Pre-erythrocytic development and associated host responses to Haemoproteus meleagridis (Haemosporina: Haemoproteidae) in experimentally infected domestic turkeys

Two generations of pre-erythrocytic schizogony occurred in skeletal and cardiac muscle of domestic turkeys infected with sporozoites of Haemoproteus meleagridis. First generation schizonts reached maturity approximately five days post-inoculation (DPI) and developed in capillary endothelial cells and myofibroblasts. The schizonts ranged from 12 to 20 microns in diameter and produced long (5-6 microns), slender merozoites. Early second generation schizonts were first detected in capillary endothelial cells between 5 and 8 DPI. They were cylindrical and ranged in size from 5 to 8 microns in diameter and up to 28 microns in length. Second generation schizonts which reached maturity by 17 DPI were surrounded by a thick, hyaline wall and were packed with numerous spherical merozoites less than 1 micron in diameter. Mature megaloschizonts were fusiform, ranged from 30 to 113 microns in diameter, and extended as much as 465 microns along the long axis of muscle fibers. Merozoites developed as buds from cytomeres that formed between 8 and 14 DPI. Infected turkeys developed a moderate to severe myositis within 5 DPI and were lame in one or both legs. The myositis was associated with the necrosis of scattered groups of muscle fibers. Muscle fibers surrounding mature megaloschizonts were swollen and hyaline. Megaloschizonts were surrounded occasionally by fibroblasts and infiltrates of mononuclear cells. The morphology and site of development of mature megaloschizonts of Haemoproteus meleagridis are contrasted with those of other avian haemosporidians.     

Myopathy associated with megaloschizonts of Haemoproteus meleagridis in a wild turkey from Florida

Necropsy of an emaciated adult wild turkey (Meleagris gallopavo osceola) that died in captivity soon after capture revealed numerous macroscopic 1-2 mm white cysts in the pectoral muscles. Microscopic, aseptate protozoan megaloschizonts, 50-150 microns in diameter, corresponded to the cysts in histological sections. The megaloschizonts were surrounded by a thick, hyaline wall and packed with spherical merozoites less than 1 micron in diameter. Muscle fibers surrounding most of the megaloschizonts exhibited early signs of dystrophic calcification. The fibers were swollen, pale and hyaline and contained scattered basophilic granules. The megaloschizonts were morphologically distinct from sarcocysts of Sarcocystis sp. and Besnoitia sp. and the thin-walled tissue cysts of Toxoplasma gondii. They were identical in structure and host reaction to the second-generation megaloschizonts of Haemoproteus meleagridis, reported previously from experimentally infected domestic turkeys. While the precise cause of death of the wild turkey could not be determined, the most prominent lesions were associated with the numerous intramuscular megaloschizonts.     

Observations on First-Generation Schizogony of Leucocytozoon caulleryi in Chickens

ABSTRACT. First and second generation schizogony of Leucocytozoon caulleryi occurred in chickens infected with sporozoites. First generation schizogony was studied by light and electron microscopy. First-generation schizonts were first detected in capillary endothelial cells in the spleen, lung, liver, and bursa of Fabricius between 3 and 6 d post-sporozoite inoculation (DPI). The schizonts ranged from 15 to 65 μm in diameter and were surrounded by a thin pellicle. Early schizonts contained numerous round or oval nuclei, endoplasmic reticulum, and mitochondria. The schizonts reached maturity 5 DPI and produced first-generation merozoites which were released into the peripheral bloodstream. The merozoites. which were infective to chickens, measured 7.1 μm in length. They were slender and had a large nucleus, a mitochondrion, and an apical complex consisting of three polar rings, rhoptries, numerous micronemes. The morphology of first-generation merozoites was different from that of second-generation merozoites.     

