Description

Opportunistic lung infection caused by Pneumocystis jiroveci (PJ). This organism is currently considered a primitive fungus based on DNA sequence analysis. It has two developmental forms (the cysts contain sporozoites that become trophozoites when excised).

• Although previously known as Pneumocystis carinii pneumonia (PCP), the acronym PCP is still in use and refers to Pneumocystis pneumonia.
• PCP occurs almost exclusively in the immunocompromised host.
• PCP is an AIDS-defining illness. It is the most common opportunistic life-threatening lung infection in infants with perinatally acquired human immunodeficiency virus (HIV) disease.
• PJ causes a diffuse pneumonitis characterized by fever, dyspnea at rest, tachypnea, hypoxemia, nonproductive cough, and bilateral diffuse infiltrates in the roentgenogram. It is a severe condition frequently leading to respiratory failure, necessitating intubation and mechanical ventilation.
• Chemoprophylaxis against this microorganism has proven successful. Therefore, early identification of the HIV-infected mother becomes essential.
• Despite advances in therapy, the infection continues to be associated with significant morbidity and mortality.

Epidemiology

• Ubiquitous in mammals worldwide, particularly rodents
• Growth on respiratory tract surfaces
• Mode of transmission is unknown:
  • Airborne person-to-person transmission is possible, but case contacts are rarely identified.
  • Environmentally acquired
• Asymptomatic infection appears early in life; >70% of healthy individuals have antibodies by age 4 years.
• Primary infection is likely to be the mechanism in infants. Reactivation of latent disease with immunosuppression was proposed as an explanation for disease later in childhood; however, animal models of PCP do not support this proposition.
• PCP in the HIV patient can occur at any time but usually presents during the 1st year of life. The highest incidence is between 3 and 6 months of age.

Risk Factors

• Immunocompromised host
  • Children with congenital or acquired immunodeficiency syndrome (AIDS) and recipients of suppressive therapy in the treatment of malignancies or after organ transplantation are at high risk.
  • In leukemic patients, the incidence of PCP has been directly related to the degree of immunodeficiency resulting from chemotherapy.
  • Epidemics of PCP were reported in premature and malnourished infants and children in resource-limited countries and during times of famine.

Pathophysiology

• In the immunodeficient child, the pathologic changes occur predominantly in the alveoli. Cysts and trophozoites are seen adhering to the alveolar lining cells or in the cytoplasm of macrophages.
• As infection progresses, the alveolar spaces are filled with a pink, foamy exudate containing fibrin, abundant desquamative cells, and a large number of organisms. Alveolar septal thickening with mononuclear cell infiltration is also seen.