Veterinary Parasitology, 47 (1993) 225-233 225 Elsevier Science Publishers B.V., Amsterdam Ultrastructure of Isospora suis during excystation and attempts to demonstrate extraintestinal stages in mice Rhonda D. Pinckney, David S. Lindsay, Maria A. Toivio-Kinnucan and Byron L. Blagburn Department of Pathobiology, College of Veterinary Medicine, Auburn University, Auburn, AL 36849-5519, USA (Accepted 18 October 1992 ) ABSTRACT Pinckney, R.D., Lindsay, D.S., Toivio-Kinnucan, M.A. and Blagburn, B.L., 1993. Ultrastructure of Isospora suis during excystation and attempts to demonstrate extraintestinal stages in mice. Vet. Parasitol., 47: 225-233. Transmission electron microscopy was used to examine the structure of the oocysts, sporocysts and sporozoites oflsospora 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 °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 micro- scopy. The oocyst wall was composed of three layers. Treatment with 1.5% (v/v) sodium hypochlor- ite 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 spo- rozoites. Tissues from experimentally inoculated outbred Swiss-Webster or inbred BALB/c mice were examined for extraintestinal stages (monozoic cysts) of L suis by immunoperoxidase staining using specific antisera. Extraintestinal stages were not observed in mice, including those given methylpred- nisolone acetate. INTRODUCTION Frenkel (1977) proposed that the species of Isospora infecting dogs and cats be placed in a genus (Cystoisospora) different from the traditional Isos- pora species that are confined to a host's intestinal tract because these para- sites could form extraintestinal stages (monozoic cysts) in both definitive Correspondence to: D.S. Lindsay, Department of Pathobiology, College of Veterinary Medicine, Auburn University, Auburn, AL 36849-5519, USA. © 1993 Elsevier Science Publishers B.V. All rights reserved 0304-4017/93/$06.00