Pathogenicity of Haemoproteus meleagridis (Haemosporina: Haemoproteidae) in experimentally infected domestic turkeys

Sporozoite-induced experimental infections of Haemoproteus meleagridis produced a moderate to severe myositis and significant effects on weight gain and growth in domestic turkey poults. Pathological effects occurred in both low- and high-dose infections (4,400 and 57,500 sporozoites, respectively). Low-dose birds weighed significantly less than controls at 3 wk postinfection (PI) when peripheral parasitemia reached a peak and had significantly shorter tarsometatarsal lengths at both 1 and 3 wk PI. High-dose birds were significantly lighter and smaller than control and low-dose birds throughout the course of the 8-wk study. Infected birds were not anemic in spite of high parasitemias that often exceeded 50% of circulating erythrocytes. The most serious pathological effects occurred prior to patency and were associated with development of megaloschizonts in skeletal muscle. Microscopic lesions in 4 high-dose birds that died between 19 and 22 days PI were characteristic of a severe, acute hemorrhagic myositis. Megaloschizonts were surrounded by a hemorrhagic inflammatory infiltrate composed of macrophages, heterophils, giant cells, and red blood cells. Muscle fibers adjacent to megaloschizonts were swollen, hyaline, and contained prominent calcium deposits. Other observations included enlargement of the spleen, deposition of pigment in macrophages of the lung and spleen, and secondary bacterial and fungal infections in the intestine and lungs. Necrotic and calcified muscle fibers and degenerating megaloschizonts were still present at 8 wk PI when the experiment ended. Our results demonstrated significant pathological changes in H. meleagridis-infected domestic turkeys that were associated primarily with preerythrocytic stages of development.     

Schizogony and Gametogony of Leucocytozoon simondi and Associated Reactions in the Avian Host*

SYNOPSIS. Stages of development of Leucocytozoon simondi in White Pekin ducklings and their reactions to the parasite were studied on successive days after infecting them artificially with sporozoites from Simulium rugglesi. The minimum prepatent period was 5 days. The first asexual cycle occurred exclusively in the parenchymal cells of the liver. Progeny of these hepatic schizonts followed one of 3 courses: (a) invaded parenchymal liver cells to give rise to another hepatic cycle, (b) penetrated blood cells to form round gametocytes, and (c) were phagocytized by macrophages and grew into megaloschizonts thruout the body. The appearance of elongating gametocytes coincided with the period of maturation and release of merozoites from the megaloschizonts. Experimental evidence supports the hypothesis that the round gametocytes arise from the hepatic schizonts and the elongate forms from the megaloschizonts. Mature megaloschizonts released millions of merozoites, but a high 2nd peak in parasitemia did not develop because of retention of developing gametocytes in the deep circulation, particularly the liver and spleen, and a pronounced host reaction.     

Endogenous Cycle of the Swine Coccidium Eimeria debliecki Douwes, 1921*

SYNOPSIS. A pure strain of Eimeria debliecki (University of Illinois strain A) established from a single oocyst was used to determine the endogenous cycle. Young parasite-free pigs 2 weeks to 3 months old were used throughout the study. The endogenous cycle was found to take place in the small intestine where the parasites were located in the distal portion of the striated simple columnar epithelial cells of the villi. The first generation schizonts were found in only the jejunum (15% of small intestine). The second generation schizonts and gametes occurred in the jejunum and ileum (70% of small intestine), a slight posterior progression occurring with each stage. The entire cycle required 6.5 days. The schizogonous cycle comprised 2 generations. The first generation schizonts required 2.5 days to reach maturity, measured 8-12 μ, contained 16 merozoites measuring 12-15 μ and had a polar residual mass. The second generation schizonts required 2 days to reach maturity, measured 13-16 μ, contained 32 rotund merozoites measuring 6–8 μ, and had only a few granules of residual material. Gametogony took place in 1.5 days. The macrogametes measured 12-16 μ, and the microgametocytes measured 9-14 μ with microgametes measuring 5–6 μ.     

