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.
In both types of patient, arterial oxygen tension is low, arterial pH usually increased, and carbon dioxide retention usually does not occur. The trophozoite is a thin-walled extracystic cell representing an excysted sporozoite. Although the mode of replication has not been described, the organism can be briefly propagated in embryonic thick epithelial lung cells, Vero cells, and WI-38 cells. The organism does not enter the host cell, but instead attaches to its surface during a phase in the replicative cycle. There is no evidence of toxin production.