The organism was long thought of as a protozoan but has been reclassified as a yeast.
Pneumocystis infection is an opportunistic infection and so is generally only seen in patients who are immunosuppressed, such as those with HIV/AIDS or on chemotherapy.
Fever is a classic initial presentation of the disease.
Hypoxemia is a common symptom of the disease. Pneumocystis invades the lungs causing a severe pneumonia with bilateral infiltrates. This leads to decreased O2 saturations.
Bronchoalveolar lavage is a medical procedure in which a bronchoscope is used to squirt fluid into a small part of the lung which is then recollected for examination. This allows Pneumocystis organisms from the lung to be collected for definitive diagnosis.
Biopsy is one of the ways to make a definitive diagnosis of this infection. Pneumocystis can be stained by methenamine silver, so using this stain on the biopsied tissue can reveal the infection.
Methenamine silver stain is used to stain Pneumocystis. Samples of affected tissue, such as lung tissue from a biopsy, can be stained to reveal the pathogen.
On microscopy, saucer or cup-shaped forms are the classic description of the yeast form of Pneumocystis jirovecii.
Pneumocystis pneumonia characteristically appears on chest X-ray (CXR) as a diffuse interstitial pneumonia. Patients have diffuse infiltrates throughout both lung fields in an interstitial pattern. A ground glass pattern is often seen especially on CT scans. Pneumatoceles can also occur.
Trimethoprim/sulfamethoxazole (TMP-SMX) is a sulfonamide antibiotic that disturbs folate metabolism in microorganisms. It is used both as treatment for Pneumocystis as well as for prophylaxis against the disease. Prophylaxis is indicated for example in HIV patients with CD4+ count below 200.
HIV patients should begin taking TMP-SMX prophylactically to prevent this disease when CD4+ cell counts drop below 200. This is because CD4+ cells (helper T cells) are crucial to normal immune system functioning, so when their numbers are depleted, patients are susceptible to opportunistic infections.
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