What Is Pneumocystis Pneumonia


Pneumocystis jirovecii is a widely present fungus which causes pneumonia in humans. Diseases other than pulmonary diseases occur occasionally.

Pneumocystis pneumonia (PCP) is a major cause of sickness and death among immunocompromised people (i.e. People who are on cancer treatment or have illnesses that decrease their immune system, like AIDS). In HIV-infected people, it is still the most common AIDS-defining infection. On the contrary, half of the patients with PCP do not have HIV.

It usually causes little to no trouble in people with healthy immune systems. But if your immune system is weakened by HIV/AIDS, cancer, organ transplant, medicines that suppress the immune system or other conditions that cause the immune system to not function properly, you can get afflicted with PCP due to being at higher risk.

Generally speaking, the number of PCP cases has been gradually increasing. There was an increase of around 7% annually every year from 2000 - 2010 in the UK. Over time, data has shown that many of these are part of the HIV-negative population, which includes transplant recipients. Another reason why the HIV-negative population is showing more cases compared to the HIV-positive population is probably because of the introduction of antiretroviral therapy (ART).1,2

Risk factors 

As mentioned previously, people who tend to get infected with PCP are usually people who are at risk. This means that PCP is commonly present in all our bodies but does not usually affect an individual. The risks that cause a suppressed immune system and, in turn, cause the salient condition to become visible are;1,4

  • If a person has HIV/AIDS
  • If a person is an organ transplant recipient
  • If a person is a cancer patient undergoing chemotherapy
  • If a person has an autoimmune disorder and requires immunosuppressive drugs

Etiology and pathogenesis

The fungus that causes PCP may survive in most people's lungs without generating symptoms. According to the Centres for Disease Control and Prevention (CDC), up to 20% of individuals in the United States have the fungus at any given time. After a few months, most people's systems naturally eliminate the fungus.

When people with compromised immune systems get the fungus that causes PCP, they can suffer severe illnesses. This is most typically found in HIV/AIDS patients. HIV/AIDS affects around 30% to 40% of PCP patients.

This organism attaches to the alveolar cells in the lungs and then further transforms from its smaller size to a larger cystic form. This act of attaching to the alveoli leads to one’s body producing an inflammatory response against the organism, which in turn also hurts the lungs and is what causes the main damage.2

Clinical presentation 

Patients presenting with PCP have subtle signs of illness that develop slowly over weeks or even months. This is mostly common for people with AIDS-associated PCP, whereas people not affected with AIDS get ill faster. The most common signs and symptoms to look out for are:1

  • Dry cough
  • Low-grade fever
  • Progressive breathlessness

Whereas there are some other symptoms that present, but are more often noticed by the doctor. These include crackles in your lungs that will be noticed by the doctor, sometimes chest pain, and finally, night sweats and weight loss.

If these symptoms are something that are noticed, and the patient is known to have a condition that suppresses the immune system. They should consult a doctor right away.

Your doctor will then run a series of tests to confirm the diagnosis; these will most likely include:3

  • A chest x-ray /or a CT scan.
  • I will also send a sample of your sputum to the lab to be grown by PCR testing to confirm PCP.
  • A blood test is needed to see if the infection has spread to the blood.
  • Finally, a bronchoscopy might be considered but is not always done unless completely necessary since it is an invasive procedure.


When patients are diagnosed with PCP, they are then started on medication; how and where they receive these medications will depend on their state of health. In milder cases, oral therapies on an outpatient basis are used. If the patient cannot tolerate oral medication and has moderate to severe infection, then they will be given a regimen that will need to be given intravenously in the hospital as an ambulatory service or as an inpatient service.

The most common and effective drug for treating PCP is ‘’Trimethoprim - Sulfamethoxazole’’ (abbreviated as TMP-SMX). This is given as a combination of different doses and formulations until improvement is noticed.

