Pneumocystis carinii pneumonia (PCP)

Date: November 1, 1994
Source: National Institutes of Health (NIH)
Author: National Institute of Allergy and Infectious Diseases (NIAID)

Pneumocystis carinii pneumonia (PCP) is a life-threatening lung infection that can affect people with weakened immune systems, such as those infected with HIV, the virus that causes AIDS. More than three-quarters of all people with HIV disease will develop PCP if they do not receive treatment to prevent it.

PCP is caused by a tiny parasite. In addition to the lungs, the parasite can infect the eyes, ears, skin, liver and other organs. The organism probably infects most people during childhood, but it usually does not cause illness in healthy people. Because the parasite remains in the body for life, it can cause disease at any time if the immune system becomes severely damaged, as in HIV infection, or is suppressed by drugs. People with HIV infection are particularly prone to PCP when their CD4+ T-cell levels fall below 200. CD4+ T cells (also called T4 cells or T helper cells) are important immune system cells targeted by HIV.

Symptoms and Diagnosis

The respiratory symptoms of PCP include a dry cough, chest tightness and chest tightness and difficulty breathing. People with this infection can experience fever, fatigue and weight loss for weeks or even months before having any respiratory symptoms. If a person with PCP is not treated, the infection can seriously impair the lungs' ability to transport oxygen from inhaled air into the blood, which can lead to death. People with respiratory symptoms usually undergo a chest x-ray to determine if signs of pneumonia are present. Doctors diagnose PCP itself by detecting the organism in sputum or in fluid removed from the lung by bronchoscopy, a procedure in which a tiny tube is threaded through the patient's airways. Rarely, doctors may have to surgically remove a sample of lung tissue for examination.


Acute. The standard treatment for people with PCP is either a combination of trimethoprim and sulfamethoxazole (TMP/SMX, also called Bactrim or Septra), or pentamidine. Both treatments are highly effective and their widespread use for both the treatment and prevention of PCP has made the likelihood of dying from a PCP infection less than ten percent.

Patients take TMP/SMX orally or through a vein for at least three weeks. More than half of the patients who receive the combination drug experience such side effects as skin rashes, a decrease in the number of red or white blood cells, nausea, vomiting or kidney impairment. Some of these side effects may be severe enough to discontinue treatment.

Pentamidine is given intravenously and, like TMP/SMX, is likely to prompt side effects in more than half the patient who receive it. These side effects include low blood sugar, low blood pressure, a depletion of red or white blood cells and inflammation of the pancreas. Most of these side effects stop once patients discontinue the drug, although pentamidine can permanently damage the pancreas.

Pentamidine and TMP/SMX are equally effective in the treatment of people with PCP, but TMP/SMX is generally preferred because it tends to cause less severe side effects. If the side effects of either treatment are intolerable, or if patients fail to improve within a reasonable time, doctors may treat them with other drugs such as atovaquone, dapsone or trimetrexate.

Maintenance. Most people with AIDS will experience another bout with PCP if they do not continue to take medication to prevent its recurrence. This is called maintenance therapy. Doctors recommend that people with AIDS who have recovered from PCP take oral TMP/SMX daily for the rest of their lives, at a lower dose than that prescribed for treatment. For people who cannot tolerate TMP/SMX, an aerosolized form of pentamidine inhaled into the lungs monthly is effective in preventing PCP recurrence. People who cannot tolerate the side effects of either TMP/SMX or aerosolized pentamidine may be given other drugs whose effectiveness at preventing PCP and side effects are not well known. These drugs include dapsone, pyrimethamine-sulfadoxine (Fansidar) or intravenous pentamidine.

Preventive. Adults whose HIV infections have substantially damaged their immune systems, as indicated by CD4+ T -cell counts of 200 or lower should receive medication to prevent PCP. A recent clinical study found that TMP/SMX, dapsone and aerosolized pentamidine were equally effective in delaying a first episode of PCP. However, for patients with fewer that 100 CD4+ T cells, TMP/SMX and dapsone were substantially more effective than aerosolized pentamidine in delaying an initial bout of PCP. Because young children normally have larger numbers of CD4+ T cells circulating in their blood than do adults, the levels of T cells used to determine when to treat children with HIV infection to prevent PCP vary with the age of the child.


The National Institute of Allergy and Infectious Diseases (NIAID) funds research aimed at finding new drugs or drug combinations for the treatment or prevention of PCP, as well as better ways to administer currently available drugs. Several new therapies for PCP are currently being evaluated for safety or effectiveness in NIAID-sponsored clinical trials.

For more information on these or other studies, call the AIDS Clinical Trials Information Service: 1-800-TRIALS-A (1-800-874-2572)

NIAID, a component of the National Institutes of Health, supports research on AIDS and other infectious diseases as well as allergies and immunology. NIH is an agency of the Public Health Service, U.S. Department of Health and Human Services.

Prepared by:
Office of Communications National Institute of Allergy and Infectious Diseases National Institutes of Health Bethesda, Maryland 20892

Public Health Service U.S. Department of Health and Human Services November 1994