Development of Eimeria meleagrimitis Tyzzer from Sporozoites and Merozoites in Turkey Kidney Cell Cultures*

SYNOPSIS. Sporozoites and 1st-, 2nd-, and 3rd-generation merozoites of Eimeria meleagrimitis were inoculated into primary cultures of turkey kidney cells. In vitro-excysted sporozoites developed into mature macrogamonts in 8 days; in vivo-excysted sporozoites developed into 2nd- or 3rd-generation schizonts within 5 to 7 days. First-generation merozoites obtained from infected turkeys produced mature 2nd-generation schizonts within 24 h. Second-generation merozoites from turkeys produced mature macrogamonts and oocysts within 72 h, whereas 3rd-generation merozoites produced these stages within 48 h. The oocysts that developed from 3rd-generation merozoites sporulated at 25 C and were infective for turkeys. The timing of the early stages and the intervals between schizogonic generations in cultures were comparable with those in turkeys. Morphologic parameters, however, indicated that some differences existed between in vitro and in vivo development. Second- and 3rd-generation schizonts and gamonts that developed after inoculation of cultures with merozoites were similar to stages in turkeys. Oocysts, however, were significantly smaller (P < 0.05) in cultures. All stages that developed after inoculation of cultures with sporozoites were smaller (P < 0.05) than their in vivo counter parts.     

Life Cycle of Isospora canis Nemeséri, 1959 in the Dog*

The life cycle of I. canis Nemeséri, 1959 was studied in experimentally infected dogs. Freshly sporulated oocysts were ovoid and 34–40 × 28–32 μm. The endogenous stages were found directly beneath the epithelium of the distal portion of the small intestinal villi. Most of the endogenous stages were in the lower 1/3 of the small intestine, but occasionally they were found in other portions of the small intestine. Three asexual generations were present. First-generation schizonts were 16–38 × 11–23 μm and contained 4–24 merozoites; mature 1st-generation merozoites were 8–11 × 3–5 μm. First-generation schizogony lasted up to 7 days after inoculation. Second-generation schizonts were 12–18 × 8–13 μm and contained up to 12 merozoites which were 11–13 × 3–5 μm. Second-generation schizogony was present on postinoculation days 6 and 7. Third-generation schizonts were formed by nuclear division of 2nd-generation merozoites. Most 2nd-generation merozoites underwent nuclear division without leaving the parasitophorous vacuole of the 2nd-generation schizont. Mature 3rd-generation schizonts were 13–38 × 8–24 μm and contained 6–72 merozoites. Third-generation merozoites were 8–13 × 1–3 μm. Third-generation schizogony was present on days 6–8 after inoculation. Mature macrogametes were 22–29 × 14–23 μm. Mature microgametocytes were 20–38 × 14–26 μm. Gametes were present on postinoculation days 7–10. Oocysts were present in tissue sections on postinoculation days 8–10 and 12. The prepatent period was 9–11 days.     

Elongate and Round Gametocytes of Leucocytozoon simondi (Mathis & Léger) in Ducks Inoculated with Megaloschizonts

SYNOPSIS. Single megaloschizonts give rise to elongate and round gametocytes, the former outnumbering the latter. Male and female elongate gametocytes develop from merozoites of a single megaloschizont. Elongate gametocytes were seen 2–7 days and round gametocytes 6–11 days after megaloschizonts had been inoculated into ducklings. Experimental evidence indicates that merozoites of megaloschizonts invade blood cells and develop into elongate gametocytes. Other merozoites infect tissue cells and develop into secondary exoerythrocytic schizonts which give rise to round gametocytes. Relapse in Leucocytozoon simondi infections is discussed in relation to megaloschizont-induced exoerythrocytic schizogony.     