Once a person improves, they are moved on to a course of medication (like Co-trimoxazole) for preventing any future reinfections, i.e. prophylaxis.1 3 4

Prevention and prophylaxis 

If someone has an illness that impairs their immune system, their doctor will check their blood count on a regular basis to see how strong their immune system is. If they have a weakened immune system, their doctor may prescribe medication to prevent PCP before it occurs.

Smokers are also more likely to get PCP. Quitting smoking will improve the health of the person’s lungs. It will also help them avoid lung infections such as PCP.

If someone has a weakened immune system, the best method to avoid PCP is to have frequent blood testing and take preventative medications as needed.

Although flu and pneumococcal immunisations protect against certain forms of pneumonia, they do not protect against PCP. Furthermore, those with compromised immune systems may not be candidates for their usage. Consult a healthcare professional about immunisations and which ones are right for you.1 3


Although most patients recuperate with adequate care, others experience progressive respiratory failure. The death rate among patients requiring critical care or mechanical breathing has been reported to be as high as 60%. According to reports, patients with mild to moderate sickness have a 10% chance of dying, and patients with more severe illness have a probability rate of more than 20%.1

Factors that are associated with poor prognosis, particularly in HIV patients, include:

  •  Increasing age
  • Prior PCP episode
  • The presence of CMV in bronchoalveolar lavage fluid
  • Increased serum lactate dehydrogenase concentration
  • Low CD4 cell count (white cell count)


What are the early symptoms of PCP? 

The earliest symptoms of PCP are: 

  • Cough
  • Fever
  • Difficulty breathing

Is PCP only associated with HIV/AIDS patients?

No, PCP is also associated with other conditions like cancer, transplant recipients and patients taking immunosuppressive therapies for other conditions.

Can PCP be prevented if you are at risk? 

Preventing PCP includes lifestyle habit changes like smoking need to be changed. People who have weak immune systems should get blood tests regularly. And finally, patients who are at risk of PCP will be given prophylactic medication.

What is the recommended treatment duration for PCP? 

Usually, people are treated with medication for up to 21 days and start showing signs of clinical improvement after 4 to 8 days.


Pneumocystis pneumonia (PCP) is a fungal infection of the lungs that can be fatal if not treated quickly. Prior to advancements in HIV therapy, PCP was the top cause of mortality among HIV-positive people in the developed world. PCP symptoms include:

  • A chronic dry cough.
  • Breathing difficulty
  • (in certain circumstances) Fever

Report any PCP symptoms as soon as possible because the illness can deteriorate quickly and without warning. Antibiotics can be used to treat PCP. If your CD4 count falls below 200, you may be prescribed antibiotics to take every day until your CD4 count returns to normal.


  1. Truong J, Ashurst JV. Pneumocystis jirovecii Pneumonia. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Aug 31]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK482370/
  2. Lu JJ, Lee CH. Pneumocystis pneumonia. Journal of the Formosan Medical Association [Internet]. 2008 Nov 1 [cited 2023 Aug 31];107(11):830–42. Available from: https://www.sciencedirect.com/science/article/pii/S0929664608601990
  3. Catherinot E, Lanternier F, Bougnoux ME, Lecuit M, Couderc LJ, Lortholary O. Pneumocystis jirovecii Pneumonia. Infectious Disease Clinics of North America [Internet]. 2010 Mar 1 [cited 2023 Aug 31];24(1):107–38. Available from: https://www.sciencedirect.com/science/article/pii/S0891552009000828
  4. Schmidt JJ, Lueck C, Ziesing S, Stoll M, Haller H, Gottlieb J, et al. Clinical course, treatment and outcome of Pneumocystis pneumonia in immunocompromised adults: a retrospective analysis over 17 years. Crit Care [Internet]. 2018 Nov 19 [cited 2023 Aug 31];22:307. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6245758/
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Shahzaman Ganai

Doctor of Medicine (MD), Medicine, Charles University

Shahzaman is a Junior Doctor currently working in India, over the last year, with future specialist interests in psychiatry. Along with his Interests in medicine, he is an ardent follower of finance, business and health tech news and events. He plans on further enhancing his knowledge in medicine with his interests in business and health tech for future endeavours.

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