The Sarcocystidae comprise a diverse, monophyletic 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 experimental 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 bradyzoites recovered from crushed
cysts derived from naturally infected opossums (Didelphis virginiana) replicated and reproduced the development of
besnoitiosis 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 tissue 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
dexamethasone at the same concentration and treated with
sulfadiazine (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 required to establish B. darlingi
infection in outbred mice inoculated with opossum-derived B. darlingi bradyzoites, 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.