The Life Cycle of Isospora felis Wenyon, 1923, a Coccidium of the Cat*

SYNOPSIS. A pure strain of Isospora felis derived from a single oocyst was used to study the endogenous cycle. One and a half to two-month-old laboratory-reared, coccidia-free kittens were used thruout the study. The endogenous stages occurred in the epithelial cells of the distal parts of the villi in the ileum and occasionally duodenum and jejunum. All stages lay above the host cell nucleus. There were 3 asexual generations. The 1st generation schizonts were 11–30 by 10–23 μ when mature and contained 16–17 banana-shaped merozoites 11–15 by 3–5 μ. They became mature in 96 or sometimes in 120 hours. The 1st generation merozoites entered new host cells, rounded up and formed 2nd generation schizonts. These formed within themselves 2–10 or more spindle-shaped bodies resembling 1st generation merozoites in shape and size. These were 2nd generation merozoites. They were uninucleate 120 hours after inoculation, but by 144 hours they became larger, multinucleate and some lost their elongate shape and became ovoid. They were then 3rd generation schizonts. They were 12–16 by 4–5 μ. Each formed up to 6 or more banana-shaped merozoites 6–8 by 1–2 μ. The 3rd generation schizonts and merozoites developed within the same host cell and parasitophorous vacuole as the 2nd generation schizonts and merozoites. Mature schizonts containing only 3rd generation merozoites appeared 144 hours after inoculation, were most abundant 168 hours after inoculation, and might be present as late as 216 hours after inoculation. They were 14–36 by 13–22 μ and contained 36 to more than 70 merozoites. The 3rd generation merozoites entered the sexual cycle. The mature microgametocytes were 24–72 by 18–32 μ and contained a central residuum and a large number of microgametes 5–7 by 0.8 μ with 2 posteriorly-directed flagella. The mature macrogametes were 16–22 by 8–13 μ. Gametogony occurred 144–216 hours after inoculation. The prepatent period was 168–192 hours and the patent period 10–11 days. Peak oocyst production occurred on the 6th day of the patent period.     

Scanning Electron Microscopy of Schizogony in Eimeria tenella

SYNOPSIS. Developing 2nd- and 3rd-generation schizonts of Eimeria tenella were found in the ceca of chicks infected orally with sporulated oocysts. Several free 2nd-generation schizonts, which varied in diameter from 11 to 21.6 μm, were found on the epithelial surface of the cecum. Some schizonts appeared to have lost merozoites. Other schizonts were intact, one of which was surrounded by an unbroken membrane that followed the contours of the merozoites. Third-generation schizonts, much smaller than 2nd-generation schizonts and with fewer merozoites, were found only on cut or fractured surfaces of the cecal tissue. Third-generation merozoites appeared shorter and thicker than those of the 2nd-generation and were attached to the schizont residuum. A form with conical protuberances and another with 4 triangular segments were found; they were believed to be developing stages 3rd-generation schizonts.     

Development and Ultrastructure of First-Generation Meronts of Sarcocystis cruzi in Calves Fed Sporocysts from Coyote Feces*

SYNOPSIS. The development of Sarcocystis cruzi Hasselmann (syn. S. fusiformis Railliet) meronts was studied in seven 7- to 10-day-old calves killed 4, 7, 11, 15, 22, 25 and 28 days postinoculation (DPI) with 5 × 10⁷ sporocysts from feces of coyotes. No meronts were found 4 and 7 DPI. Young and intermediate meronts with 1–16 nuclei were found in endothelial cells of arteries in mesenteric lymph nodes, but not in kidneys 11 DPI. Mature meronts were noted in endothelial cells of arteries, arterioles, or capillaries of many organs of calves killed 15 to 25 DPI. No first-generation meronts were found 28 DPI. By electron microscopy, all stages of the first-generation merogony were found free within the host cell cytoplasm and not within a parasitophorous vacuole. The appearance of intranuclear spindles preceded the formation of merozoites by endopolygeny. Mature meronts measured 41.0 × 17.5 (34–50 × 15–24) μm, contained ～ 100–350 merozoites, and had 2 to 4 relatively small residual bodies, 2.8 μm in diameter. Merozoites measured 6.3 × 1.5 (5.5–7 × 1 μm) and contained most of the organelles characteristically found in coccidian merozoites. Micropores were observed in merozoites, but not in young and intermediate meronts. Merozoites were seen free in the lumen of blood vessels, in intracellular areas, and free within the host cell cytoplasm.     

