The pneumonia rarely, if ever, occurs in healthy individuals. Pneumocystis carinii , an extracellular protozoan, has been observed in three forms. Diagnosis requires identification of P carinii in lung tissue, obtained by invasive techniques, or in lower airway fluids. Experimental studies have shown that the organism can be transmitted by inhalation. Clinical Manifestations Pneumocystis carinii causes bilateral diffuse pneumonitis in immunocompromised patients and no discernible disease in otherwise healthy individuals.
Clinical features are to some extent age-dependent.
In premature and debilitated infants, onset is subtle, starting with mild tachypnea. Within a week or so, respiratory distress is apparent, with marked tachypnea, flaring of the nasal alae, retractions, and cyanosis. The illness may last 4 to 6 weeks and has a mortality rate of 25 to 50 percent. In the immunodeficient child or adult onset is abrupt, with fever, tachypnea, and respiratory distress. Deterioration progresses to death in almost all cases if no treatment is given.