Do Cysts of Endamoeba histolytica Formed in the Intestine Excyst Before Evacuation and Cause Internal Autoinfection?

Abstract

The following study was designed to determine whether internal antoinfection from cysts of Endamoeba histolytica occurs. By internal autoinfection is meant the excystation of cysts formed in the intestine and their invasion of the tissues of the host without leaving the body. The term does not refer to “anus to mouth” transmission of cysts nor to reinvasion of the gut by the trophozoites. Sellards and Theiler claim to have infected 6 of 8 kittens by injecting material containing cysts only into the colon following laparotomy. In each case a silk ligature was tied around the large bowel and the inoculum injected above the point of ligation. Hoare exhibited sections of the intestine of a kitten, presumably infected with Endamoeba histolytica by the rectal injection of cysts alone. According to Wenyon, Drbohlav has stated that he was able to confirm these observations. However, these results are open to question, since the information for the most part is incomplete, there was a possibility of trophozoites being in the inoculum or the results were obtained under abnormal conditions. Negative results were obtained by Izar, who, in a single instance, inoculated one adult cat per rectum with a patient's stool containing cysts and presumably no trophozoites. No symptoms developed. Hegner, Johnson and Stabler failed to produce infection in a brown howler monkey after injection of a human strain of cysts per anum into the colon. In the present study 24 healthy young dogs, in addition to controls, were used. All animals were carefully examined before use to insure that none were naturally infected with ameæ. Precautions were taken to eliminate the possibility of the inoculum's reaching the mouth and thus causing infection by the normal route.