Development of Eimeria meleagrimitis Tyzzer from sporozoites and merozoites in turkey kidney cell cultures

Sporozoites and 1st-, 2nd-, and 3rd-generation merozoites of Eimeria meleagrimitis were inoculated into primary cultures of turkey kidney cells. In vitro-excysted sporozoites developed into mature macrogamonts in 8 days; in vivo-excysted sporozoites developed into 2nd- or 3rd-generation schizonts within 5 to 7 days. First-generation merozoites obtained from infected turkeys produced mature 2nd-generation schizonts within 24 h. Second-generation merozoites from turkeys produced mature macrogamonts and oocysts within 72 h, whereas 3rd-generation merozoites produced these stages within 48 h. The oocysts that developed from 3rd-generation merozoites sporulated at 25 C and were infective for turkeys. The timing of the early stages and the intervals between schizogonic generations in cultures were comparable with those in turkeys. Morphologic parameters, however, indicated that some differences existed between in vitro and in vivo development. Second- and 3rd-generation schizonts and gamonts that developed after inoculation of cultures with merozoites were similar to stages in turkeys. Oocysts, however, were significantly smaller (P less than 0.05) in cultures. All stages that developed after inoculation of cultures with sporozoites were smaller (P less than 0.05) than their in vivo counter parts.     

In Vitro Cultivation of the Vascular Phase of Sarcocystis singaporensis

To establish an in vitro culture system for the precystic phase of Sarcocystis singaporensis, we initially tested various excysting fluids for sporocysts. An excysting fluid containing 2.5% bovine taurocholate and 10% bile of the specific intermediate host, Rattus norvegicus, in RPMI medium was the most suitable resulting in excystation of 80% of the sporozoites. Subsequently, we identified brain endothelial cells and pneumonocytes of the rat to promote growth of sporozoites to schizonts. Hepatoma, fibroblastic, or myoblastic cells were not suitable for the parasite's development. First-generation schizonts were seen at days 3-10 postinoculation (PI); a distinct second peak of schizogonic development only occurred in endothelial cells at days 14-18 PI. First-generation schizonts were 26.0 (± 3.8) μm in diameter and contained 32-50 merozoites, second-generation schizonts measured 34.4 (± 10.6) μm and contained 54-72 merozoites. Merozoite yield at large-scale culture conditions (75 cm² flasks) using pneumonocytes as host cells was relatively low. Ultrastructurally, sporozoites and merozoites were quite similar to corresponding stages of other Sarcocystis species. With regard to host cell specificity and developmental kinetics, in vitro cultivation showed close similarities to the situation in vivo.     

Schizogonic Development of Leucocytozoon smithi

ABSTRACT The schizogonic development of Leucocytozoon smithi in the liver of experimentally infected turkey poults was examined by electron microscopy. Following intraperitoneal injection, sporozoites migrated to the liver and entered hepatic cells to become intracellular trophozoites. Three to four days post inoculation (PI), trophozoites underwent asexual multiple fission known as merogony or schizogony. Two generations of schizonts were observed. The primary or first generation schizonts, abundant on day 4 PI, appeared as interconnected cytoplasmic masses (pseudocytomeres). Each pseudocytomere was enclosed by a membranous vacuole and contained varying numbers of nuclei. As nuclear division and growth of the schizonts continued, larger discrete cytoplasmic masses or cytomeres were formed with rhoptries and multiple nuclei in various stages of division. Synchronous multiple cytoplasmic cleavage of the schizont resulted in the formation of numerous uninucleate merozoites. Second generation schizonts, which developed from hepatic merozoites released from primary schizonts, were abundant in hepatocytes on day 6 PI. Although tissue samples from liver, lung, spleen, kidney, intestine, brain, blood vessels and lymph nodes were examined, schizogonous forms were observed in liver only. No megaloschizonts were detected in any host tissue examined. Schizogonic development was completed by day 7 PI as merozoites developed into gametocytes within mononuclear phagocytes.     

Schizogonic development of Leucocytozoon smithi.

