Pneumocystis pneumonia is a disease caused by a microorganism called Pneumocystis jiroveci. It has been reclassified in recent years as a fungus due to its characteristic nucleotide sequences and cell wall composition. This type of pneumonia is mainly seen in immunocompromised patients, such as those with acquired immunodeficiency disorder (or AIDS).
Smear of Pneumocystis jirovecii, formerly known as Pneumocystis carinii, concentrated from human lung. Toluidine blue staining. Pneumocystis jirovecii cysts, concentrated from human lung. Toluidine blue staining. Image Credit: CDC
Recent advances in the prophylaxis, diagnosis and treatment of Pneumocystis pneumonia resulted in decreased morbidity and associated mortality. However, the disease continues to be an important clinical entity in patients with severe immunosuppression or whose adherence to specific prophylactic regimens is inadequate.
Pathogenesis of the disease
Pneumocystis jiroveci reproduces in association with the type 1 alveolar cell lining the lung tissue. For this reason, the active disease is limited to the lungs. If the ensuing disease process is not limited by the host’s immune activity or appropriate antimicrobial therapy, the affected alveoli will eventually be filled with this microorganism, as well as protein fluid.
The classic lesion of Pneumocystis pneumonia is therefore an interstitial infiltrate of plasma cells and lymphocytes, as well as hyperplasia of specific pneumocytes. In addition, the alveoli are filled with a characteristic foamy exudate, while the organisms appear as tiny bubbles in a background of protein fluid.
Such progressive filling of the alveoli prevents adequate gas exchange, so that the patient suffocates slowly. It can also lead to thickening of the interstitium of the lungs and potentially to pulmonary fibrosis. Due to this mechanism of pathogenesis and its unique life cycle, Pneumocystis jiroveci cannot be grown routinely, but instead is diagnosed by specific staining showing the trophozoite or cyst wall.
Clinical presentation of Pneumocystis pneumonia
The classic triad of symptoms associated with Pneumocystis pneumonia includes fever, non-productive cough, and progressive dyspnea on effort. These are only noted about 50 percent of the time. In other words, the signs and symptoms of the disease can be very subtle. This means that in order to make a correct diagnosis it is necessary to have a high index of clinical suspicion and increased awareness. This is all the more true as the risk factors for HIV and other immunosuppression situations may be denied or unknown.
Other symptoms associated with this type of pneumonia may include chills, weight loss, nonspecific chest discomfort. In rare cases, the patient may have hemoptysis (lood’s cough). If present, these symptoms usually show slow progress over weeks or months. A careful history often reveals progressive and prolonged dyspnea on exertion.
The physical signs on examination are also non-specific. During auscultation, fine crackles and dry rales are sometimes heard, but normal results are seen in up to half of affected individuals. In children with severe clinical presentations, findings may include nasal enlargement, cyanosis, and retraction (inward movement) of the rib muscles.
Extrapulmonary diseases caused by Pneumocystis jiroveci are rarely seen, but have been described in the medical literature. They occur mostly in patients with advanced HIV infection without prophylaxis. They are known to involve organs such as the skin, spleen, heart, eyes, or thyroid gland. Such presentations can occur even in the absence of lung disease.
A serious potential complication of Pneumocystis pneumonia is a lung attack akin to acute respiratory distress syndrome (ARDS) that may require intubation. This rarely happens and leaves physicians with the dilemma of choosing between differential diagnoses of Pneumocystis pneumonia, classical bacterial pneumonia, tuberculosis of the lungs, as well as a myriad of other respiratory disorders.