Pneumocystis pneumonia

Pneumocystis pneumonia is a lung infection that occurs primarily in people with weakened immune systems especially people who are HIV-positive. The disease agent is an organism whose biological classification is still uncertain. Pneumocystis carinii was originally thought to be a one-celled organism(a protozoan), but more recent research suggests that it is a fungus. Although its life cycle is known to have three stages, its method of reproduction isnot yet completely understood. The complete name of the disease is Pneumocystis carinii pneumonia (PCP) and is also sometimes called pneumocystosis.

Pneumonia as a general term refers to a severe lung inflammation. In pneumocystis pneumonia, this inflammation is caused by the growth of Pneumocystiscarinii, a fungus-like organism that is widespread in the environment. PCP is ordinarily a rare disease, affecting only people with weakened immune systems. Many of these people are patients receiving drugs for organ transplants or cancer treatment. With the rising incidence of AIDS, however, PCP has become primarily associated with AIDS patients. In fact, as many as 75% of AIDSpatients have developed PCP. It has also been the leading cause of death inAIDS patients.

The organism that causes PCP is widely distributed in nature and is transmitted through the air. When the organism is inhaled, it enters the upper respiratory tract and infects the tiny air sacs at the ends of the smaller air tubes(bronchioles) in the lungs. These tiny air sacs are called alveoli. Under amicroscope, alveoli look like groups of hollow spheres resembling grape clusters. The exchange of oxygen with the blood takes place in the alveoli. It appears that P. carinii lives in the fluid in the lining of the alveoli.

Person-to-person infection does not appear to be very common; however, clusters of PCP outbreaks in hospitals and groups of immunocompromised people indicate that patients with active PCP should not be exposed to others with weakened immune systems. It is thought that many people actually acquire mild Pneumocystis carinii infections from time to time, but are protected by their immune systems from developing a full-blown case of the disease.

P. carinii is an opportunistic organism. This means that it causes disease only under certain conditions, as when a person is immunocompromised. Under these circumstances, P. carinii can multiply and cause pneumonia.The mechanisms of the organism's growth within the alveoli are not fully understood. As the pneumocystis organism continues to replicate, it gradually fills the alveoli. As the pneumonia becomes more severe, fluid accumulates and tissue scarring occurs. These changes result in decreased respiratory functionand lower levels of oxygen in the blood.

Some patients are at greater risk of developing PCP. These high-risk groups include:

  • Premature infants
  • Patients with immunodeficiency diseases, including severe combined immunodeficiency disease (SCID) and acquired immunodeficiency syndrome (AIDS).
  • Patients receiving immunosuppressive drugs, especially cortisone-like drugs (corticosteroids)
  • Patients suffering from protein malnutrition.

AIDS is currently the most common risk factor for PCP in the United States. PCP is, however, also found in countries with widespread hunger and poor hygiene.

The incubation period of PCP is not definitely known, but is thought to be between four and eight weeks. The major symptoms include shortness of breath, fever, and a nonproductive cough. Less common symptoms include production of sputum, blood in the sputum, difficulty breathing, and chest pain. Most patients will have symptoms for one to two weeks before seeing a physician. Occasionally, the disease will spread outside of the lung to other organs, includingthe lymph nodes, spleen, liver, or bone marrow.

The diagnosis of PCP begins with a thorough physical examination and blood tests. Although imaging studies are helpful in identifying abnormal areas in the lungs, the diagnosis of PCP must be confirmed by microscopic identificationof the organism in the lung. Samples may be taken from the patient's sputum,or may be obtained via bronchoscopy or lung biopsy. Because of the severityof the disease, many physicians will proceed to treat patients with symptomsof pneumocystis pneumonia if they belong to a high-risk group, without the formality of an actual diagnosis. The severity of PCP can be measured by x-raystudies and by determining the amount of oxygen and carbon dioxide present inthe patient's blood.

Treatment for PCP involves the use of antibiotics. These include trimethoprim-sulfamethoxazole (TMP-SMX, Bactrim, Septra) and pentamidine isoethionate (Nebupent, Pentam 300). Both of these anti-microbial drugs are equally effective. AIDS patients are typically treated for 21 days, whereas non-AIDS patientsare treated for 14 days. TMP-SMX may be highly toxic in AIDS patients, causing severe side effects that include fever, rash, decreased numbers of white blood cells and platelets, and hepatitis. Pentamidine also causes side effectsin immunocompromised patients. These side effects include decreased blood pressure, irregular heart beats, the accumulation of nitrogenous waste productsin the blood, and electrolyte imbalances. Pentamidine can be given in aerosolform to minimize side effects. Alternative drugs can be used for patients experiencing these side effects.

P. carinii appears to be developing resistance to TMP-SMX. In addition, some patients are allergic to the standard antibiotics given for PCP. As aresult, other antibiotics for the treatment of PCP are continually under investigation. Some drugs proven to be effective against P. carinii include dapsone (DDS) with trimethoprim (Trimpex), clindamycin (Cleocin) with primaquine, as well as atovaquone (Mepron). Paradoxically, corticosteroids have been found to improve the ability of TMP-SMX or pentamidine to treat PCP.

If left untreated, PCP will cause breathing difficulties that will eventuallycause death. The prognosis for this disease depends on the amount of damageto the patient's lungs prior to treatment. Prognosis is usually better at a facility that specializes in caring for AIDS patients. Antibiotic treatment ofPCP is about 80% effective.

Patients who have previously had PCP often experience a recurrence. Healthy lifestyle choices, including exercising, eating well, and giving up smoking may keep the disease at bay.

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