Blastocystis hominis: pathogenic potential in human patients and in gnotobiotes.

Germfree guinea pigs were inoculated orally, in some experiments, and intracecally, in others, with Blastocystis hominis and the enteric flora from symptomatic patients. Other germfree guinea pigs received the parasite from axenic culture and still others from monoxenic culture with Proteus vulgaris. Fourteen of 43 animals inoculated orally with B. hominis and patient's enteric flora developed B. hominis infections and those with particularly heavy infections developed watery diarrhea of more than 1 week's duration immediately prior to sacrifice. Similar results were obtained from intracecal inoculations in that 13 of 28 animals developed infections and those with the greatest numbers of B. hominis had watery diarrhea for more than 1 week prior to sacrifice. Gross pathologic changes in these animals were mostly unremarkable, with only a slight hyperemia observed in several of the symptomatic animals. Microscopic examination, however, revealed frequent penetration of intestinal epithelium by B. hominis and the parasites in significant numbers were observed within the epithelium. There was a slight increase in cellularity in the lamina propria but parasites were not observed therein and their presence in the epithelium did not provoke an inflammatory response. Only one of eight animals inoculated from monoxenic cultures developed B. hominis infection (asymptomatic), and infections were not produced in animals inoculated from axenic culture. As a result of our observations of diarrhea in patients with particularly heavy infections with B. hominis together with the demonstration of similar symptoms in animals heavily infected with this parasite, we believe B. hominis may occasionally be related causally to the production of such symptoms by a mechanism not completely understandable.