Classification and Antigenic Types Antigenic differences have been demonstrated between organisms obtained from humans and from lower mammals such as the rat, rabbit, and ferret. The taxonomy of P carinii has not been established. It is either a protozoan or a fungus, although membership in a heretofore undescribed category cannot be excluded. Recent studies show rRNA sequences, thymidylate synthase, dihydrofolate reductase, beta tubulin, mitochondrial DNA and chitin in the cell wall of P carinii more closely resemble fungi than protozoa. The mode of replication of P carinii has not been established.
Sporozoites excyst through breaks in the cyst wall and then are termed trophozoites.
The means by which the trophozoite form progresses to the cyst phase is not known. Airborne transmission has been demonstrated in animals. In debilitated infants with Pneumocystis pneumonia, the alveolar septum is thickened and there is an interstitial plasma cell and lymphocyte infiltration. The infection results in impaired ventilation and severe hypoxia. Severe protein-calorie malnutrition alone may provoke the disease.