Sarcocystis Idahoensis Sp. N. In Deer Mice Peromyscus Maniculatus (Wagner) and Geopher Snakes Pituophis Melanoleucus (Daudin) * , †

SYNOPSIS Deer mice Peromyscus maniculatus (Wagner) were trapped near Hammett, Idaho, as a possible source of Besnoitia jellisoni Frenkel and species of Sarcocystis to be used for life cycle studies. Forty‐nine deer mice were necropsied; 20 (40.8%) were positive for sarcocysts structurally identical with those of Sarcocystis idahoensis sp. N. the source of S. idahoensis used for life cycle studies was a Great Basin gopher snake Pituophis melanoleucus deserticola Stejneger killed near Hammett, Idaho; 20 sporulated sporocysts measured 11.1 × 13.4 (11‐12 × 13‐14) μm. Structurally identical sporocysts were found in 7 of 14 Pacific gopher snakes P. m. catenifer (Blainville), and in 6 of 10 San Diego gopher snakes, P. m. annectens Baird & Girard. Totals of 148 deer mice and 17 gopher snakes were necropsied in the course of life cycle studies. Development of the first generation meronts took place within the hepatocytes of deer mice 2‐10 days post‐inoculation (PI) with sporulated sporocysts. Rosette‐shaped meronts (6‐8 days PI) contained tachyzoites attached by their posterior poles to a residual body. After release from the residual body, tachyzoites were initially retained in a meront wall and later released from the hot cells Within muscle cells a single tachyzoite‐shaped structure was found 11 days PI and PAS‐negative metrocyte‐containing sarcocysts (2nd generation meronts) 13‐34 days PI. PAS‐positive material was first seen in sarcocysts 34 days II at which time bradyzoite formation became apparent. At 160 days PI, 10 sarcocysts measured 0.4 × 5.8 (0.2‐0.9 × 1.8‐9.9) μ and appeared to be mature and structurally identical with those from naturally infected deer mice. After ingestion of S. idahoensis ‐infected deer mice by gopher snakes, bradyzoites developed directly into microgamonts and macrogametes. These stages were first seen 5 days PI. Microgamonts were generally located above and macrogametes below the epithelial host cell nucleus. Seven to 11 days PI microgamonts were seen with mature microgametes, and oocysts which had not yet begun sporogony were found with oocyst walls. Clinical signs of illness were generally not observed in infected gopher snakes; however, one snake developed anorexia and cachexia, and became moribund after repeated ingestion of heavily infected deer mice. Acute hepatitis associated with developing meronts often was noted in deer mice given over 15,000 sporocysts each. Five to 6 days PI anorexia, weakness, ataxia, and dyspnea were observed: these clinical signs increased in severity until 6‐8 days PI, when mice became recumbent and died, or were killed while moribund. Hepatosplenomegaly, petechial hemorrhage o the serosal and cut surfaces of the liver, and icterus were common. Diffuse coagulative necrosis with cellular infiltration (primarily neutrophils) was noted on microscopic examination.