Ultrastructure of Isospora suis during excystation and attempts to demonstrate extraintestinal stages in mice

Abstract

Transmission electron microscopy was used to examine the structure of the oocysts, sporocysts and sporozoites of Isospora suis during in vitro excystation. Oocysts were ground in a teflon-coated tissue grinder to free most sporocysts and to allow for exposure of oocysts and sporocysts to excystation medium. The suspension of oocysts and sporocysts was incubated at 37 degrees C for 0-45 min in excystation medium. After incubation, the intact oocysts and sporocysts, excysted sporocysts, and sporozoites in the excystation medium were pelleted by centrifugation and fixed for transmission electron microscopy. The oocyst wall was composed of three layers. Treatment with 1.5% (v/v) sodium hypochlorite solution removed the outer layer. The sporocyst wall was composed of two layers, the inner layer of which was interrupted by sutures. During excystation these sutures separated, allowing release of the sporozoites. Sporozoites were elongate and possessed all of the organelles typical of coccidial sporozoites. Tissues from experimentally inoculated outbred Swiss-Webster or inbred BALB/c mice were examined for extraintestinal stages (monozoic cysts) of I. suis by immunoperoxidase staining using specific antisera. Extraintestinal stages were not observed in mice, including those given methylprednisolone acetate.