An Intra-Axonal Protozoon in the Spinal Cord of the Toad Bufo arenarum (Hensel)

SYNOPSIS. A protozoon was found in myelinated axons of the spinal cord and brain of the toad, Bufo arenarum. Examination with the light microscope using Giemsa, Feulgen, PAS and methylene blue technics revealed a primary cell as large as 30 μ in diameter and containing up to 80 nuclei. Electron micrographs showed that the protozoon ranged from 2 μ to 30 μ in diameter and that larger specimens contained numerous secondary cells (2 μ) in addition to multiple nuclei. A few specimens were found in which the secondary cells had long processes with microtubules. Multiple nuclei together with secondary cells suggest that it may be a schizont form of a sporozoon.
The protozoon was found most frequently in axons of the perimedullary plexus just beneath the pia. These axons are without degenerative changes, are up to 3 times the diameter of the largest normal myelinated fibers. The myelin appears normal altho there are fewer laminae than in myelin of other large nerve fibers. The protozoon apparently causes axonal swelling but does not block the fibers completely.
Light microscopic attempts to locate similar forms or other stages in the life cycle by examining blood, skin lesions, spleen, liver, small intestine, dorsal and ventral roots, or sensory ganglia were unsuccessful.
Examination of spinal cords which had been mechanically severed excluded the possibility of confusing the protozoa with multinucleated macrophages. Altho observations do not prove their mode of entrance to the nervous system, the preponderance of protozoa in the peripherally located perimedullary plexus suggests that the path may be by way of the cerebrospinal fluid or along the endoneurium.