2) levels, stimulates hyperventilatory effort, thereby causing dyspnea (breathlessness). In addition, in symptomatic cases of P. jirovecii pneumonia, the overgrowth of the fungus is associated to a co-infection with trichomonads, unicellular flagellated parabasalid protist (Parabasalia) of the family Trichomonadidae. These parasites (including the commensal Trichomonas tenax, Trichomonas vaginalis and Tritrichomonas foetus) exhibit an amoeboid form, without flagellum, which makes it difficult to identify them under the microscope. Amoeboid transformation is an argument in favor of a deleterious action, which nevertheless remains conjectural.

Diagnosis The diagnosis can be confirmed by the characteristic appearance of the chest X-ray and an arterial oxygen level (PaO2) that is strikingly lower than would be expected from symptoms. Gallium 67 scans are also useful in the diagnosis. They are abnormal in about 90% of cases and are often positive before the chest X-ray becomes abnormal. Chest X-ray typically shows widespread pulmonary infiltrates. CT scan may show pulmonary cysts (not to be confused with the cyst-forms of the pathogen).

The diagnosis can be definitively confirmed by histological identification of the causative organism in sputum or bronchoalveolar lavage (lung rinse). Staining with toluidine blue, silver stain, periodic acid-Schiff stain, or an immunofluorescence assay shows the characteristic cysts. The cysts resemble crushed ping-pong balls and are present in aggregates of two to eight (and not to be confused with Histoplasma or Cryptococcus, which typically do not form aggregates of spores or cells). A lung biopsy would show thickened alveolar septa with fluffy eosinophilic exudate in the alveoli. Both the thickened septa and the fluffy exudate contribute to dysfunctional diffusion capacity that is characteristic of this pneumonia. Pneumocystis infection can also be diagnosed by immunofluorescent or histochemical staining of the specimen, and more recently by molecular analysis of polymerase chain reaction products comparing DNA samples. Notably, simple molecular detection of P. jirovecii in lung fluids does not mean that a person has PCP or infection by HIV. The fungus appears to be present in healthy individuals in the general population. A blood test to detect β-D-glucan (a part of the cell wall of many different types of fungi) can also help in the diagnosis of PCP.

Prevention In immunocompromised people, prophylaxis with co-trimoxazole (trimethoprim/sulfamethoxazole), atovaquone, or regular pentamidine inhalations may help prevent PCP.

Treatment Antipneumocystic medication is used with concomitant steroids to avoid inflammation, which causes an exacerbation of symptoms about 4 days after treatment begins if steroids are not used. By far, the most commonly used medication is trimethoprim/sulfamethoxazole, but patients are often unable to tolerate this treatment due to adverse reactions, especially if they are HIV positive. Other medications that are used, alone or in combination, include pentamidine, trimetrexate, dapsone, atovaquone, primaquine, pafuramidine maleate (under investigation), and clindamycin. Treatment is usually for a period of about 21 days. Pentamidine is less often used, as its major limitation is the high frequency of side effects. These include acute pancreatic inflammation, kidney failure, liver toxicity, decreased white blood cell count, rash, fever, and low blood sugar.

Epidemiology

Current epidemiology

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