The schizogonic development of Leucocytozoon smithi in the liver of experimentally infected turkey poults was examined by electron microscopy. Following intraperitoneal injection, sporozoites migrated to the liver and entered hepatic cells to become intracellular trophozoites. Three to four days post inoculation (PI), trophozoites underwent asexual multiple fission known as merogony or schizogony. Two generations of schizonts were observed. The primary or first generation schizonts, abundant on day 4 PI, appeared as interconnected cytoplasmic masses (pseudocytomeres). Each pseudocytomere was enclosed by a membranous vacuole and contained varying numbers of nuclei. As nuclear division and growth of the schizonts continued, larger discrete cytoplasmic masses or cytomeres were formed with rhoptries and multiple nuclei in various stages of division. Synchronous multiple cytoplasmic cleavage of the schizont resulted in the formation of numerous uninucleate merozoites. Second generation schizonts, which developed from hepatic merozoites released from primary schizonts, were abundant in hepatocytes on day 6 PI. Although tissue samples from liver, lung, spleen, kidney, intestine, brain, blood vessels and lymph nodes were examined, schizogonous forms were observed in liver only. No megaloschizonts were detected in any host tissue examined. Schizogonic development was completed by day 7 PI as merozoites developed into gametocytes within mononuclear phagocytes.     

Eimeria apsheronica in the goat: endogenous development and host cellular response

Most first generation schizonts of Eimeria apsheronica developed in the jejunum; others were distributed throughout the small intestine and occasionally in the caecum. Some were also found in the mesenteric lymph nodes, which were oedematous and haemorrhagic. In the intestine, haemorrhage and congestion were seen before parasites were detected, and continued throughout all later stages. Schizonts occurred in the lamina propria and occasionally in the submucosa, where they sometimes caused a cellular inflammatory response. Schizonts were first seen at 8 days post-infection (DPI); they had poorly defined nuclei and were enclosed in a capsule-like wall. At 16 DPI, many had matured, had a mean size of 125 x 82 microns, and were filled with numerous spindle-shaped merozoites, which were in ranks and loops. At 18 and 20 DPI, when small white lesions (1-3 mm in diameter) were observed in the jejunum and elsewhere in the small intestine, a second generation of schizonts, macrogametes, microgametocytes and maturing oocysts were seen, in the epithelial cells of the small intestine and caecum. Their mean sizes, respectively, were: 26.2 x 18.9, 24.7 x 18.5, 30.2 x 21.7 and 26.6 x 19.3 microns. Macrogametes contained basophilic central and eosinophilic peripheral granules. The sexual stages were associated with a generalized cellular inflammatory response.     

The Life Cycle of Eimeria dispersa Tyzzer, 1929 in Turkeys

SYNOPSIS. The life cycle of a turkey strain of Eimeria dispersa Tyzzer was studied in Beltsville Small White turkeys. There were 4 asexual generations. Mature schizonts of the first generation were present 30 h postinoculation (PI); those of the 2nd, 3rd, and 4th generations were present 48, 72, and 96 h PI, respectively. Average size of schizonts and number and size of merozoites for each generation were as follows: first , 14.3 × 13.0 μm with 19.2 merozoites, each 4.5 × 1.2 μm; second , 8.0 × 7.2 μm with 13.5 merozoites, each 4.5 × 1.1 μm; third , 8.9 × 8.9 μm with 15.1 merozoites, each 5.6 × 2.1 μm; fourth , 11.6 × 10.5 μm with 6.7 merozoites, each 8.2 × 2.0 μm. Sporozoites and developmental stages of the first generation were in close association with an epithelial cell nucleus and located between the brush border and the "row" of epithelial cell nuclei; developmental stages of the other 3 generations were not associated with a nucleus and were located just under the brush border. Early macrogametes and microgametocytes were present 96 h PI. Development was confined to the epithelial cells of the villus and extended from the tip of the villus to ∼ 1/2 the distance down the sides in all areas of the intestine except the cecum. The prepatent period was between 114 and 120 h. Percentage of sporulation was 15, 57, and 90, at 24, 36, and 48 h, respectively. Sporulated oocysts averaged 24.5 × 20.2 μm.     

