ORIGINAL PAPER Hany M. Elsheikha Æ Benjamin M. Rosenthal Linda S. Mansfield Dexamethasone treatment induces susceptibility of outbred Webster mice to experimental infection with Besnoitia darlingi isolated from opossums (Didelphis virginiana) Received: 8 June 2004 / Accepted: 23 November 2004 / Published online: 10 March 2005 Ó Springer-Verlag 2005 Abstract The Sarcocystidae comprise a diverse, mono- phyletic apicomplexan parasite family, most of whose members form intracellular cysts in their intermediate hosts. The extent of pathology associated with such cyst formation can range widely. We currently lack experi- mental animal models for many of these infections. Here we explored dexamethasone treatment as a means to render outbred mice susceptible to Besnoitia darlingi infection and demonstrated that this approach allows viable parasites to be subsequently isolated from these mice and maintained in tissue culture. Besnoitia brad- yzoites recovered from crushed cysts derived from nat- urally infected opossums (Didelphis virginiana) replicated and reproduced the development of besnoiti- osis in mice treated with dexamethasone (0.5 mg/ml drinking water) daily for 12 days post infection (DPI). Isolates recovered from the peritoneal exudates of these mice were viable and were maintained in long-term tis- sue cultures. In contrast, control mice given saline without dexamethasone and challenged with similar bradyzoites remained clinically normal for up to 70 DPI. An additional group of mice challenged with the same inoculum of bradyzoites and given dexameth- asone at the same concentration and treated with sul- fadiazine (1 mg/ml drinking water) daily for 12 DPI also remained normal for up to 70 DPI. Severe disease developed more rapidly in dexamethasone-treated mice inoculated with culture-derived B. darlingi tachyzoites than in those inoculated with cyst-derived bradyzoites. B. darlingi tachyzoite-infected, untreated control mice developed signs of illness at 18 DPI. In contrast, mice treated with sulfadiazine showed no clinical signs up to 50 DPI. Although dexamethasone treatment was re- quired to establish B. darlingi infection in outbred mice inoculated with opossum-derived B. darlingi bradyzo- ites, no such treatment was required for mice inoculated with culture-derived B. darlingi tachyzoites. Finally, sulfadiazine was highly effective in protecting mice from infection with the tachyzoite stage of B. darlingi. Introduction Apicomplexan parasites belonging to the coccidian genus Besnoitia are prevalent in some domestic and wild animals and can cause the clinical disease termed bes- noitiosis. Besnoitia darlingi is a heteroxenous coccidian employing opossums (Didelphis virginiana) as the inter- mediate host and cats as the definitive host (Dubey et al. 2002). Opossums become infected with B. darlingi by ingesting infected tissues containing cysts via carnivo- rism, by ingesting food or water contaminated with oocysts excreted by infected cats, or by experimental inoculation of tachyzoites (Smith and Frenkel 1977). In the United States, B. darlingi has been reported from opossums from many regions (Elsheikha et al. 2003). H. M. Elsheikha Æ L. S. Mansfield Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824, USA B. M. Rosenthal Animal Diseases Parasite Laboratory, Agricultural Research Service, United States Department of Agriculture, Agricultural Research Center, Animal and Natural Resources Institute, Building 1080, BARC-East, Beltsville, MD 20705-2350, USA L. S. Mansfield Department of Microbiology and Molecular Genetics, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824, USA L. S. Mansfield (&) Michigan State University, B43 Food Safety and Toxicology Bldg., East Lansing, MI 48824, USA E-mail: mansfie4@cvm.msu.edu Tel.: +1-517-4323100 Fax: +1-517-4322310 Parasitol Res (2005) 95: 413–419 DOI 10.1007/s00436-004-1286-2