Pneumocystis pneumonia (PCP) is a lung infection caused by the fungus Pneumocystis jirovecii. This fungus is common worldwide, and most people develop immunity during childhood without illness. As an opportunistic infection, it takes advantage of a weakened immune system to cause disease. It primarily affects individuals with compromised immune systems, leading to inflammation and fluid buildup in the lungs.
Causes and High-Risk Populations
Pneumocystis pneumonia develops from the reactivation of a latent Pneumocystis jirovecii infection when the body’s defenses can no longer keep the fungus in check. The condition is seen almost exclusively in specific, vulnerable populations.
A primary group at risk includes people with HIV, particularly when their CD4+ T cell count falls below 200 cells per microliter of blood. Before effective antiretroviral therapy, PCP was a common and often fatal complication of HIV.
Cancer patients are another high-risk population, especially those with hematologic cancers or undergoing chemotherapy. These treatments suppress the immune system, creating an opportunity for the fungus to multiply.
Organ transplant recipients are also at high risk, as they must take immunosuppressive drugs to prevent organ rejection. Individuals on long-term, high-dose corticosteroids for autoimmune conditions like lupus or rheumatoid arthritis also face an increased risk.
Recognizing the Symptoms
The onset of Pneumocystis pneumonia is slow, with symptoms developing gradually over several weeks. Unlike the sudden illness associated with bacterial pneumonia, PCP’s initial signs can be mild and easily dismissed.
The most characteristic symptom is progressive shortness of breath (dyspnea). Initially, this may only be noticeable during physical activity, but it can occur even at rest as the infection worsens. This is accompanied by a persistent dry, non-productive cough and a low-grade fever.
As the infection advances, patients often report chest tightness, fatigue, chills, and unexplained weight loss. These symptoms are a direct result of the lungs filling with fluid and inflammatory cells, which impairs their ability to transfer oxygen into the bloodstream.
The Diagnostic Process
Diagnosis begins with a physical examination, where a provider listens to the lungs and may use a pulse oximeter to measure blood oxygen saturation. A low oxygen level can be an early indicator of PCP, even if a chest X-ray appears normal.
Imaging tests are a key part of the diagnostic workup. A chest X-ray often reveals diffuse, bilateral infiltrates, which look like hazy areas spreading from the center of the chest. If the X-ray appears normal, a more sensitive computed tomography (CT) scan may be ordered to show faint, ground-glass opacities suggestive of PCP.
While symptoms and imaging provide clues, a definitive diagnosis requires identifying the Pneumocystis jirovecii fungus in a lung fluid sample. A common method is induced sputum, where the patient inhales a sterile saline mist to help them cough up fluid from deep within their lungs.
If an induced sputum sample is negative, a bronchoscopy is performed. During this procedure, a thin tube is passed into the airways to perform a bronchoalveolar lavage (BAL). A small amount of sterile fluid is washed into a lung section and then suctioned out, collecting cells and organisms for laboratory analysis.
Treatment and Management
The standard treatment for an active PCP infection is a 21-day course of antibiotics. The primary medication is trimethoprim and sulfamethoxazole (TMP-SMX). For moderate to severe pneumonia with low oxygen levels, this is administered intravenously in a hospital. As the patient’s condition improves, they can switch to an oral form to complete the course.
For individuals with a sulfa allergy or who experience side effects from TMP-SMX, alternative medications are available. These include clindamycin with primaquine, atovaquone, or intravenous pentamidine. Pentamidine use can be limited by potential toxic effects on the kidneys and blood sugar.
Corticosteroids, such as prednisone, are a component of managing moderate to severe PCP. These anti-inflammatory drugs are given to patients with significant hypoxemia (a low level of oxygen in the blood). The fungus causes an intense inflammatory response in the lungs that can worsen when antibiotic treatment begins. Corticosteroids help reduce this inflammation, which can prevent further lung damage and improve the illness’s outcome.
Supportive care is also part of managing the illness. Because PCP impacts lung function, many patients require oxygen therapy to maintain adequate blood oxygen levels. This can range from a nasal cannula to more intensive respiratory support.
Preventative Measures
Preventing Pneumocystis pneumonia, known as prophylaxis, is a standard part of care for individuals at high risk. The goal is to stop the infection from developing. This proactive approach is for people with severely weakened immune systems who are vulnerable to this opportunistic fungus.
The decision to start preventative medication is based on clinical markers. For people with HIV, prophylaxis is recommended when their CD4 cell count drops below 200 cells/mm³. Preventative therapy is also prescribed for organ transplant recipients, patients undergoing certain chemotherapies, and individuals on long-term, high-dose corticosteroids.
The medication most commonly used for prevention is the same one used for treatment: trimethoprim-sulfamethoxazole (TMP-SMX). For prophylaxis, it is given at a much lower dose, typically one pill taken daily or three times a week. This lower dose is effective at suppressing the fungus and preventing an active infection. For patients unable to tolerate TMP-SMX, alternatives like dapsone or atovaquone are available.