Morphologic resemblance of zygomycete spores to Pneumocystis carinii cysts in tissue.

Structures resembling the cyst form of Pneumocystis carinii were noted in the lungs of alloxan-diabetic rabbits to whom spores of several species of fungi belonging to the class zygomycetes had been administered by intranasal instillation. At 24 to 168 hours after the instillation of spores, these rabbits succumbed to diabetes or zygomycete infection, or both, and the possibility of a mixed infection with both fungus and P. carinii could not be ruled out on morphologic criteria alone. To resolve the dilemma, rabbit lung tissue was examined by the fluorescent antibody procedure with labeled P. carinii antibodies. The labeled antibodies did not stain structures that resembled P. carinii but stained P. carinii cells in control rat lung infected with this organism. Investigators should be aware of the difficulty that may arise in differentiating the cyst form of P. carinii from spores of some fungi. The direct fluorescent antibody procedure for P. carinii appears to be very useful for differentiating P. carinii cysts from fungal spores.