Mouse-to-Mouse Transmission of Caryospora simplex (Apicomplexa: Eimeriidae)

Abstract

Caryospora simplex Leger, 1904 (Apicomplexa: Eimeriidae) is a heteroxenous coccidium that infects the intestinal epithelium of viperid snakes (Leger, 1911, Archiv fur Protistenkunde 22: 71-88) and, experimentally, dermis of rodents (Upton et al., 1984, Journal of Protozoology 31: 392-398). When mice ingest sporulated oocysts of C. simplex of snake origin merogony, gamogony, and sporogony occur in fibroblastlike cells of the facial and tongue dermis. Unlike snake-derived oocysts, however, oocysts developing in the dermis of mice possess a much thinner wall and lack a distinct sporocyst wall, Stieda, or substieda bodies (Upton et al., 1984, loc. cit.). Sporozoites appear to exit from these thin-walled oocysts on days 12-14 post-inoculation (PI), enter macrophage-like cells within the dermis and remain dormant as a cystic stage that is presumed to be infective for the snake host. These monozoic cysts of Caryospora spp. have been termed caryocysts (Wacha and Christiansen, 1982, Journal of Protozoology 29: 272-278). The present study was undertaken to determine if caryocysts of C. simplex are infective to mice and to determine if cannibalism may play a role in the transmission and maintenance of the parasite within the rodent population. Three separate groups of 4-6 wk old, female, coccidiafree Swiss-Webster mice were used for the experiment. All mice were housed in groups of 2 in metal bottom cages, with experimentally infected animals maintained separately from controls. Bedding consisted of wood shavings and was changed at 48 hr intervals. All animals were given commercial rodent chow and water ad lib. Group A consisted of 8 mice inoculated by stomach tube with 250,000 sporulated oocysts obtained from the feces of an Ottoman viper, Vipera x. xanthina (see Upton et al., 1984, loc. cit.) and 2 uninoculated controls. Two inoculated mice were killed by cervical dislocation on days 10, 15, 18, and 25 PI and portions of the cheek dermis were examined for developmental stages of C. simplex by Nomarski interference contrast (NIC) microscopy. Examination of tissues at these intervals insured that parasite stages were developing at the same intervals as reported previously (see Upton et al., 1984, loc. cit.). Control mice killed on day 25 PI were examined similarly. Portions of cheek dermis from each of the 2 inoculated mice killed on day 25 PI were minced and equal portions were fed separately to each of 2 coccidia-free mice (Group B). Two additional coccidia-free mice in Group B received similar portions of cheek dermis from the 2 uninoculated control mice in Group A. All mice in group B were killed on day 25 post-feeding and examined for the presence of parasites as described above. Equal portions of cheek dermis from infected mice in Group B were then fed separately to 2 coccidia-free mice (Group C). Two additional mice in Group C were fed similar portions of cheek dermis from the 2 control mice in Group B. All 4 mice in Group C were examined for the presence of parasites 25 days after being fed infected or uninfected cheek tissues, as described above. Mice in Group A inoculated with oocysts from the snake had developmental stages of C. simplex on days 10, 15, 18, and 25 PI similar to those reported by Upton et al. (1984, loc. cit.). Caryocysts on day 25 PI (Fig. 1) appeared viable. No parasite stages were found in the 2 control mice of Group A. Both mice in Group B inoculated with infected cheek dermis from mice in Group A and necropsied on day 25 PI had numerous caryocysts within the cheek dermis and, although quantitative comparisons were not made, caryocysts appeared more numerous than in mice of Group A. Control mice in Group B fed cheek dermis from control mice in Group A were not infected. Mice in Group C receiving