The finding of several infections of Balantidium coli in man (Young, 1939) afforded the opportunity of attempting some transmission experiments. Such experiments appeared justified because of the lack of information on the transmission of this parasite.
The sources of the balantidia were several very active human infections. Fecal material containing trophozoites and cysts was put into gelatin capsules and swallowed by the volunteers. The trophozoites were motile. The cysts were viable as indicated by movements of the encysted parasites.
The pertinent data are shown in Table 1.
Volunteer My received a small number of cysts and did not develop an infection during a 40-day observation period. Transmission was again attempted in this volunteer as well as in a new volunteer, Jk. On this trial approximately 250 cysts and 250 trophozoites were given each volunteer. During the following observation period of more than 10 years, no B. coli parasites appeared in the stools of either person.
Laboratory of Tropical Diseases, National Institutes of Health, U. S. Public Health Service, P. O. Box 717, Columbia, S. C.