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Pneumocystis carinii Pneumonia

Pneumocystis carinii Pneumonia


Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues



Pneumocystis carinii pneumonia (PCP) is the most common opportunistic infection occurring among people with AIDS (acquired immunodeficiency syndrome). Although rates of the infection have fallen with the advent of drugs against HIV, PCP is still a leading cause of mortality in this patient group in the United States.

P. carinii is a fungus that is found in the respiratory tract of humans and other mammals. Its distribution is widespread and most children will have been exposed to it by the age of three or four. However, P. carinii only causes disease among those with impaired immunity. Its appearance among previously healthy homosexual men in the early 1980s was an early warning sign of the emergence of HIV/AIDS. Other groups at risk include those who have cancer or are receiving immunosuppressive drugs following an organ transplant.

Fortunately, PCP is treatable with antibiotics and survival rates have improved in recent years. People at risk can also be given preventive drugs to stop PCP infection taking hold.

Disease History, Characteristics, and Transmission

P. carinii was first thought to be a trypanosome, then a protozoan. More detailed biochemical studies have now established that the organism is a fungus. The organism that actually causes PCP was recently re-named Pneumocystis jiroveci after Otto Jirovec, the researcher who first isolated it from human subjects (although the new name is not yet in common use).

P. carinii is harmless in a healthy person, but if immunity is weakened for any reason, it can invade the lungs, causing pneumonia. The symptoms of PCP may be of gradual onset and include breathlessness, fever, chills, weight loss, a non-productive cough, and weight loss. PCP proves fatal in 10–20% of cases. Survival rates have improved in recent years for those with HIV (human immunodeficiency virus), but not for non-HIV patients.

P. carinii is spread by airborne transmission. Most people have been infected by P. carinii in early childhood. Research suggests that PCP occurs through new infection, by inhalation, rather than by re-activating an old infection.


AIRBORNE TRANSMISSION: Airborne transmission refers to the ability of a disease-causing (pathogenic) microorganism to be spread through the air by droplets expelled during sneezing or coughing.

CD4+ T CELLS: CD4 cells are a type of T cell found in the immune system, which are characterized by the presence of a CD4 antigen protein on their surface. These are the cells most often destroyed as a result of HIV infection.

MORTALITY: Mortality is the condition of being susceptible to death. The term “mortality” comes from the Latin word mors, which means “death.” Mortality can also refer to the rate of deaths caused by an illness or injury, i.e., “Rabies has a high mortality.”

OPPORTUNISTIC INFECTION: An opportunistic infection is so named because it occurs in people whose immune systems are diminished or are not functioning normally; such infections are opportunistic insofar as the infectious agents take advantage of their hosts’ compromised immune systems and invade to cause disease.

Scope and Distribution

PCP was first noted during World War II among malnourished and premature infants in Central and Eastern Europe. Prior to the 1980s, PCP was rare in the United States, with fewer than 100 cases a year occurring— mainly during cancer chemotherapy and after solid organ transplantation. In 1981, the U.S. Centers for Disease Control and Prevention reported an unusual finding—five cases of PCP in previously healthy homosexual men. It was the first warning of the advent of AIDS. Since the causative agent, HIV, attacks and destroys the immune system, P. carinii is able to cause PCP, an opportunistic infection.

However, PCP is less of a problem than it was in the past. Before the introduction of highly active antiretroviral therapy (HAART), 70–80% of those with HIV/AIDS would develop PCP, but these rates have been much reduced. Similarly, 88% of lung transplant recipients developed PCP, but the disease is now rare in this patient group.

It used to be assumed that PCP was less common in the developing world, but this may not be so. Apparently lower rates could merely reflect lack of access to diagnostic facilities. It now appears that PCP could be on the increase in Africa, with the infection affecting around 80% of HIV-positive children who present with pneumonia.

Treatment and Prevention

Although it is a fungus, P. carinii does not actually respond to anti-fungal drugs. However, there are a number of antibiotics that can be used to treat PCP. These include trimethoprim-sulfamethoxazole and pentamidine. Steroids may also be used in severe cases. Prophylactic treatment with these drugs, and with other drugs, can help those at risk avoid developing PCP. Giving up smoking is also essential in helping prevent PCP.

Impacts and Issues

PCP is an ongoing threat to people with HIV/AIDS. It mainly seems to affect those whose CD4+ t-cell count is less than 200 per microliter. CD4+ t-cells are a type of white blood cell that is targeted by the HIV virus. CD4+ t-cell counts are an essential monitor of the condition of someone with HIV and a drop in the count indicates a vulnerability to PCP that should be addressed. Although treatment, both to prevent and cure the infection, is available, PCP is still a significant cause of mortality (death) among those with HIV/AIDS.

See AlsoAIDS (Acquired Immunodeficiency Syndrome); Opportunistic Infection; Pneumonia.



Wilks, Robert, Mark Farrington, and David Rubenstein. The Infectious Diseases Manual. 2nd ed. Malden, UK: Blackwell Publishing, 2003.

Web Sites

McLean, Joseph. “Pneumocystis carinii Pneumonia.” eMedicine, September 11, 2006. <> (accessed April 18, 2007).

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