Development of Eimeria callospermophili and E. bilamellata from the Uinta Ground Squirrel Spermophilus armatus in Cultured Cells*

SYNOPSIS. Cell lines or established cell lines of bovine, ovine or human origin and primary cells from whole embryos of groundsquirrels were used in a study of the in vitro development of Eimeria callospermophili and E. bilamellata from the Uinta ground squirrel, Spermophilus armatus. Monolayers in Leighton tube cultures were inoculated with sporozoites of either of these 2 species and examined with phase-contrast microscopy at various intervals. After such examination, coverslips were fixed in Schaudinn's or Zenker's fluid and variously stained. E. callospermophi sporozoites penetrated cells and underwent development to mature 1st generation schizonts in most cell types. At different times after inoculation, both species formed sporozoite-shaped schizonts, which later became spheroidal. Intracellular movements of sporo zoite-shaped schizonts of E. callospermophili were observed and such schizonts penetrated cells when freed by mechanical disintegration of the host cells. Merozoites were formed at the periphery of the schizont in both species. Mature 1st generation schizonts of E. callospermophili, with 6–14 merozoites, were first seen 15 hr after inoculation; the corresponding values for E. bilamellata were 12–27 merozoites and 4 days. Merozoites of both had anterior and posterior refractile bodies. Exposure to a trypsin-bile solution stimulated motility in merozoites of E. callospermophili. Second generation trophozoites and immature schizonts of E. callospermophili were seen in cultures of primary cells of whole ground-squirrel embryos 20–24 hr and 44–48 hr, respectively, after inoculation of sporozoites.     

Development of Ox-Coyote Cycle of Sarcocystis cruzi1

The ox-coyote cycle of Sarcocystis cruzi was studied by killing 38 calves between 4 and 153 days postinoculation (DPI) with 55 × 10³-5 × 10⁸ sporocysts from the intestines of coyotes. At 4 DPI, a zoite was found within the lumen of a mesenteric lymph node artery. At 7 DPI, zoites were found in mononuclear cells and in endothelial cells in mesenteric arteries. First generation meronts (41.0 × 17.5 μm in diameter) occurred 7–26 DPI in mesenteric lymph nodes. At 19–46 DPI, second generation meronts occurred in kidneys, muscles, and other tissues: renal meronts were 19.6 × 11.0 μm, and intramuscular meronts were 25.0 × 11.1 μm. Merozoites were found in the peripheral blood 17 DPI and later at 24–46 DPI. They divided by endodyogeny in mononuclear cells. Sarcocysts were seen first in the heart at 45 DPI and contained one or two metrocytes. At 55 DPI, sarcocysts containing only metrocytes were found in striated muscles, heart, and in smooth muscles of the urinary bladder, rumen, omasum, abomasum, and small intestine. At 67, 87, 112, and 153 DPI, sarcocysts were found only in striated muscles and in the heart. At 67 DPI, sarcocysts were up to 360 μm long. They contained only metrocytes and were not infective to the dog. At 86 DPI, sarcocysts contained mostly bradyzoites, a few metrocytes, and were infective to a coyote. The thin-walled sarcocysts grew to a maximum length of 800 μm and contained bradyzoites that were 10.9 × 3.0 μm. At 90 DPI, two mature sarcocysts were found in 2 of 73 sections of brain and spinal cord; hundreds of sarcocysts were present in sections of tongue and heart of this calf. Gametogony occurred in the small intestine of the coyote. Macro-and microgamonts were found in goblet cells of the small intestines of coyotes 6 h after the ingestion of infected meat. Microgamonts were few and contained 3–11 slender gametes. Oocysts were seen at 12 h and sporulation was completed 9 DPI. The prepatent period in the coyote was 8 days. The ox-coyote cycle is compared with ox-dog cycle.