Eighty-one experimental Entamoeba histolytica infections were studied in human volunteers at a state penitentiary. Inoculation of 2,000 to 4,000 cysts derived directly from a non-symptomatic “carrier” or after one or two passages through volunteers, produced infections in 42 of 42 individuals not medicated or previously infected; heavier inocula did so in 4 of 4 others.
Prepatent periods, based on zinc sulfate flotation and saline smear fecal examinations, varied from 2 days to 4 months. In all but 2 of 48 individuals the stools were positive within 2 weeks; 68.7 per cent were positive in 7 days or less. An extraordinarily long prepatency of 4 months in one case was associated with abnormally pale, mucoid stools.
From 33 individuals followed for more than 1 month after infection, 86 per cent of the stools examined were positive. In one there were 3 periods of 4 to 7 weeks when amebae could not be demonstrated. Among 23 infected individuals observed for 9 to 14 months there were 7 who apparently lost their infections spontaneously after 3 weeks to 8 months. One of 3 individuals who had spontaneously lost infections was refractory to reinfection when given two inoculations of 4,000 cysts.
None of the volunteers developed symptoms that could be associated with the amebic infections. However, the strain of amebae used in the human experiments could not be regarded as non-pathogenic, since it produced typical amebic lesions in dogs, guinea pigs and rats, and was as pathogenic in guinea pigs and rats as a recently isolated strain from a frankly dysenteric patient.
Address Medical Dept., State Penitentiary, Parchman, Miss.